Ergonomics, 2014 Vol. 57, No. 12, 1806–1816, http://dx.doi.org/10.1080/00140139.2014.952681

Speech intelligibility and speech quality of modified loudspeaker announcements examined in a simulated aircraft cabin Sibylle Pennig*, Julia Quehl and Martin Wittkowski German Aerospace Center (DLR), Institute of Aerospace Medicine, Cologne, Germany (Received 13 November 2013; accepted 2 August 2014) Acoustic modifications of loudspeaker announcements were investigated in a simulated aircraft cabin to improve passengers’ speech intelligibility and quality of communication in this specific setting. Four experiments with 278 participants in total were conducted in an acoustic laboratory using a standardised speech test and subjective rating scales. In experiments 1 and 2 the sound pressure level (SPL) of the announcements was varied (ranging from 70 to 85 dB(A)). Experiments 3 and 4 focused on frequency modification (octave bands) of the announcements. All studies used a background noise with the same SPL (74 dB(A)), but recorded at different seat positions in the aircraft cabin (front, rear). The results quantify speech intelligibility improvements with increasing signal-to-noise ratio and amplification of particular octave bands, especially the 2 kHz and the 4 kHz band. Thus, loudspeaker power in an aircraft cabin can be reduced by using appropriate filter settings in the loudspeaker system. Practitioner Summary: Acoustic modifications of loudspeaker announcements were examined in a simulated aircraft cabin via psychological methods with the aim of improving speech intelligibility and subjective speech quality. The findings led to recommendations for improvements of announcement systems concerning sound pressure level and frequencies according to the noise in different cabin sections. Keywords: speech intelligibility; sound design; loudspeaker announcements; aircraft cabin

1.

Introduction

Cabin noise is an important physical variable that interferes with activities during a flight, such as relaxation, concentration or communication. Noise-induced communication impairments concern listening, understanding and speaking. This may lead to a loss of information or to misinformation, to an increased listening and speaking effort and may result in annoyance (Lazarus 1990). In the aircraft cabin, speech intelligibility and speech quality are especially important with regard to the loudspeaker announcements in flight. In general, there are different approaches to measure speech intelligibility in background noise. Numeric methods estimate speech intelligibility quantitatively by calculating indices from speech stimuli and acoustic characteristics of the setting (e.g. background noise, reverberation). A basic index is the signal-to-noise ratio (SNR). Mapp (2008) suggests a ‘rule of thumb’ of 6 dB(A) as a general minimum and at least 10 dB(A) as a target for good speech intelligibility. The improvement gained is less at higher SNRs and depends on the test conditions and signals. Beyond this simple index there are further numeric speech indices such as the Articulation Index (AI; French and Steinberg 1947; Kryter 1962), the (Coherence) Speech Intelligibility Index ((C)SII; ANSI S3.5-1997 1997; Kates and Arehart 2005) or the (Rapid) Speech Transmission Index ((RA)STI; Houtgast and Steeneken 1984; Steeneken and Houtgast 1980). These objective indices, however, cannot completely reflect speech intelligibility. They do not measure but rather predict intelligibility based on a certain model. Psychological test methods measure the subjective perception of speech during a given acoustic background condition. Speech intelligibility can be determined by standardised test procedures which address the reproduction of speech stimuli, such as sentences (meaningful or nonsense sentences) or syllables. Meaningful sentences are easier to understand than nonsense sentences or words. The speech intelligibility in a communication situation, however, turns only into a quality feature when the listener evaluates how well he/she understood speech in the situation (Lazarus et al. 2007). Therefore, to assess and improve communication conditions subjective measurement methods should be included in addition to standardised speech intelligibility tests. A five-point scale for subjective evaluation of speech intelligibility was proposed in the ISO standard (ISO 9921 2003). It ranges from ‘bad’ to ‘excellent’ and corresponds to particular levels of SNR and speech indices. This classification depends on the study conditions (noise level, speech material, hearing situation). A specification of speech intelligibility in terms of SNR and speech quality features for the particular communication conditions in an aircraft cabin has not yet been examined.

