International Journal of Audiology 2015; 54: S9–S18
Original Article
The interaction of hearing loss and level-dependent hearing protection on speech recognition in noise Christian Giguère, Chantal Laroche & Véronique Vaillancourt Audiology and Speech-Language Pathology Program, University of Ottawa, Ottawa, Ontario, Canada
Abstract Objectives: To determine the effects of different control settings of level-dependent hearing protectors on speech recognition performance in interaction with hearing loss. Design: Controlled laboratory experiment with two level-dependent devices (PeltorÒ PowerCom PlusÔ and Nacre QuietProÒ) in two military noises. Study sample: Word recognition scores were collected in protected and unprotected conditions for 45 participants grouped into four hearing profile categories ranging from within normal limits to moderate-to-severe hearing loss. Results: When the level-dependent mode was switched off to simulate conventional hearing protection, there were large differences across hearing profile categories regarding the effects of wearing the devices on speech recognition in noise; participants with normal hearing showed little effect while participants in the most hearing-impaired category showed large decrements in scores compared to unprotected listening. Activating the level-dependent mode of the devices produced large speech recognition benefits over the passive mode at both low and high gain pass-through settings. The category of participants with the most impaired hearing benefitted the most from the level-dependent mode. Conclusions: The findings indicate that level-dependent hearing protection circuitry can provide substantial benefits in speech recognition performance in noise, compared to conventional passive protection, for individuals covering a wide range of hearing losses.
Key Words: Hearing loss; hearing protection; speech recognition; noise Noise is one of the most common problems in the workplace. It has been estimated that 16% of adult-onset hearing loss worldwide is due to occupational noise exposure (Nelson et al, 2005). In the USA, approximately 22 million workers are exposed to daily hazardous occupational noise levels (Tak et al, 2009). In a sample of over 1.1 million American workers, 18% had hearing loss; prevalence ratios were especially high in mining, wood product manufacturing and construction of buildings (Masterson et al, 2013). In the European Union, about 30% of workers indicate that they are exposed to loud noises at least a quarter of their work shift, and 7% report a workrelated hearing disorder (Parent-Thirion et al, 2007). Noise-induced hearing loss is particularly prevalent in the military. In the Canadian Forces, 42% of service members show at least a mild hearing loss and 26% develop a moderate to severe hearing loss by midlife, with some military trades such as infantry, artillery, and flight engineers being the most at risk (Abel, 2005). As convincingly argued by Suter (2012), engineering noise control is the essential pillar of hearing loss prevention programs to better protect hearing and promote a safer workplace for everyone. While engineering control at the source is the preferred mitigation method, it is not always feasible to reduce noise to safe levels in
some working environments or for some occupations. In these cases, short-term or long-term supplementary methods such as the use of personal hearing protection devices (HPDs) can become necessary (Gerges & Casali, 2007; Canetto, 2009). Conventional HPDs are passive devices providing a fixed amount of attenuation across a wide range of noise levels in the work environment. Since they also reduce the level of speech and other important sounds, a compromise must be achieved in the attenuation provided for optimal protection and communication needs (CSA Z94.2-02 R2011; EN 458: 2004). However, this goal is difficult to ascertain in practice due to a range of factors, including the uncertainty in the effective attenuation achieved across workers or over time in a given worker, the variation in signal and noise levels occurring in the work day, and the differences in hearing acuity among workers. When properly selected and fitted, conventional HPDs are adequate for most situations involving workers with normal hearing. However, they may interfere with aural communication tasks and work performance in workers with hearing loss (Morata et al, 2005; Abel, 2008; Canetto, 2009; Casali, 2010; Giguère et al, 2010; Themann et al, 2013a,b). For these individuals, the combined effect of hearing loss and hearing protection may be such that important sounds from the
Correspondence: Christian Giguère, Audiology and Speech-Language Pathology Program, University of Ottawa, 451 Smyth Road, Room 3056, Ottawa, Ontario K1H 8M5, Canada. E-mail: [email protected] (Received 25 September 2014; accepted 1 October 2014 ) ISSN 1499-2027 print/ISSN 1708-8186 online © 2015 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2014.973540
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Abbreviations HINT HPD L NRR PTA SNR SR TT
Hearing in noise test Hearing protection device Level Noise reduction rating Pure-tone average Signal-to-noise ratio Surround Talk-through
environment become inaudible or masked, compromising auditory situational awareness. In focus group meetings with industrial workers and military personnel (Morata et al, 2005; Abel, 2008), serious concerns were raised about job performance and safety when working in noise with a hearing loss, especially when HPDs are used. Methods to promote the selection of the most appropriate HPDs are needed as well as the identification of the types of hearing protection and/or amplification devices that are most beneficial to the population of hearing-impaired workers (Themann et al, 2013a,b). Powered-electronic hearing protectors are rapidly being introduced into the marketplace, often with the dual purpose of achieving protection against noise and enhancing situational awareness. In principle, workers with pre-existing hearing loss would particularly benefit from hearing protectors with level-dependent attenuation to maintain or enhance signal audibility in all noise conditions. Such devices typically provide a small amount of amplification in low noise conditions while maintaining exposure to a safe limit in very noisy conditions throughout the day. Unfortunately, the available research on the effects of level-dependent powered electronic HPDs during communication tasks is rather limited (see Casali, 2010, and Giguère et al, 2011a for reviews). This is especially the case for hearing-impaired users, the most likely beneficiaries, for which only a handful of studies are available (Abel et al, 1993, 1996; Dolan & O’Loughlin, 2005; Tufts et al, 2011; Williams, 2011; Talcott et al, 2012). Current level-dependent HPDs often have different modes of operation and several user-adjustable settings and options, which can complicate the selection of the best solution for each particular situation. Moreover, detailed information on the electroacoustic characteristics of the devices is often not supplied by the manufacturers or made readily available, making it difficult to gain a full understanding of the behavior of the devices in various signal and noise conditions, which compounds the problem. This paper reports results from a speech recognition experiment conducted with two level-dependent HPDs, one earmuff and one insert-type device, involving participants with a wide range of hearing profiles. The research project builds on a previous study on the effects of conventional hearing protectors on speech perception in noise for individuals with hearing loss (Giguère et al, 2010). The focus of the present research is on the interaction between the level-dependent amplification setting of the device and the degree of hearing loss on speech recognition outcome.
Methods Participants A total of 45 English-speaking adults (24 males and 21 females) between the ages of 23 and 81 years old (mean 48; SD 16) participated in the study. No restriction was placed on the degree of
hearing loss and on whether the hearing loss was of conductive, sensorineural, or mixed origin. Recruitment was carried out by means of posters displayed in various settings, including the University of Ottawa, audiology clinics, community centres and medical centres. The experimentation took place in the Research Unit in Noise and Communication at the Faculty of Health Sciences. Testing with human participants was approved by the Office of Research Ethics and Integrity at the University of Ottawa. The hearing assessment was carried out in an IAC double-wall audiometric booth using a portable tympanometer (Grason-Stadler GSI 38) and a clinical audiometer (Interacoustics AC40) equipped with insert earphones (3M E-A-RTONE 3A) and a bone vibrator (Radioear B-71). The participants were grouped into four previously defined hearing profile categories (Giguère et al, 2010) based on the audiogram (250–8000 Hz) and the five-frequency (500–4000 Hz) pure-tone average (PTA), as follows: (1) Within normal limits (hearing thresholds 25 dB HL at all frequencies in both ears), (2) Slight-to-mild (best-ear PTA 25 dB HL with hearing thresholds greater than 25 dB HL at one or more individual frequencies, (3) Mild-to-moderate (best-ear PTA between 26 and 40 dB HL), and (4) Moderate-to-severe (best-ear PTA between 41 and 55 dB HL). The goal was to recruit 12 participants in each category. This was achieved for all categories, except for the Moderate-to-severe profile where only nine participants were recruited. The headphone version of the American English hearing-in-noise test (HINT) was also administered (Vermiglio, 2008) as part of the hearing assessment to document supra-threshold hearing performance in each hearing loss category. The HINT is a binaural speech recognition threshold test carried out in quiet and in three spatial conditions of fixed 65-dBA speech-spectrum noise (front, left, right), with speech incidence always from the front. Testing was performed using four different HINT lists, counterbalanced across participants, among the twelve available test lists. The speech levels were adaptively varied to measure the SRT (SNR at which 50% of the sentences are repeated correctly) for each of the three noise conditions. The composite score was then computed as the weighted average of the SRTs for the noise front (NF), noise left (NL), and noise right (NR) conditions, as follows: composite score (2 NF NL NR)/4.
Devices Two tactical hearing protection devices were selected for the study: the PeltorÒ PowerCom Plus™ (3M Personal Safety Division, St. Paul, USA) and the Nacre QuietProÒ (Nacre A/S, Trondheim, Norway). In addition to two-way radio communication capabilities, both devices feature level-dependent stereo pass-through circuitry for improved listening in the immediate environment. Only this latter mode of operation, labeled “surround” (SR) by Peltor and “talk-through” (TT) by Nacre, is addressed in this paper. Depending on the chosen pass-through setting and the prevailing noise level, this feature can amplify or attenuate external sounds. The Peltor PowerCom Plus is an analog circumaural device with five different user-selectable level-dependent pass-through settings (referred to here as SR1 to SR5). At each setting, the maximum gain is achieved at low-to-moderate noise levels to facilitate situational listening, but with increasing noise levels the system gradually reduces the gain then shuts down transmission to re-establish passive protection. The level-dependent mode can also be deactivated (SR off), in which case the device provides conventional passive attenuation at a manufacturer-listed noise reduction rating (NRR)
Giguère: Hearing loss and level-dependent HPDs of 25 dB at all times. The Nacre QuietPro is a digital in-ear device with an external control unit. The user can manually select among ten level-dependent pass-through settings (referred to here as TT2 to TT11). At the lowest setting (TT1), the device provides passive attenuation at a manufacturer-listed NRR of 29 dB. At very high noise levels, once transmission of the level-dependent circuitry has been shut down, the device can provide an additional 6–8 dB of attenuation through active noise reduction. Unfortunately, technical specifications and/or quantitative details on the operation of the above features are not supplied in the product literature or made readily available by the two manufacturers. Electroacoustic measurements were carried out using an acoustic manikin (Brüel & Kjaer Type 4128) conforming to ANSI S3.36-1985 R2006, in order to gain more insight into the level-dependent characteristics of the two devices and guide the choice of experimental settings for the level-dependent pass-through systems. Speech-spectrum noise ranging in level from 40 dBA to 90 dBA was presented in front of the manikin at a 1-m distance. Sound measurements were made with the right ear microphone of the ear simulator assembly without (open-ear, unprotected) and with (occluded-ear, protected) the devices fitted on the manikin at different pass-through settings.