*Corresponding author. Email: [email protected] q 2014 Taylor & Francis

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The quality of a communication situation should not be reduced to mere comprehensibility. The perception processes during communication, such as the listeners’ experience of stress and strain, needs to be considered as well (Sust et al. 2009). Such measurements comprise ratings of a global impression of the speech situation and quality as well as specific attributes including the listener’s effort to cope with the situation, the concentration needed to understand the speech material and the perceived disturbance/annoyance during communication (Lazarus 1990; Lazarus-Mainka and Leushacke 1985; Lazarus-Mainka, Pasligh, and Raschdorf 1985; Lazarus-Mainka and Raschdorf 1985; Sust et al. 2009; Volberg et al. 2006). These parameters of subjective speech quality have been shown to be more sensitive in cases where speech intelligibility is nearly constant, i.e. in situations with high SNRs (Sust et al. 2009). An SNR between 9 and 16 dB(A) was reported to be the requirement for good to very good speech quality. Sato, Bradley, and Morimoto (2005) concluded that word intelligibility tests should be applied to conditions with an SNR , 0 dB(A) whereas listening difficulty ratings are more appropriate for conditions with an SNR between 0 and 15 dB(A). Speech intelligibility of loudspeaker announcements in cabin noise has primarily been measured by numeric indices. There is a lack of investigations using psychological methods. Experimental studies concerning communication, speech intelligibility, and noise annoyance within a vehicle interior have only been conducted in train coaches (Khan 2003; Kuwano, Namba, and Okamoto 2004; Patsouras et al. 2000). Studies from other vehicle surroundings, however, are not easily transferable to the acoustic conditions of an aircraft cabin. For example, speech intelligibility decreases at constant SNR with rising sound pressure levels (SPLs) of noise and signal (Goshorn and Studebaker 1994; Hagermann 1982; Studebaker et al. 1999). In an aircraft cabin SPLs are usually higher than those in a train coach and differ in their noise characteristics (Soeta and Shimokura 2009). There are two main possibilities for improving the speech intelligibility of announcements in the aircraft cabin. It is possible either to modify the background noise or the announcements themselves; the latter solution being more feasible. Varying the SPL of the announcements appears to be the simplest acoustic optimisation. An enhancement of the announcements’ SPL, however, implies the installation of loudspeakers with higher performance and therefore bigger size and weight. Hence, the modification of other acoustic parameters such as the spectral composition is a desirable strategy. The enhancement of specific frequencies of the announcement signal could improve speech intelligibility without increasing the overall SPL. Human speech uses mainly the frequency range from 100 Hz to 10 kHz (Lazarus et al. 2007). Research efforts aimed to identify a span of frequencies that is particularly relevant for speech intelligibility. Studies based on the filtering of different kinds of speech stimuli defined a range between 1.5 and 4 kHz (Bell, Dirks, and Trine 1992; Chari, Herman, and Danhauer 1977; DePaolis, Janota, and Frank 1996). The frequency band centred near 2 kHz was found particularly important for the intelligibility of sentences (e.g. DePaolis, Janota, and Frank 1996). It was frequently observed (French and Steinberg 1947; Hirsh, Reynolds, and Joseph 1954; Pollack 1948) that a limitation of the speech material’s frequency range cannot adequately be compensated by a higher SPL. Furthermore, these studies show that limitations of the frequency range constrain the utmost possible speech intelligibility. High speech quality requires a wide frequency range including high frequencies up to 6 kHz (Lazarus et al. 2007). Nevertheless, the significance of the signal’s frequency range is limited with respect to the spectral composition of the background noise that might affect speech intelligibility as well. Recently, there has been an increased effort in the development of speech modification algorithms to enhance intelligibility in noisy environments (Cooke et al. 2013). Some strategies were inspired by characteristics of Lombard speech, a speaking style which naturally occurs in noise and is more intelligible, and propose, for example, boosting the consonant-vowel power ratio (Skowronski and Harris 2006; Yoo et al. 2007). Besides noise-independent approaches there are also noise-dependent algorithms, for example using a spectral audio power reallocation based on the Speech Intelligibility Index (Sauert and Vary, 2010) or a transfer of speech energy based on the local SNR in the frequency region of 1.8–7.5 kHz (Tang and Cooke 2010). This work investigated speech intelligibility and speech quality of modified loudspeaker announcements in a simulated aircraft cabin by subjective methods. Based on the results, recommendations for sound design should be specified to achieve an improvement of passengers’ speech comprehension during short-haul flights. The research focused on application related solutions that can be easily integrated in announcement systems currently used in aircrafts. Four experimental studies were conducted in an acoustic laboratory setting. Studies 1 and 2 focused on variation of the SPL of the loudspeaker announcements and studies 3 and 4 examined the effects of simple frequency variations of octave bands. The studies used realistic background noise recorded in different locations of an aircraft cabin in flight. 2. 2.1

Variation of loudspeaker announcements with respect to sound pressure level: studies 1 and 2 Methods

2.1.1 Participants A total of 60 volunteers participated in each study. The sample in study 1 consisted of 36 men and 24 women with a mean age of 30 years (SD ¼ 9) and in study 2 of 29 men and 31 women (mean age ¼ 49 years, SD ¼ 16). Participants reported normal hearing and very good German language skills.

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2.1.2 Experimental design and setting The SPL of the loudspeaker announcements was varied between 70, 73, 76, 79, 82 and 85 dB(A). The effects were tested by a within-subjects design with the repeated measures factor SPL. The order of presentation followed a balanced Latin square design (Jones and Kenward 2003). Thus, each SPL appeared in the experiment at each position once, and six study sessions had to be performed. Participants were randomly assigned to one of these study groups. During the announcements a constant background noise of 74 dB(A) was played back. This noise level is realistic for a short-haul flight (Soeta and Shimokura 2009). The six experimental conditions corresponded to the following levels of SNR: – 4, – 1, 2, 5, 8 and 11 dB (A). Background noise samples were recorded at different seat locations in the aircraft cabin to provide cabin noise samples with different frequency spectra. Study 1 used background noise from the front section of the cabin and study 2 used noise from the rear section. The experiments were conducted in an acoustic laboratory (Figure 1). The facility was designed to simulate the acoustic environment of an aircraft cabin as realistically as possible. An array of multiple loudspeakers and sub-woofers arranged at ceiling height behind simulated overhead compartments provided an equal distribution of SPL and frequency spectra at each seat of the laboratory. Additional flat panel loudspeakers were placed at both sides of the laboratory behind simulated aircraft windows. At the bottom of the overhead compartments, original aircraft loudspeakers for announcements were installed above each row of seats according to their positions in the real cabin. Each loudspeaker in the laboratory could be triggered and calibrated separately. For the study, 17 loudspeakers were used for the presentation of cabin noise and 7 for the announcements. Each seat position in the laboratory was acoustically calibrated using a high definition microphone array. Cabin noise could be presented evenly at all seats with a maximum deviation of ^ 0.93 dB(A) for the front noise and ^ 1.44 dB(A) for the rear noise. The impression of an aircraft cabin was promoted by the installation of 18 original aircraft seats arranged in a typical cabin setting (four rows of three seats on the right side and three rows of two seats on the left side). Air temperature in the laboratory was kept at a constant 238C. 2.1.3

Measures of speech intelligibility and speech quality

A standardised speech intelligibility test was performed to compare different acoustic settings. Providing a more realistic communication situation, a sentence test in favour of a test using syllables was more appropriate for the current setting. Here the ¨ SA; Kollmeier and Wesselkamp 1997) was applied. This test is characterised by high precision in Go¨ttinger sentence test (GO measuring speech intelligibility in noise and is recommended for applications such as clinical audiology or the evaluation of communication systems. It consists of 20 sentence lists that are highly homogeneous with respect to intelligibility. These lists comprise 10 short sentences each. Six lists were randomly selected for the six experimental conditions Furthermore, subjective speech quality was assessed by a questionnaire measuring the dimensions coping (how well the comprehension process can generally be coped with), concentration, subjective speech intelligibility and annoyance (disturbing conditions under which the sentences had to be understood) (Sust et al. 2009; Volberg et al. 2006). In addition, the questionnaire was extended by the assessment of subjective speech quality. Each dimension was measured by a statement with a five-point rating scale (1 ¼ ‘not at all’, 2 ¼ ‘not very’, 3 ¼ ‘moderately’, 4 ¼ ‘fairly’, 5 ¼ ‘very’ [e.g. ‘I had to concentrate . . . to understand the sentences’]; or 1 ¼ ‘bad’, 2 ¼ ‘poor’, 3 ¼ ‘fair’, 4 ¼ ‘good’, 5 ¼ ‘very good’ [e.g. ‘I understood the sentences . . . ’]).