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Figure 1 shows the at-ear manikin sound levels as a function of the free field stimulus level for the five pass-through settings of the Peltor PowerCom Plus and for eight of the ten pass-through settings of the Nacre QuietPro, as well as for the open-ear condition. The open-ear sound levels are about 7 dB above the free field levels, owing to the natural transformation of sound by the torso, head, and external ear (Shaw, 1974). Data points above the open-ear curve indicate conditions for which the devices amplify external sounds, and conversely, data points below the curve indicate attenuation relative to the open-ear condition. The vertical difference with respect to the open-ear curve is the amount of insertion gain (in the case of amplification) or insertion loss (in the case of attenuation) provided by the devices. Both devices show well-defined level-dependent characteristics. At low free field levels ( 60 dBA), the Peltor PowerCom Plus provides linear amplification (Figure 1, A). As further described in Giguère et al (2011b), the gain-frequency response curve is relatively uniform (within 5 dB across mid-frequency octave bands) and the insertion gain ranges from about 6 to 13 dB across settings SR1 to SR5 (averaged over four octave bands 500-4000 Hz). At higher free field levels ( 60 dBA), the device shows gain
Figure 1. Input-output gain functions for the (A) Peltor PowerCom Plus, and (B) Nacre QuietPro devices measured on an acoustic manikin for frontal speech spectrum noise at different pass-through gain settings.
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compression, i.e. the increase in manikin sound level is less than the increase in free field sound level (Figure 1, A). The device behaves like an input-sensitive automatic gain controller with a compression ratio of 4:1 and compression threshold of 60 dBA. At a free field level of 85 dBA, the device provides 6 to 25 dB of insertion loss (attenuation) for settings SR5 to SR1, respectively. The Nacre QuietPro provides similar characteristics to the Peltor device, with some differences. At low free field levels, the Nacre also provides linear amplification (Figure 1, B); the gain-frequency response increases at a rate of about 3 dB/octave from 125 to 4000 Hz and the insertion gain ranges from 12 to 12 dB for settings TT2 to TT11 (averaged over four octave bands 500–4000 Hz) (Giguère et al, 2011b). The compression ratio is the same as the Peltor device at 4:1, but the input-sensitive compression threshold is higher at 80 dBA (Figure 1, B). The Nacre device also shows output-sensitive compression characteristics that appear to limit atear sound levels to about 92 dBA. At a free field level of 85 dBA, it provides 1 to 14 dB of insertion loss (attenuation) for settings TT11 to TT2, respectively.
Noises Two noises developed for a previous study (Giguère et al, 2010) were used in the present experiment. The two noises were recorded from light-armored vehicles in the Canadian Forces. Noise 1 is from a LAVIII vehicle at the driver and crew commander positions while operating over rough terrain and highway. The A-weighted equivalent sound pressure level of Noise 1 is 95.3 dBA, with sizeable fluctuations in sound levels over time (L10 L90 14.6 dB, calculated over 4-s segments). Noise 2 is from a Bison vehicle at the driver and crew commander positions with the engine in idle. The A-weighted equivalent sound pressure level of Noise 2 is 89.5 dBA and is relatively steady over time (L10 L90 4.2 dB). The average one-third octave band levels of the two noises, measured in the field, are displayed in Figure 2. The average spectral slope, calculated over third-octave bands from 31.5 Hz to 8000 Hz, is 4.1 dB/octave and 2.6 dB/octave for Noises 1 and 2, respectively.
Experimental protocol Speech recognition data (% word recognition) were collected in the two military noises using the two level-dependent HPDs. Four listening conditions and operation of the devices were investigated:
(1) unprotected, (2) passive attenuation, (3) level-dependent attenuation at a low pass-through gain setting, and (4) level-dependent attenuation at a high pass-through gain setting, as summarized in Table 1. The first two listening conditions (No device, pass-through off) allowed a comparison with previous findings concerning the adverse effects of conventional passive attenuation on speech recognition in noise in individuals with hearing loss (Giguère et al, 2010). The two level-dependent conditions (pass-through low and high) allowed assessing the potential benefits of the transmission system in each device for this population. Note that the pass-through settings were selected to achieve approximately the same amount of low-level gain across the two devices in the range 500–4000 Hz, about 4 dB in the pass-through low condition and 10 dB in the pass-through high condition (Table 1). Each participant was tested with only one of the two devices (22 participants were assigned to the Peltor PowerCom Plus and 23 participants to the Nacre QuietPro). For each device, half of the participants were tested in Noise 1 and the second half in Noise 2. Within each hearing profile category, participants were assigned randomly to one of four possible device-noise combinations (two devices two noises), such that there were three participants from each hearing loss category for each device-noise combination, except in the Moderate-to-severe category (with only two participants for most device-noise combinations). Percent correct word recognition was measured at two fixed signal-to-noise ratios (SNRs) in each of the four listening conditions (Table 1), for a total of eight conditions per participant using their assigned device and noise. The unprotected and pass-through off conditions were tested first in this order, followed by the pass-through low or high conditions; the latter were counterbalanced. The speech material consisted of the remaining eight lists of HINT sentences not used during the hearing assessment. All lists were counterbalanced across participants and experimental conditions. Scoring was performed at the word level. Due to the very wide range of hearing profiles and speech recognition abilities in the participant sample, it was not possible to test all participants at the same SNRs as this would have led to a preponderance of ceiling and floor effects. Instead, SNRs were individually selected so as to achieve a percent correct score between 20% and 80% unprotected, as in our previous study (Giguère et al, 2010). The HINT test results obtained during the hearing assessment were used to guide the choice of the SNRs for each individual. If the score obtained at the first SNR fell within the 50–80% range, the second SNR was lowered in order to achieve a score in the 20–50% range, and vice-versa, the second SNR was increased if the first SNR yielded a score in the 20–50% range. Within participants, the same two SNRs were targeted in the four listening conditions, when
Table 1. Listening conditions. SR # or TT # refers to the leveldependent pass-through gain settings of the Peltor PowerCom Plus and Nacre QuietPro, respectively. Label
Device setting
Unprotected Pass-through off
No device SR off TT 1 SR 1 TT 4 SR 4 TT 10
Pass-through low
Figure 2. One-third octave band spectra of the two noises (field measurements.
Pass-through high
Comment Open ear Passive protection Level-dependent (low gain » 4 dB) Level-dependent (high gain » 10 dB)
Giguère: Hearing loss and level-dependent HPDs possible, to be able to compare the effects of the different device settings to unprotected performance under the exact same conditions. Different SNRs were required and used across participants to ensure a similar level of difficulty, irrespective of hearing profile.
Simulation room The experiment was conducted in a noise simulation room with inner dimensions of 4.29 m 3.65 m 2.42 m (length width height). The simulation room is lined with 35 wall and ceiling panels that can be changed from sound reflective to sound absorbent to modify the reverberation characteristics and sound field. The speech was presented from a single loudspeaker at 1 m in front of the participants in quasi-free sound field conditions. The noise was presented over a set of five loudspeakers and a subwoofer in quasi-diffuse field conditions. Further details on the sound field specifications and test qualification procedures are described in Giguère et al (2010). The 20 sentences in each HINT list were individually presented in different 4-s long noise segments. A sampling procedure was devised to extract a subset of 20 noise segments from each noise environment (Noise 1 or 2) that closely matched the underlying acoustical characteristics of the entire set of recordings from that noise environment (Figure 2). The sampling procedure was repeated independently for each participant and noise environment to use as much of the available noise data as possible. A software interface controlled the adjustment of the SNR that was selected for each HINT list and the stimulus presentation sequence. To change the SNR, only the speech level was varied; the noise level was not altered, to ensure the simulated noise conditions were closely reflecting the real noise environments recorded. Noise began 0.5 s before each sentence and typically lasted 1.0 s or more after the sentence ended. The participants were asked to repeat the sentences heard. Guessing was encouraged, but no feedback was provided.
Results
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categories defined earlier. Twelve participants had hearing thresholds within 25 dB HL at all frequencies in both ears and formed the normal hearing category, while the remaining 33 had sensorineural hearing losses and made up the Slight-to-mild, Mild-to-moderate and Moderate-to-severe categories. A total of 22 participants had symmetrical hearing (12 Normal, four Slight-to-mild, four Mild-tomoderate, and two Moderate-to-severe), and 23 had asymmetrical hearing (eight Slight-to-mild, eight Mild-to-moderate, seven Moderate-to-severe) defined as interaural threshold differences 10 dB at three frequencies, or 15 dB at two frequencies, or 20 dB at one frequency between 250 and 8000 Hz. Figure 3 presents additional data on the mean hearing thresholds across frequency and ear in each hearing profile category. Mean thresholds range from 8 dB HL to 28 dB HL at 250 Hz, and from 4 dB HL to 79 dB HL at 8000 Hz across categories. As shown, the four hearing categories are nearly equidistant on the dB HL scale within each frequency. Except for the normal hearing category, mean thresholds increase nearly monotonically with frequency. Table 2 also presents summary statistics for scores on the HINT test by hearing profile category. The speech reception threshold for 50% sentence recognition is reported in dBA for the quiet condition and in dB SNR for the noise front and composite score. There is a gradual increase in the mean speech reception threshold in quiet from 17.7 dBA to 43.5 dBA for the normal hearing to the Moderate-to-severe hearing loss category, reflecting differences in absolute hearing thresholds (Figure 3) across categories. Likewise, the noise front and composite SRT scores are also progressively higher from the normal hearing to the Moderate-to-severe hearing loss category, reflecting a gradual decrease in supra-threshold performance due to hearing loss and/or age. Considering the normative value of 6.4 dB for the composite score of the American English HINT (Vermiglio, 2008), the mean SNR loss in each category is as follows: Normal hearing (0.4 dB), Slight-to-mild (1.6 dB), Mild-to-moderate (3.9 dB,) and Moderate-to-severe (6.2 dB).
Speech recognition data set
Description of hearing profile categories Table 2 presents summary statistics for age and PTA threshold for the 45 participants grouped according to the four hearing profile
Figure 4 shows the complete data set of word recognition scores in the two military noises by level-dependent device. The recognition scores in the three protected listening conditions (pass-through off,
Table 2. Summary statistics for age, PTA, and HINT scores by hearing category. Normative data for the American English HINT are Quiet: 15.6 dBA (SD 3.1 dB), Noise Front: 2.6 dB SNR (SD 1.0 dB), Composite score: 6.4 dB SNR (SD 0.9 dB) (Vermiglio, 2008). HINT scores
Statistics
Age (years)
Best-ear PTA (dB HL)
Quiet (dBA)
Noise front (dB SNR)
Composite (dB SNR)
Normal (n 12)
Minimum Mean Maximum
23 29 47
0.0 4.9 14.0
13.7 17.7 21.9
3.7 2.1 1.1
6.8 6.0 5.0
Slight-to-mild (n 12)
Minimum Mean Maximum
36 54 72
9.0 20.1 25.0
20.1 26.7 33.6
3.0 1.5 0.1
5.9 4.8 2.9
Mild-to-moderate (n 12)
Minimum Mean Maximum
45 58 69
27.0 31.9 39.0
29.4 35.4 44.3
2.4 0.6 2.5
6.1 2.5 2.5
Moderate/severe (n 9)
Minimum Mean Maximum
42 61 81
42.0 48.1 55.0
27.9 43.5 51.2
0.0 1.7 3.5
1.5 0.2 1.7
Hearing category
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Figure 3. Mean pure-tone thresholds for the left (dashed line) and right ear (solid line) of participants by hearing profile category (▴ Normal, ◾ Slight-to-mild, ⚫ Mild-to-moderate, ♦ Moderate-tosevere). low or high) are contrasted to scores in the unprotected condition in the form of a scatter diagram. Each data point represents a pair of protected-unprotected measurements at the same participant-
specific SNR (two per participant). In total, 243 such measurement pairs could be collected out of a maximum of 270 pairs; the other 27 protected-unprotected pairs are missing due to exhausting the sentence lists in some instances. In Figure 4, the protected-unprotected difference directly reflects the performance increase (point above diagonal) or decrease (point below diagonal) due to wearing the device. Altogether, the data set demonstrates the wide range of speech recognition effects obtained in the present study across experimental conditions and participants. The panels in Figure 5 show the protected-unprotected results by hearing profile category and level-dependent device. It is clearly apparent that the difference between the protected and unprotected conditions is largely dependent on the device listening condition in interaction with the hearing profile. For participants with hearing thresholds within normal limits, the passive attenuation condition (pass-through off) did not affect speech recognition performance in the majority of cases. Except for three data points (two below diagonal, one above), the data lie in and around the diagonal for both devices for this group (Figure 5, A-B). Furthermore, in the level-dependent conditions, the devices either did not affect recognition performance (pass-through high for Nacre) or improved scores (pass-through low for Nacre, pass-through low and high for Peltor) for most participants in the normal-hearing category (Figure 5, C-F).
Figure 4. Protected versus unprotected speech recognition scores for all participants and SNR conditions in the two noises and for the three pass-through settings with the Peltor PowerCom Plus and Nacre QuietPro devices.
Giguère: Hearing loss and level-dependent HPDs
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Figure 5. Protected versus unprotected speech recognition scores by hearing profile category for the three pass-through settings (top row: off; middle row: low gain; bottom row: high gain) with the Peltor PowerCom Plus and Nacre QuietPro devices (▴ Normal, ◾ Slight-to-mild, ⚫ Mild-to-moderate, ♦ Moderate-to-severe). For participants in the hearing loss categories (Slight-to-mild, Mildto-moderate, Moderate-to-severe), passive attenuation (pass-through off) reduced speech recognition scores; all data points except two fall below the diagonal (Figure 5, A-B). This effect is progressively more evident with increasing degree of hearing loss from the Slight-to-mild to the Moderate-to-severe category for both the Peltor and Nacre devices. In several cases (9 data points with the Peltor, 4 with the Nacre), the passive attenuation combined with the hearing loss of participants resulted in a drop in recognition performance down to 0%. In all hearing loss categories, use of the higher level-dependent pass-through gain setting almost fully restored speech recognition to unprotected values with the Nacre (Figure 5, F) or provided significant improvements over unprotected performance for all participants
with the Peltor (Figure 5, E). Similarly, except for a few data points belonging to the two categories with the most hearing loss, the lower pass-through gain setting also largely restored recognition scores to unprotected performance for both the Peltor (Figure 5, C) and Nacre (Figure 5, D) devices.
Summary results In Figure 6, the average protected-unprotected difference scores over all participants, irrespective of hearing profile, are presented to highlight the overall effects of noise type (Noise 1, Noise 2) and pass-through gain setting (off, low, high) for each device. For the Peltor (Figure 6, A), the pass-through off condition (passive
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attenuation) led to a mean decrease in performance of 27–29% across Noise l and Noise 2, compared to unprotected listening. The conditions with the level-dependent pass-through circuitry activated, on the other hand, yielded an average overall benefit of 11–15% and 23–24% across the two noises in the low and high gain settings, respectively. Results were similar for the Nacre device (Figure 6, B) in the pass-through off condition, with a mean decrement in performance of 17–25% compared to unprotected listening across the two noises. In the conditions with the level-dependent pass-through circuitry activated, the Nacre had only a small effect on performance, a mean benefit of 6–7% in the low gain setting and 0–2% in the high gain setting across the two noises. Within devices, the protected-unprotected difference scores were nearly identical across the two noises. It should be noted however that, over all participants, the testing SNRs were on average 2.9 dB higher in Noise 1 than in Noise 2 for equivalent word recognition scores, a value identical to the difference in SRT between these two noises found in a previous study (Giguère et al, 2010). In other words, while the masking effects from these two noises differ, the device effects are the same in the two noises. Figure 7 presents the average protected-unprotected difference scores by hearing profile category and pass-through setting for each device. The most interesting feature is the very similar difference scores across all hearing categories when the level-dependent passthrough is used at the high setting. For the Peltor (Figure 7, A), wearing the device at the high gain setting yielded a 19–34% mean benefit compared to unprotected listening across the four hearing categories. For the Nacre (Figure 7, B), there was virtually no effect
on the mean speech recognition scores when wearing the device at the high setting compared to unprotected listening; although some differences were seen at the individual level (Figure 5, F), a mean difference of only 1 to 4% was found across the four hearing categories. The results in the level-dependent conditions (pass-through low and high) are in sharp contrast to the passive condition (passthrough off). In the latter, the normal-hearing category showed little effect compared to the unprotected condition, but all categories of participants with hearing loss were affected. The mean decrement in performance increased with the degree of hearing loss and reached 50% and 42% for the Peltor and Nacre, respectively, for individuals in the Moderate-to-severe category. Setting the devices to passthrough low yielded a large improvement over the off position and even offered a benefit compared to unprotected listening in all but the Moderate-to-severe category.
Figure 6. Protected-unprotected difference score by device setting and noise environment (mean and standard deviation). Data averaged over all participants.
Figure 7. Protected-unprotected difference score by device setting and hearing profile category (mean and standard deviation). Data averaged across noise environments.
Discussion and Conclusions In contrast to conventional HPDs, powered-electronic devices offer a range of user-adjustable control settings. However, few research
Giguère: Hearing loss and level-dependent HPDs reports or guidelines are available to identify the best settings for each user or group of users. In this study, the interaction between the passthrough gain setting and the degree of hearing loss was investigated for two level-dependent devices (Peltor PowerCom Plus and Nacre QuietPro) in a speech recognition task involving two military noises and carried out in a simulation room. Participants covered hearing profiles ranging from normal hearing to moderate-to-severe hearing loss grouped into four distinct categories. The normal-hearing group was within normative limits for both at-threshold (HINT in quiet) and supra-threshold (HINT in noise) performance, while the three categories of participants with hearing loss showed progressive amounts of performance deficits in both types of measures (Table 2). Given the wide range of speech recognition performance to be expected from such a heterogeneous sample, testing had to be performed at a different set of SNRs for each participant. Results were reported as scatterplots of word recognition scores with the device fitted (protected) and not fitted (unprotected) at the same participant-specific SNR. The difference between the recognition score in the protected conditions and the baseline unprotected condition was used to document the device effect at different level-dependent gain settings. When the devices were used as passive protectors, by switching off the level-dependent mode, the normal-hearing group achieved speech recognition scores comparable to unprotected listening, but all categories of participants with hearing loss showed decrements in scores that progressively worsened with the degree of hearing loss. The decrement reached as much as 50% and 42% with the Peltor and Nacre devices, respectively, for the Moderate-to-severe category (Figure 7). Such results for the passive mode of the level-dependent devices are in line with previous results on the effects of conventional HPDs under the same experimental conditions (Giguère et al, 2010). In that study, decreases in performance of up to 57% and 22% were found for one earmuff and one earplug for the same categories of participants. Loss of audibility due to the combined effects of passive attenuation and elevated hearing thresholds is clearly at play for the poor speech recognition performance of participants with hearing loss in these studies. When the two devices were used with the level-dependent mode activated, performance was markedly superior for all categories of participants compared to the passive mode (Figure 7). At the highest pass-through gain setting, a 36–69% benefit over the passive mode (pass-through off) was found with the Peltor device across hearing categories; a 7–20% benefit was found with the Nacre. Furthermore, mean speech recognition scores were as good (Nacre) or better (Peltor) than unprotected performance from the normal-hearing group up to the Mild-to-moderate hearing loss category at both low and high pass-through gain settings, as well as for the Moderate-to-severe category at the highest setting. Interestingly, for participants with normal hearing or hearing loss of up to about a mild-moderate degree, the protected-unprotected difference score was not too sensitive to the choice of the level-dependent gain setting (pass-through low or high). Participants with more severe losses benefitted from a higher gain setting, particularly with the Peltor, which likely further improved overall audibility. A question arises concerning the origin of the greater benefits found for the Peltor across the two noises (Figure 6) and the four hearing groups (Figure 7) when the level-dependent mode is activated. The Peltor is a circumaural device with external microphones mounted directly in front of the earcups. Such a mounting arrangement can provide a directional effect and may favor frontal sound pick-up (such as the speech stimuli in this study) over other sound
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incidences (such as the diffuse noise stimuli). This was further investigated with an acoustic manikin and it was found that fitting the Peltor could enhance the at-ear SNR by about 5–6 dB for frontal speech in diffuse pink noise compared to the unprotected ear. This effect very likely explains the sizeable positive protected-unprotected differences in speech recognition scores observed for the Peltor in the two level-dependent settings. In contrast, the Nacre is an in-ear device and the external microphones are located at the level of the concha, closer to the ears. Manikin sound measurements revealed that for this device the at-ear SNR for frontal speech in diffuse pink noise is within 1–2 dB of that in the unprotected ear. Thus, the differences between devices in the level-dependent mode found in this study appear related to geometrical differences in external sound pick-up and the choice of a frontal speech in diffuse noise listening methodology. While the latter reflects a very common face-to-face communication situation, the differences between devices cannot be generalized to other speech and noise field spatial distributions. Data is provided in Giguère et al (2011a) showing that while external microphones mounted frontally on circumaural devices can provide a benefit in a frontal speech situation, the converse can occur when speech incidence is from the back. In contrast, in-ear devices are expected to show less variability with speech incidence. All in all, the main finding of the study is that level-dependent hearing protection can yield much less variation in speech recognition performance between protected and unprotected listening, compared to passive protection, across a heterogeneous group of users with widely different hearing profiles (Figures 5 and 7). It is important to note, however, that the results do not imply that all groups of users will achieve the same absolute recognition performance under the same listening conditions when level-dependent protection is used. Because the SNR was individually selected to achieve a score in the 20–80% range unprotected for all participants, the distribution of SNRs used in the listening experiment was quite different across the different hearing categories. Averaged over all experimental conditions, the mean listening SNR selected was as follows: Normal hearing ( 10.3 dB), Slight-to-mild ( 8.5 dB), Mild-to-moderate ( 7.0 dB), and Moderate-to-severe ( 5.0 dB). As such, participants in the Moderate-to-severe hearing loss category were listening on average at a 5.3 dB more favourable SNR than participants in the normalhearing category. This value is almost equal to the difference of 5.8 dB in the average SNR loss between these two hearing profiles (Table 2), as measured by the HINT composite score. The leveldependent circuitry thus appeared to overcome the loss of speech audibility that occurs with passive conventional HPDs for individuals with elevated hearing thresholds loss, but the effects of suprathreshold deficits due to hearing loss and/or age among participants remained. Finally, situational awareness in the noisy workplace is not only dependent on speech communication; it also involves a wider range of aural communication skills, including signal detection and sound localization (Abel et al, 2009; Casali et al, 2009; Talcott et al, 2012; Clasing & Casali, 2014). Further work should continue to address all these dimensions in drawing up guidelines to obtain the best overall solution for each particular user given the task demands. From a design standpoint, advanced hearing protection devices are increasingly incorporating sound processing functions and features typically found in hearing aids. Further integration of technologies, such as bilateral gain-frequency shaping, can be expected. This holds promise in providing better solutions tailored to the individual needs of hearing-impaired workers to achieve both protection and good situational awareness in noisy environments. One obstacle to future
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progress is the very sparse disclosure of electroacoustic specifications by manufacturers of advanced hearing protectors, in sharp contrast to the situation found in the hearing-aid industry. Important parameters affecting speech communication such as the directional characteristics of the microphone mounting, and the frequency response, compression dynamics, and harmonic distortion of the level-dependent circuitry, among others, are not conveyed. Hence, it remains difficult to relate study findings to the technical specifications of the devices. This information is however essential to develop predictive models and tools to assist in the selection of the most appropriate device in the field.
Acknowledgements Work based on Defence R&D Canada Contractor Report DRDC Toronto CR-2011-101. Part of this study was presented at the 21st International Congress on Acoustics in Montréal, Canada (2–7 June, 2013) and the 39th Annual Conference of the National Hearing Conservation Association in Las Vegas, USA (13–15 March, 2014). Declaration of interests: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
References Abel S.M., Armstrong N.M. & Giguère C. 1993. Auditory perception with level-dependent hearing protectors: The effects of age and hearing loss. Scand Audiol, 22, 71–85. Abel S.M. & Hay V.H. 1996. Sound localization: The interaction of aging, hearing loss, and hearing protection. Scand Audiol, 25, 3–12. Abel S.M. 2005. Hearing loss in military aviation and other trades: Investigation of prevalence and risk factors. Aviat Space Environ Med, 76, 1128–1135. Abel S.M. 2008. Barriers to hearing conservation programs in combat arms occupations. Aviat Space Environ Med, 79, 591–598. Abel S.M., Boyne S. & Roesler-Mulroney H. 2009. Sound localization with an army helmet worn in combination with an in-ear advanced communications system. Noise Health, 11:45, 199–205. ANSI S3.36-1985 R2006. Specification for a Manikin for In-situ Airborne Acoustic Measurements. New York: American National Standards Institute. CAN/CSA Z94.2-02 R2011. Hearing protection devices - performance, selection, care, and use. Toronto: Canadian Standards Association. Canetto P. 2009. Hearing protectors: Topicality and research needs. Int J Occup Saf Ergon, 15, 141–153. Casali J.G., Ahroon W.A. & Lancaster J.A. 2009. A field investigation of hearing protection and hearing enhancement in one device: For soldiers whose ears and lives depend upon it. Noise Health, 11:42, 69–90. Casali J.G. 2010. Electronic augmentations in hearing protection technology circa 2010 including active noise reduction, electronically-modulated sound transmission, and tactical communications devices: Review of design, testing, and research. Int J of Acoustics Vibration, 15, 168–186. Clasing J.E. & Casali J.G. 2014. Warfighter auditory situational awareness: Effects of augmented hearing protection/enhancement devices and TCAPS for military ground combat applications. Int J Audiol, 53, S43–S52.
Dolan T.G. & O’Loughlin D. 2005. Amplified earmuffs: Impact of speech intelligibility in industrial noise for listeners with hearing loss. Am J Audiol, 14, 80–85. EN 458: 2004. Hearing protectors - recommendations for selection, use, care and maintenance - Guidance document. Brussels: European Committee for Standardization. Gerges S.N.Y. & Casali J.G. 2007. Hearing protectors. In: M.J. Crooker (ed.) Handbook of Noise and Vibration Control. New York: Wiley, pp. 364–376. Giguère C., Laroche C. & Vaillancourt V. 2010. Modelling speech intelligibility in the noisy workplace for normal-hearing and hearing-impaired listeners using hearing protectors. Int J of Acoustics and Vibration, 15, 156–167. Giguère C., Laroche C., Brammer A.J. et al. 2011a. Advanced hearing protection and communication: Progress and challenges. Proceedings of the 10th International Congress on Noise as a Public Health Problem, London, UK, 24–28 July, pp. 225–233. Giguère C., Laroche C. & Vaillancourt V. 2011b. Research on Modelling the Effects of Personal Hearing Protection and Communication Devices on Speech Intelligibility in Noise. Final Report DRDC Toronto CR 2011101, Canada, 48 pp. [Available at http://cradpdf.drdc-rddc.gc.ca/PDFS/ unc114/p535476_A1b.pdf ] Masterson E.A., Tak S., Themann C.L. et al. 2013. Prevalence of hearing loss in the United States by industry. Am J Ind Med, 56, 670–81. Morata T.C., Themann C.L., Randolph R.F. et al. 2005. Working in noise with a hearing loss: perceptions for workers, supervisors, and hearing conservation program managers. Ear Hear, 26, 529–545. Nelson D.I., Nelson R.Y., Concha-Barrientos M. et al. 2005. The global burden of occupational noise-induced hearing loss. Am J Ind Med, 48, 446–58. Parent-Thirion A., Fernandes Macias E., Hurley J. & Vermeylen G. 2007, Fourth European Working Conditions Survey. Dublin: European Foundation for the Improvement of Living and Working Conditions, Catalogue no: TJ-76-06-497-EN-C, ISBN 92-897-0974-X, 139p. [Available at http://www.eurofound.europa.eu/publications/htmlfiles/ef0698.htm] Shaw E.A.G. 1974. Transformation of sound pressure level from the free field to the eardrum in the horizontal plane. J Acoust Soc Am, 56, 1848–61. Suter A.H. 2012. Engineering controls for occupational noise exposure. Sound and Vibration, 46, 24–31. Tak S., Davis R. & Calvert G. 2009. Exposure to hazardous workplace noise and use of hearing protection devices among US workers – NHANES, 1999–2004. Am J Ind Med, 52, 358–371. Talcott K.A., Casali J.G., Keady J.P. et al. 2012. Azimuthal auditory localization of gunshots in a realistic field environment: Effects of openear versus protection-enhancement devices (HPEDs), military vehicle noise, and hearing impairment. Int J Audiol, 51, S20–S30. Themann C., Suter A.H. & Stephenson M.R. 2013a. National research agenda for the prevention of occupational hearing loss – Part 1. Seminars in Hearing, 34, 145–207. Themann C., Suter A.H. & Stephenson M.R. 2013b. National research agenda for the prevention of occupational hearing loss – Part 2. Seminars in Hearing, 34, 208–251. Tufts J.B., Hamilton M.A., Ucci A.J. et al. 2011. Evaluation by industrial workers of passive and level-dependent hearing protection devices. Noise Health, 13, 26–36. Vermiglio A.J. 2008. The American English hearing in noise test. Int J Audiol, 47, 386–387. Williams W. 2011. A qualitative assessment of the performance of electronic, level-dependent earmuffs when used on firing ranges. Noise & Health, 13, 189–194.
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Original Article
The interaction of hearing loss and level-dependent hearing protection on speech recognition in noise Christian Giguère, Chantal Laroche & Véronique Vaillancourt Audiology and Speech-Language Pathology Program, University of Ottawa, Ottawa, Ontario, Canada
Abstract Objectives: To determine the effects of different control settings of level-dependent hearing protectors on speech recognition performance in interaction with hearing loss. Design: Controlled laboratory experiment with two level-dependent devices (PeltorÒ PowerCom PlusÔ and Nacre QuietProÒ) in two military noises. Study sample: Word recognition scores were collected in protected and unprotected conditions for 45 participants grouped into four hearing profile categories ranging from within normal limits to moderate-to-severe hearing loss. Results: When the level-dependent mode was switched off to simulate conventional hearing protection, there were large differences across hearing profile categories regarding the effects of wearing the devices on speech recognition in noise; participants with normal hearing showed little effect while participants in the most hearing-impaired category showed large decrements in scores compared to unprotected listening. Activating the level-dependent mode of the devices produced large speech recognition benefits over the passive mode at both low and high gain pass-through settings. The category of participants with the most impaired hearing benefitted the most from the level-dependent mode. Conclusions: The findings indicate that level-dependent hearing protection circuitry can provide substantial benefits in speech recognition performance in noise, compared to conventional passive protection, for individuals covering a wide range of hearing losses.
Key Words: Hearing loss; hearing protection; speech recognition; noise Noise is one of the most common problems in the workplace. It has been estimated that 16% of adult-onset hearing loss worldwide is due to occupational noise exposure (Nelson et al, 2005). In the USA, approximately 22 million workers are exposed to daily hazardous occupational noise levels (Tak et al, 2009). In a sample of over 1.1 million American workers, 18% had hearing loss; prevalence ratios were especially high in mining, wood product manufacturing and construction of buildings (Masterson et al, 2013). In the European Union, about 30% of workers indicate that they are exposed to loud noises at least a quarter of their work shift, and 7% report a workrelated hearing disorder (Parent-Thirion et al, 2007). Noise-induced hearing loss is particularly prevalent in the military. In the Canadian Forces, 42% of service members show at least a mild hearing loss and 26% develop a moderate to severe hearing loss by midlife, with some military trades such as infantry, artillery, and flight engineers being the most at risk (Abel, 2005). As convincingly argued by Suter (2012), engineering noise control is the essential pillar of hearing loss prevention programs to better protect hearing and promote a safer workplace for everyone. While engineering control at the source is the preferred mitigation method, it is not always feasible to reduce noise to safe levels in
some working environments or for some occupations. In these cases, short-term or long-term supplementary methods such as the use of personal hearing protection devices (HPDs) can become necessary (Gerges & Casali, 2007; Canetto, 2009). Conventional HPDs are passive devices providing a fixed amount of attenuation across a wide range of noise levels in the work environment. Since they also reduce the level of speech and other important sounds, a compromise must be achieved in the attenuation provided for optimal protection and communication needs (CSA Z94.2-02 R2011; EN 458: 2004). However, this goal is difficult to ascertain in practice due to a range of factors, including the uncertainty in the effective attenuation achieved across workers or over time in a given worker, the variation in signal and noise levels occurring in the work day, and the differences in hearing acuity among workers. When properly selected and fitted, conventional HPDs are adequate for most situations involving workers with normal hearing. However, they may interfere with aural communication tasks and work performance in workers with hearing loss (Morata et al, 2005; Abel, 2008; Canetto, 2009; Casali, 2010; Giguère et al, 2010; Themann et al, 2013a,b). For these individuals, the combined effect of hearing loss and hearing protection may be such that important sounds from the
Correspondence: Christian Giguère, Audiology and Speech-Language Pathology Program, University of Ottawa, 451 Smyth Road, Room 3056, Ottawa, Ontario K1H 8M5, Canada. E-mail: [email protected] (Received 25 September 2014; accepted 1 October 2014 ) ISSN 1499-2027 print/ISSN 1708-8186 online © 2015 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2014.973540
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Abbreviations HINT HPD L NRR PTA SNR SR TT
Hearing in noise test Hearing protection device Level Noise reduction rating Pure-tone average Signal-to-noise ratio Surround Talk-through
environment become inaudible or masked, compromising auditory situational awareness. In focus group meetings with industrial workers and military personnel (Morata et al, 2005; Abel, 2008), serious concerns were raised about job performance and safety when working in noise with a hearing loss, especially when HPDs are used. Methods to promote the selection of the most appropriate HPDs are needed as well as the identification of the types of hearing protection and/or amplification devices that are most beneficial to the population of hearing-impaired workers (Themann et al, 2013a,b). Powered-electronic hearing protectors are rapidly being introduced into the marketplace, often with the dual purpose of achieving protection against noise and enhancing situational awareness. In principle, workers with pre-existing hearing loss would particularly benefit from hearing protectors with level-dependent attenuation to maintain or enhance signal audibility in all noise conditions. Such devices typically provide a small amount of amplification in low noise conditions while maintaining exposure to a safe limit in very noisy conditions throughout the day. Unfortunately, the available research on the effects of level-dependent powered electronic HPDs during communication tasks is rather limited (see Casali, 2010, and Giguère et al, 2011a for reviews). This is especially the case for hearing-impaired users, the most likely beneficiaries, for which only a handful of studies are available (Abel et al, 1993, 1996; Dolan & O’Loughlin, 2005; Tufts et al, 2011; Williams, 2011; Talcott et al, 2012). Current level-dependent HPDs often have different modes of operation and several user-adjustable settings and options, which can complicate the selection of the best solution for each particular situation. Moreover, detailed information on the electroacoustic characteristics of the devices is often not supplied by the manufacturers or made readily available, making it difficult to gain a full understanding of the behavior of the devices in various signal and noise conditions, which compounds the problem. This paper reports results from a speech recognition experiment conducted with two level-dependent HPDs, one earmuff and one insert-type device, involving participants with a wide range of hearing profiles. The research project builds on a previous study on the effects of conventional hearing protectors on speech perception in noise for individuals with hearing loss (Giguère et al, 2010). The focus of the present research is on the interaction between the level-dependent amplification setting of the device and the degree of hearing loss on speech recognition outcome.
Methods Participants A total of 45 English-speaking adults (24 males and 21 females) between the ages of 23 and 81 years old (mean 48; SD 16) participated in the study. No restriction was placed on the degree of
hearing loss and on whether the hearing loss was of conductive, sensorineural, or mixed origin. Recruitment was carried out by means of posters displayed in various settings, including the University of Ottawa, audiology clinics, community centres and medical centres. The experimentation took place in the Research Unit in Noise and Communication at the Faculty of Health Sciences. Testing with human participants was approved by the Office of Research Ethics and Integrity at the University of Ottawa. The hearing assessment was carried out in an IAC double-wall audiometric booth using a portable tympanometer (Grason-Stadler GSI 38) and a clinical audiometer (Interacoustics AC40) equipped with insert earphones (3M E-A-RTONE 3A) and a bone vibrator (Radioear B-71). The participants were grouped into four previously defined hearing profile categories (Giguère et al, 2010) based on the audiogram (250–8000 Hz) and the five-frequency (500–4000 Hz) pure-tone average (PTA), as follows: (1) Within normal limits (hearing thresholds 25 dB HL at all frequencies in both ears), (2) Slight-to-mild (best-ear PTA 25 dB HL with hearing thresholds greater than 25 dB HL at one or more individual frequencies, (3) Mild-to-moderate (best-ear PTA between 26 and 40 dB HL), and (4) Moderate-to-severe (best-ear PTA between 41 and 55 dB HL). The goal was to recruit 12 participants in each category. This was achieved for all categories, except for the Moderate-to-severe profile where only nine participants were recruited. The headphone version of the American English hearing-in-noise test (HINT) was also administered (Vermiglio, 2008) as part of the hearing assessment to document supra-threshold hearing performance in each hearing loss category. The HINT is a binaural speech recognition threshold test carried out in quiet and in three spatial conditions of fixed 65-dBA speech-spectrum noise (front, left, right), with speech incidence always from the front. Testing was performed using four different HINT lists, counterbalanced across participants, among the twelve available test lists. The speech levels were adaptively varied to measure the SRT (SNR at which 50% of the sentences are repeated correctly) for each of the three noise conditions. The composite score was then computed as the weighted average of the SRTs for the noise front (NF), noise left (NL), and noise right (NR) conditions, as follows: composite score (2 NF NL NR)/4.
Devices Two tactical hearing protection devices were selected for the study: the PeltorÒ PowerCom Plus™ (3M Personal Safety Division, St. Paul, USA) and the Nacre QuietProÒ (Nacre A/S, Trondheim, Norway). In addition to two-way radio communication capabilities, both devices feature level-dependent stereo pass-through circuitry for improved listening in the immediate environment. Only this latter mode of operation, labeled “surround” (SR) by Peltor and “talk-through” (TT) by Nacre, is addressed in this paper. Depending on the chosen pass-through setting and the prevailing noise level, this feature can amplify or attenuate external sounds. The Peltor PowerCom Plus is an analog circumaural device with five different user-selectable level-dependent pass-through settings (referred to here as SR1 to SR5). At each setting, the maximum gain is achieved at low-to-moderate noise levels to facilitate situational listening, but with increasing noise levels the system gradually reduces the gain then shuts down transmission to re-establish passive protection. The level-dependent mode can also be deactivated (SR off), in which case the device provides conventional passive attenuation at a manufacturer-listed noise reduction rating (NRR)
Giguère: Hearing loss and level-dependent HPDs of 25 dB at all times. The Nacre QuietPro is a digital in-ear device with an external control unit. The user can manually select among ten level-dependent pass-through settings (referred to here as TT2 to TT11). At the lowest setting (TT1), the device provides passive attenuation at a manufacturer-listed NRR of 29 dB. At very high noise levels, once transmission of the level-dependent circuitry has been shut down, the device can provide an additional 6–8 dB of attenuation through active noise reduction. Unfortunately, technical specifications and/or quantitative details on the operation of the above features are not supplied in the product literature or made readily available by the two manufacturers. Electroacoustic measurements were carried out using an acoustic manikin (Brüel & Kjaer Type 4128) conforming to ANSI S3.36-1985 R2006, in order to gain more insight into the level-dependent characteristics of the two devices and guide the choice of experimental settings for the level-dependent pass-through systems. Speech-spectrum noise ranging in level from 40 dBA to 90 dBA was presented in front of the manikin at a 1-m distance. Sound measurements were made with the right ear microphone of the ear simulator assembly without (open-ear, unprotected) and with (occluded-ear, protected) the devices fitted on the manikin at different pass-through settings.
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Figure 1 shows the at-ear manikin sound levels as a function of the free field stimulus level for the five pass-through settings of the Peltor PowerCom Plus and for eight of the ten pass-through settings of the Nacre QuietPro, as well as for the open-ear condition. The open-ear sound levels are about 7 dB above the free field levels, owing to the natural transformation of sound by the torso, head, and external ear (Shaw, 1974). Data points above the open-ear curve indicate conditions for which the devices amplify external sounds, and conversely, data points below the curve indicate attenuation relative to the open-ear condition. The vertical difference with respect to the open-ear curve is the amount of insertion gain (in the case of amplification) or insertion loss (in the case of attenuation) provided by the devices. Both devices show well-defined level-dependent characteristics. At low free field levels ( 60 dBA), the Peltor PowerCom Plus provides linear amplification (Figure 1, A). As further described in Giguère et al (2011b), the gain-frequency response curve is relatively uniform (within 5 dB across mid-frequency octave bands) and the insertion gain ranges from about 6 to 13 dB across settings SR1 to SR5 (averaged over four octave bands 500-4000 Hz). At higher free field levels ( 60 dBA), the device shows gain
Figure 1. Input-output gain functions for the (A) Peltor PowerCom Plus, and (B) Nacre QuietPro devices measured on an acoustic manikin for frontal speech spectrum noise at different pass-through gain settings.
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compression, i.e. the increase in manikin sound level is less than the increase in free field sound level (Figure 1, A). The device behaves like an input-sensitive automatic gain controller with a compression ratio of 4:1 and compression threshold of 60 dBA. At a free field level of 85 dBA, the device provides 6 to 25 dB of insertion loss (attenuation) for settings SR5 to SR1, respectively. The Nacre QuietPro provides similar characteristics to the Peltor device, with some differences. At low free field levels, the Nacre also provides linear amplification (Figure 1, B); the gain-frequency response increases at a rate of about 3 dB/octave from 125 to 4000 Hz and the insertion gain ranges from 12 to 12 dB for settings TT2 to TT11 (averaged over four octave bands 500–4000 Hz) (Giguère et al, 2011b). The compression ratio is the same as the Peltor device at 4:1, but the input-sensitive compression threshold is higher at 80 dBA (Figure 1, B). The Nacre device also shows output-sensitive compression characteristics that appear to limit atear sound levels to about 92 dBA. At a free field level of 85 dBA, it provides 1 to 14 dB of insertion loss (attenuation) for settings TT11 to TT2, respectively.
Noises Two noises developed for a previous study (Giguère et al, 2010) were used in the present experiment. The two noises were recorded from light-armored vehicles in the Canadian Forces. Noise 1 is from a LAVIII vehicle at the driver and crew commander positions while operating over rough terrain and highway. The A-weighted equivalent sound pressure level of Noise 1 is 95.3 dBA, with sizeable fluctuations in sound levels over time (L10 L90 14.6 dB, calculated over 4-s segments). Noise 2 is from a Bison vehicle at the driver and crew commander positions with the engine in idle. The A-weighted equivalent sound pressure level of Noise 2 is 89.5 dBA and is relatively steady over time (L10 L90 4.2 dB). The average one-third octave band levels of the two noises, measured in the field, are displayed in Figure 2. The average spectral slope, calculated over third-octave bands from 31.5 Hz to 8000 Hz, is 4.1 dB/octave and 2.6 dB/octave for Noises 1 and 2, respectively.
Experimental protocol Speech recognition data (% word recognition) were collected in the two military noises using the two level-dependent HPDs. Four listening conditions and operation of the devices were investigated:
(1) unprotected, (2) passive attenuation, (3) level-dependent attenuation at a low pass-through gain setting, and (4) level-dependent attenuation at a high pass-through gain setting, as summarized in Table 1. The first two listening conditions (No device, pass-through off) allowed a comparison with previous findings concerning the adverse effects of conventional passive attenuation on speech recognition in noise in individuals with hearing loss (Giguère et al, 2010). The two level-dependent conditions (pass-through low and high) allowed assessing the potential benefits of the transmission system in each device for this population. Note that the pass-through settings were selected to achieve approximately the same amount of low-level gain across the two devices in the range 500–4000 Hz, about 4 dB in the pass-through low condition and 10 dB in the pass-through high condition (Table 1). Each participant was tested with only one of the two devices (22 participants were assigned to the Peltor PowerCom Plus and 23 participants to the Nacre QuietPro). For each device, half of the participants were tested in Noise 1 and the second half in Noise 2. Within each hearing profile category, participants were assigned randomly to one of four possible device-noise combinations (two devices two noises), such that there were three participants from each hearing loss category for each device-noise combination, except in the Moderate-to-severe category (with only two participants for most device-noise combinations). Percent correct word recognition was measured at two fixed signal-to-noise ratios (SNRs) in each of the four listening conditions (Table 1), for a total of eight conditions per participant using their assigned device and noise. The unprotected and pass-through off conditions were tested first in this order, followed by the pass-through low or high conditions; the latter were counterbalanced. The speech material consisted of the remaining eight lists of HINT sentences not used during the hearing assessment. All lists were counterbalanced across participants and experimental conditions. Scoring was performed at the word level. Due to the very wide range of hearing profiles and speech recognition abilities in the participant sample, it was not possible to test all participants at the same SNRs as this would have led to a preponderance of ceiling and floor effects. Instead, SNRs were individually selected so as to achieve a percent correct score between 20% and 80% unprotected, as in our previous study (Giguère et al, 2010). The HINT test results obtained during the hearing assessment were used to guide the choice of the SNRs for each individual. If the score obtained at the first SNR fell within the 50–80% range, the second SNR was lowered in order to achieve a score in the 20–50% range, and vice-versa, the second SNR was increased if the first SNR yielded a score in the 20–50% range. Within participants, the same two SNRs were targeted in the four listening conditions, when
Table 1. Listening conditions. SR # or TT # refers to the leveldependent pass-through gain settings of the Peltor PowerCom Plus and Nacre QuietPro, respectively. Label
Device setting
Unprotected Pass-through off
No device SR off TT 1 SR 1 TT 4 SR 4 TT 10
Pass-through low
Figure 2. One-third octave band spectra of the two noises (field measurements.
Pass-through high
Comment Open ear Passive protection Level-dependent (low gain » 4 dB) Level-dependent (high gain » 10 dB)
Giguère: Hearing loss and level-dependent HPDs possible, to be able to compare the effects of the different device settings to unprotected performance under the exact same conditions. Different SNRs were required and used across participants to ensure a similar level of difficulty, irrespective of hearing profile.
Simulation room The experiment was conducted in a noise simulation room with inner dimensions of 4.29 m 3.65 m 2.42 m (length width height). The simulation room is lined with 35 wall and ceiling panels that can be changed from sound reflective to sound absorbent to modify the reverberation characteristics and sound field. The speech was presented from a single loudspeaker at 1 m in front of the participants in quasi-free sound field conditions. The noise was presented over a set of five loudspeakers and a subwoofer in quasi-diffuse field conditions. Further details on the sound field specifications and test qualification procedures are described in Giguère et al (2010). The 20 sentences in each HINT list were individually presented in different 4-s long noise segments. A sampling procedure was devised to extract a subset of 20 noise segments from each noise environment (Noise 1 or 2) that closely matched the underlying acoustical characteristics of the entire set of recordings from that noise environment (Figure 2). The sampling procedure was repeated independently for each participant and noise environment to use as much of the available noise data as possible. A software interface controlled the adjustment of the SNR that was selected for each HINT list and the stimulus presentation sequence. To change the SNR, only the speech level was varied; the noise level was not altered, to ensure the simulated noise conditions were closely reflecting the real noise environments recorded. Noise began 0.5 s before each sentence and typically lasted 1.0 s or more after the sentence ended. The participants were asked to repeat the sentences heard. Guessing was encouraged, but no feedback was provided.
Results
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categories defined earlier. Twelve participants had hearing thresholds within 25 dB HL at all frequencies in both ears and formed the normal hearing category, while the remaining 33 had sensorineural hearing losses and made up the Slight-to-mild, Mild-to-moderate and Moderate-to-severe categories. A total of 22 participants had symmetrical hearing (12 Normal, four Slight-to-mild, four Mild-tomoderate, and two Moderate-to-severe), and 23 had asymmetrical hearing (eight Slight-to-mild, eight Mild-to-moderate, seven Moderate-to-severe) defined as interaural threshold differences 10 dB at three frequencies, or 15 dB at two frequencies, or 20 dB at one frequency between 250 and 8000 Hz. Figure 3 presents additional data on the mean hearing thresholds across frequency and ear in each hearing profile category. Mean thresholds range from 8 dB HL to 28 dB HL at 250 Hz, and from 4 dB HL to 79 dB HL at 8000 Hz across categories. As shown, the four hearing categories are nearly equidistant on the dB HL scale within each frequency. Except for the normal hearing category, mean thresholds increase nearly monotonically with frequency. Table 2 also presents summary statistics for scores on the HINT test by hearing profile category. The speech reception threshold for 50% sentence recognition is reported in dBA for the quiet condition and in dB SNR for the noise front and composite score. There is a gradual increase in the mean speech reception threshold in quiet from 17.7 dBA to 43.5 dBA for the normal hearing to the Moderate-to-severe hearing loss category, reflecting differences in absolute hearing thresholds (Figure 3) across categories. Likewise, the noise front and composite SRT scores are also progressively higher from the normal hearing to the Moderate-to-severe hearing loss category, reflecting a gradual decrease in supra-threshold performance due to hearing loss and/or age. Considering the normative value of 6.4 dB for the composite score of the American English HINT (Vermiglio, 2008), the mean SNR loss in each category is as follows: Normal hearing (0.4 dB), Slight-to-mild (1.6 dB), Mild-to-moderate (3.9 dB,) and Moderate-to-severe (6.2 dB).
Speech recognition data set
Description of hearing profile categories Table 2 presents summary statistics for age and PTA threshold for the 45 participants grouped according to the four hearing profile
Figure 4 shows the complete data set of word recognition scores in the two military noises by level-dependent device. The recognition scores in the three protected listening conditions (pass-through off,
Table 2. Summary statistics for age, PTA, and HINT scores by hearing category. Normative data for the American English HINT are Quiet: 15.6 dBA (SD 3.1 dB), Noise Front: 2.6 dB SNR (SD 1.0 dB), Composite score: 6.4 dB SNR (SD 0.9 dB) (Vermiglio, 2008). HINT scores
Statistics
Age (years)
Best-ear PTA (dB HL)
Quiet (dBA)
Noise front (dB SNR)
Composite (dB SNR)
Normal (n 12)
Minimum Mean Maximum
23 29 47
0.0 4.9 14.0
13.7 17.7 21.9
3.7 2.1 1.1
6.8 6.0 5.0
Slight-to-mild (n 12)
Minimum Mean Maximum
36 54 72
9.0 20.1 25.0
20.1 26.7 33.6
3.0 1.5 0.1
5.9 4.8 2.9
Mild-to-moderate (n 12)
Minimum Mean Maximum
45 58 69
27.0 31.9 39.0
29.4 35.4 44.3
2.4 0.6 2.5
6.1 2.5 2.5
Moderate/severe (n 9)
Minimum Mean Maximum
42 61 81
42.0 48.1 55.0
27.9 43.5 51.2
0.0 1.7 3.5
1.5 0.2 1.7
Hearing category
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Figure 3. Mean pure-tone thresholds for the left (dashed line) and right ear (solid line) of participants by hearing profile category (▴ Normal, ◾ Slight-to-mild, ⚫ Mild-to-moderate, ♦ Moderate-tosevere). low or high) are contrasted to scores in the unprotected condition in the form of a scatter diagram. Each data point represents a pair of protected-unprotected measurements at the same participant-
specific SNR (two per participant). In total, 243 such measurement pairs could be collected out of a maximum of 270 pairs; the other 27 protected-unprotected pairs are missing due to exhausting the sentence lists in some instances. In Figure 4, the protected-unprotected difference directly reflects the performance increase (point above diagonal) or decrease (point below diagonal) due to wearing the device. Altogether, the data set demonstrates the wide range of speech recognition effects obtained in the present study across experimental conditions and participants. The panels in Figure 5 show the protected-unprotected results by hearing profile category and level-dependent device. It is clearly apparent that the difference between the protected and unprotected conditions is largely dependent on the device listening condition in interaction with the hearing profile. For participants with hearing thresholds within normal limits, the passive attenuation condition (pass-through off) did not affect speech recognition performance in the majority of cases. Except for three data points (two below diagonal, one above), the data lie in and around the diagonal for both devices for this group (Figure 5, A-B). Furthermore, in the level-dependent conditions, the devices either did not affect recognition performance (pass-through high for Nacre) or improved scores (pass-through low for Nacre, pass-through low and high for Peltor) for most participants in the normal-hearing category (Figure 5, C-F).
Figure 4. Protected versus unprotected speech recognition scores for all participants and SNR conditions in the two noises and for the three pass-through settings with the Peltor PowerCom Plus and Nacre QuietPro devices.
Giguère: Hearing loss and level-dependent HPDs
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Figure 5. Protected versus unprotected speech recognition scores by hearing profile category for the three pass-through settings (top row: off; middle row: low gain; bottom row: high gain) with the Peltor PowerCom Plus and Nacre QuietPro devices (▴ Normal, ◾ Slight-to-mild, ⚫ Mild-to-moderate, ♦ Moderate-to-severe). For participants in the hearing loss categories (Slight-to-mild, Mildto-moderate, Moderate-to-severe), passive attenuation (pass-through off) reduced speech recognition scores; all data points except two fall below the diagonal (Figure 5, A-B). This effect is progressively more evident with increasing degree of hearing loss from the Slight-to-mild to the Moderate-to-severe category for both the Peltor and Nacre devices. In several cases (9 data points with the Peltor, 4 with the Nacre), the passive attenuation combined with the hearing loss of participants resulted in a drop in recognition performance down to 0%. In all hearing loss categories, use of the higher level-dependent pass-through gain setting almost fully restored speech recognition to unprotected values with the Nacre (Figure 5, F) or provided significant improvements over unprotected performance for all participants
with the Peltor (Figure 5, E). Similarly, except for a few data points belonging to the two categories with the most hearing loss, the lower pass-through gain setting also largely restored recognition scores to unprotected performance for both the Peltor (Figure 5, C) and Nacre (Figure 5, D) devices.
Summary results In Figure 6, the average protected-unprotected difference scores over all participants, irrespective of hearing profile, are presented to highlight the overall effects of noise type (Noise 1, Noise 2) and pass-through gain setting (off, low, high) for each device. For the Peltor (Figure 6, A), the pass-through off condition (passive
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attenuation) led to a mean decrease in performance of 27–29% across Noise l and Noise 2, compared to unprotected listening. The conditions with the level-dependent pass-through circuitry activated, on the other hand, yielded an average overall benefit of 11–15% and 23–24% across the two noises in the low and high gain settings, respectively. Results were similar for the Nacre device (Figure 6, B) in the pass-through off condition, with a mean decrement in performance of 17–25% compared to unprotected listening across the two noises. In the conditions with the level-dependent pass-through circuitry activated, the Nacre had only a small effect on performance, a mean benefit of 6–7% in the low gain setting and 0–2% in the high gain setting across the two noises. Within devices, the protected-unprotected difference scores were nearly identical across the two noises. It should be noted however that, over all participants, the testing SNRs were on average 2.9 dB higher in Noise 1 than in Noise 2 for equivalent word recognition scores, a value identical to the difference in SRT between these two noises found in a previous study (Giguère et al, 2010). In other words, while the masking effects from these two noises differ, the device effects are the same in the two noises. Figure 7 presents the average protected-unprotected difference scores by hearing profile category and pass-through setting for each device. The most interesting feature is the very similar difference scores across all hearing categories when the level-dependent passthrough is used at the high setting. For the Peltor (Figure 7, A), wearing the device at the high gain setting yielded a 19–34% mean benefit compared to unprotected listening across the four hearing categories. For the Nacre (Figure 7, B), there was virtually no effect
on the mean speech recognition scores when wearing the device at the high setting compared to unprotected listening; although some differences were seen at the individual level (Figure 5, F), a mean difference of only 1 to 4% was found across the four hearing categories. The results in the level-dependent conditions (pass-through low and high) are in sharp contrast to the passive condition (passthrough off). In the latter, the normal-hearing category showed little effect compared to the unprotected condition, but all categories of participants with hearing loss were affected. The mean decrement in performance increased with the degree of hearing loss and reached 50% and 42% for the Peltor and Nacre, respectively, for individuals in the Moderate-to-severe category. Setting the devices to passthrough low yielded a large improvement over the off position and even offered a benefit compared to unprotected listening in all but the Moderate-to-severe category.
Figure 6. Protected-unprotected difference score by device setting and noise environment (mean and standard deviation). Data averaged over all participants.
Figure 7. Protected-unprotected difference score by device setting and hearing profile category (mean and standard deviation). Data averaged across noise environments.
Discussion and Conclusions In contrast to conventional HPDs, powered-electronic devices offer a range of user-adjustable control settings. However, few research
Giguère: Hearing loss and level-dependent HPDs reports or guidelines are available to identify the best settings for each user or group of users. In this study, the interaction between the passthrough gain setting and the degree of hearing loss was investigated for two level-dependent devices (Peltor PowerCom Plus and Nacre QuietPro) in a speech recognition task involving two military noises and carried out in a simulation room. Participants covered hearing profiles ranging from normal hearing to moderate-to-severe hearing loss grouped into four distinct categories. The normal-hearing group was within normative limits for both at-threshold (HINT in quiet) and supra-threshold (HINT in noise) performance, while the three categories of participants with hearing loss showed progressive amounts of performance deficits in both types of measures (Table 2). Given the wide range of speech recognition performance to be expected from such a heterogeneous sample, testing had to be performed at a different set of SNRs for each participant. Results were reported as scatterplots of word recognition scores with the device fitted (protected) and not fitted (unprotected) at the same participant-specific SNR. The difference between the recognition score in the protected conditions and the baseline unprotected condition was used to document the device effect at different level-dependent gain settings. When the devices were used as passive protectors, by switching off the level-dependent mode, the normal-hearing group achieved speech recognition scores comparable to unprotected listening, but all categories of participants with hearing loss showed decrements in scores that progressively worsened with the degree of hearing loss. The decrement reached as much as 50% and 42% with the Peltor and Nacre devices, respectively, for the Moderate-to-severe category (Figure 7). Such results for the passive mode of the level-dependent devices are in line with previous results on the effects of conventional HPDs under the same experimental conditions (Giguère et al, 2010). In that study, decreases in performance of up to 57% and 22% were found for one earmuff and one earplug for the same categories of participants. Loss of audibility due to the combined effects of passive attenuation and elevated hearing thresholds is clearly at play for the poor speech recognition performance of participants with hearing loss in these studies. When the two devices were used with the level-dependent mode activated, performance was markedly superior for all categories of participants compared to the passive mode (Figure 7). At the highest pass-through gain setting, a 36–69% benefit over the passive mode (pass-through off) was found with the Peltor device across hearing categories; a 7–20% benefit was found with the Nacre. Furthermore, mean speech recognition scores were as good (Nacre) or better (Peltor) than unprotected performance from the normal-hearing group up to the Mild-to-moderate hearing loss category at both low and high pass-through gain settings, as well as for the Moderate-to-severe category at the highest setting. Interestingly, for participants with normal hearing or hearing loss of up to about a mild-moderate degree, the protected-unprotected difference score was not too sensitive to the choice of the level-dependent gain setting (pass-through low or high). Participants with more severe losses benefitted from a higher gain setting, particularly with the Peltor, which likely further improved overall audibility. A question arises concerning the origin of the greater benefits found for the Peltor across the two noises (Figure 6) and the four hearing groups (Figure 7) when the level-dependent mode is activated. The Peltor is a circumaural device with external microphones mounted directly in front of the earcups. Such a mounting arrangement can provide a directional effect and may favor frontal sound pick-up (such as the speech stimuli in this study) over other sound
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incidences (such as the diffuse noise stimuli). This was further investigated with an acoustic manikin and it was found that fitting the Peltor could enhance the at-ear SNR by about 5–6 dB for frontal speech in diffuse pink noise compared to the unprotected ear. This effect very likely explains the sizeable positive protected-unprotected differences in speech recognition scores observed for the Peltor in the two level-dependent settings. In contrast, the Nacre is an in-ear device and the external microphones are located at the level of the concha, closer to the ears. Manikin sound measurements revealed that for this device the at-ear SNR for frontal speech in diffuse pink noise is within 1–2 dB of that in the unprotected ear. Thus, the differences between devices in the level-dependent mode found in this study appear related to geometrical differences in external sound pick-up and the choice of a frontal speech in diffuse noise listening methodology. While the latter reflects a very common face-to-face communication situation, the differences between devices cannot be generalized to other speech and noise field spatial distributions. Data is provided in Giguère et al (2011a) showing that while external microphones mounted frontally on circumaural devices can provide a benefit in a frontal speech situation, the converse can occur when speech incidence is from the back. In contrast, in-ear devices are expected to show less variability with speech incidence. All in all, the main finding of the study is that level-dependent hearing protection can yield much less variation in speech recognition performance between protected and unprotected listening, compared to passive protection, across a heterogeneous group of users with widely different hearing profiles (Figures 5 and 7). It is important to note, however, that the results do not imply that all groups of users will achieve the same absolute recognition performance under the same listening conditions when level-dependent protection is used. Because the SNR was individually selected to achieve a score in the 20–80% range unprotected for all participants, the distribution of SNRs used in the listening experiment was quite different across the different hearing categories. Averaged over all experimental conditions, the mean listening SNR selected was as follows: Normal hearing ( 10.3 dB), Slight-to-mild ( 8.5 dB), Mild-to-moderate ( 7.0 dB), and Moderate-to-severe ( 5.0 dB). As such, participants in the Moderate-to-severe hearing loss category were listening on average at a 5.3 dB more favourable SNR than participants in the normalhearing category. This value is almost equal to the difference of 5.8 dB in the average SNR loss between these two hearing profiles (Table 2), as measured by the HINT composite score. The leveldependent circuitry thus appeared to overcome the loss of speech audibility that occurs with passive conventional HPDs for individuals with elevated hearing thresholds loss, but the effects of suprathreshold deficits due to hearing loss and/or age among participants remained. Finally, situational awareness in the noisy workplace is not only dependent on speech communication; it also involves a wider range of aural communication skills, including signal detection and sound localization (Abel et al, 2009; Casali et al, 2009; Talcott et al, 2012; Clasing & Casali, 2014). Further work should continue to address all these dimensions in drawing up guidelines to obtain the best overall solution for each particular user given the task demands. From a design standpoint, advanced hearing protection devices are increasingly incorporating sound processing functions and features typically found in hearing aids. Further integration of technologies, such as bilateral gain-frequency shaping, can be expected. This holds promise in providing better solutions tailored to the individual needs of hearing-impaired workers to achieve both protection and good situational awareness in noisy environments. One obstacle to future
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progress is the very sparse disclosure of electroacoustic specifications by manufacturers of advanced hearing protectors, in sharp contrast to the situation found in the hearing-aid industry. Important parameters affecting speech communication such as the directional characteristics of the microphone mounting, and the frequency response, compression dynamics, and harmonic distortion of the level-dependent circuitry, among others, are not conveyed. Hence, it remains difficult to relate study findings to the technical specifications of the devices. This information is however essential to develop predictive models and tools to assist in the selection of the most appropriate device in the field.
Acknowledgements Work based on Defence R&D Canada Contractor Report DRDC Toronto CR-2011-101. Part of this study was presented at the 21st International Congress on Acoustics in Montréal, Canada (2–7 June, 2013) and the 39th Annual Conference of the National Hearing Conservation Association in Las Vegas, USA (13–15 March, 2014). Declaration of interests: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
References Abel S.M., Armstrong N.M. & Giguère C. 1993. Auditory perception with level-dependent hearing protectors: The effects of age and hearing loss. Scand Audiol, 22, 71–85. Abel S.M. & Hay V.H. 1996. Sound localization: The interaction of aging, hearing loss, and hearing protection. Scand Audiol, 25, 3–12. Abel S.M. 2005. Hearing loss in military aviation and other trades: Investigation of prevalence and risk factors. Aviat Space Environ Med, 76, 1128–1135. Abel S.M. 2008. Barriers to hearing conservation programs in combat arms occupations. Aviat Space Environ Med, 79, 591–598. Abel S.M., Boyne S. & Roesler-Mulroney H. 2009. Sound localization with an army helmet worn in combination with an in-ear advanced communications system. Noise Health, 11:45, 199–205. ANSI S3.36-1985 R2006. Specification for a Manikin for In-situ Airborne Acoustic Measurements. New York: American National Standards Institute. CAN/CSA Z94.2-02 R2011. Hearing protection devices - performance, selection, care, and use. Toronto: Canadian Standards Association. Canetto P. 2009. Hearing protectors: Topicality and research needs. Int J Occup Saf Ergon, 15, 141–153. Casali J.G., Ahroon W.A. & Lancaster J.A. 2009. A field investigation of hearing protection and hearing enhancement in one device: For soldiers whose ears and lives depend upon it. Noise Health, 11:42, 69–90. Casali J.G. 2010. Electronic augmentations in hearing protection technology circa 2010 including active noise reduction, electronically-modulated sound transmission, and tactical communications devices: Review of design, testing, and research. Int J of Acoustics Vibration, 15, 168–186. Clasing J.E. & Casali J.G. 2014. Warfighter auditory situational awareness: Effects of augmented hearing protection/enhancement devices and TCAPS for military ground combat applications. Int J Audiol, 53, S43–S52.
Dolan T.G. & O’Loughlin D. 2005. Amplified earmuffs: Impact of speech intelligibility in industrial noise for listeners with hearing loss. Am J Audiol, 14, 80–85. EN 458: 2004. Hearing protectors - recommendations for selection, use, care and maintenance - Guidance document. Brussels: European Committee for Standardization. Gerges S.N.Y. & Casali J.G. 2007. Hearing protectors. In: M.J. Crooker (ed.) Handbook of Noise and Vibration Control. New York: Wiley, pp. 364–376. Giguère C., Laroche C. & Vaillancourt V. 2010. Modelling speech intelligibility in the noisy workplace for normal-hearing and hearing-impaired listeners using hearing protectors. Int J of Acoustics and Vibration, 15, 156–167. Giguère C., Laroche C., Brammer A.J. et al. 2011a. Advanced hearing protection and communication: Progress and challenges. Proceedings of the 10th International Congress on Noise as a Public Health Problem, London, UK, 24–28 July, pp. 225–233. Giguère C., Laroche C. & Vaillancourt V. 2011b. Research on Modelling the Effects of Personal Hearing Protection and Communication Devices on Speech Intelligibility in Noise. Final Report DRDC Toronto CR 2011101, Canada, 48 pp. [Available at http://cradpdf.drdc-rddc.gc.ca/PDFS/ unc114/p535476_A1b.pdf ] Masterson E.A., Tak S., Themann C.L. et al. 2013. Prevalence of hearing loss in the United States by industry. Am J Ind Med, 56, 670–81. Morata T.C., Themann C.L., Randolph R.F. et al. 2005. Working in noise with a hearing loss: perceptions for workers, supervisors, and hearing conservation program managers. Ear Hear, 26, 529–545. Nelson D.I., Nelson R.Y., Concha-Barrientos M. et al. 2005. The global burden of occupational noise-induced hearing loss. Am J Ind Med, 48, 446–58. Parent-Thirion A., Fernandes Macias E., Hurley J. & Vermeylen G. 2007, Fourth European Working Conditions Survey. Dublin: European Foundation for the Improvement of Living and Working Conditions, Catalogue no: TJ-76-06-497-EN-C, ISBN 92-897-0974-X, 139p. [Available at http://www.eurofound.europa.eu/publications/htmlfiles/ef0698.htm] Shaw E.A.G. 1974. Transformation of sound pressure level from the free field to the eardrum in the horizontal plane. J Acoust Soc Am, 56, 1848–61. Suter A.H. 2012. Engineering controls for occupational noise exposure. Sound and Vibration, 46, 24–31. Tak S., Davis R. & Calvert G. 2009. Exposure to hazardous workplace noise and use of hearing protection devices among US workers – NHANES, 1999–2004. Am J Ind Med, 52, 358–371. Talcott K.A., Casali J.G., Keady J.P. et al. 2012. Azimuthal auditory localization of gunshots in a realistic field environment: Effects of openear versus protection-enhancement devices (HPEDs), military vehicle noise, and hearing impairment. Int J Audiol, 51, S20–S30. Themann C., Suter A.H. & Stephenson M.R. 2013a. National research agenda for the prevention of occupational hearing loss – Part 1. Seminars in Hearing, 34, 145–207. Themann C., Suter A.H. & Stephenson M.R. 2013b. National research agenda for the prevention of occupational hearing loss – Part 2. Seminars in Hearing, 34, 208–251. Tufts J.B., Hamilton M.A., Ucci A.J. et al. 2011. Evaluation by industrial workers of passive and level-dependent hearing protection devices. Noise Health, 13, 26–36. Vermiglio A.J. 2008. The American English hearing in noise test. Int J Audiol, 47, 386–387. Williams W. 2011. A qualitative assessment of the performance of electronic, level-dependent earmuffs when used on firing ranges. Noise & Health, 13, 189–194.
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