Figure 1. Acoustic laboratory with flat panel loudspeakers and loudspeakers for announcements at the bottom of a simulated overhead compartment in an aircraft cabin environment.

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¨ SA as a function of loudspeaker announcements’ SPL at a constant Figure 2. Percentage of correctly understood words in the GO background noise of 74 dB(A) for study 1: noise recorded at seat positions in the front, and study 2: noise recorded at seat positions in the rear (mean values ^ standard error).

2.1.4

Experimental procedure

Six different study sessions each were conducted in study 1 and study 2. In each study session a randomly composed group of 7 –11 participants was exposed at the same time to the modifications of the announcements’ SPL in the acoustic laboratory. Demographic data were collected. The investigator gave instructions and started the background noise and speech signals for each condition using a computer in the laboratory. After a training session the announcements were ¨ SA was played presented with the constant cabin noise at 74 dB(A). In each acoustic condition a different list of the GO ¨ SA lists was the same in every study session. After each sentence subjects were required to back. The sequence of the GO write down the sentence or every word they had understood. The completion of each test list was followed by the assessment of the communication situation by using the subjective speech quality rating scales. Participants performed all tests and rating scales on small laptop computers. 2.2 Results 2.2.1 Go¨ttinger sentence test ¨ SA were prepared for analysis by determining the percentage of correctly understood words Data acquired from the GO using the written responses for each acoustic condition. Figure 2 shows the percentage of correctly understood words in the ¨ SA as a function of SPL for both studies. These descriptive data indicate that speech intelligibility rose for the front GO background noise from 43.7% at 70 dB(A) to 98.2% at 85 dB(A). For the rear noise the percentage of correctly understood words was 58.2% at 70 dB(A) and reached 99.7% at 85 dB(A). Thus, at the same SPL, loudspeaker announcements were better understood in the acoustic situation at the rear of the cabin than in the front. The impact of the SPL modifications on the proportion of recognised words was analysed using a linear regression setting via generalised estimation equations (GEE; Liang and Zeger 1986) to account for the within-subject correlation caused by the repeated measurements and non-normally distributed residuals. Since the separate analysis of studies 1 and 2 showed similar effects of the SPL, the data were pooled for the statistical analysis to compare the influence of the two ¨ SA. The repeated factor SPL and the between-subjects factor study different background noises on the results of the GO (cabin noise) were included in the statistical analysis and treated as covariates. The GEE results underlined the descriptive observations. Speech intelligibility significantly enhanced with increasing SPL of the announcements and under the rear cabin noise condition compared to the front cabin noise (Table 1). ¨ SA as a function of SPL Table 1. Summary of generalised estimating equation model for the percentage of understood words in the GO and study (background noise).

Intercept SPL Study (cabin noise)

b

SE

x 2 (1 df)

p

2 140.02 2.86 8.07

11.04 0.14 1.43

160.98 456.33 32.00

,0.001 ,0.001 ,0.001

Note: Study parameter coding: study 1 (front noise) ¼ 0; study 2 (rear noise) ¼ 1.

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Table 2. Summary of generalised estimating equation model for the overall subjective speech assessment as a function of SPL and study (background noise).

Intercept SPL Study (cabin noise)

b

SE

x 2 (1 df)

p

2 12.17 0.16 0.38

0.32 0.004 0.08

1488.47 1506.00 25.03

,0.001 ,0.001 ,0.001

Note: Study parameter coding: study 1 (front noise) ¼ 0; study 2 (rear noise) ¼ 1.

Furthermore, improvements of speech intelligibility between particular acoustic conditions were examined to identify a possible ceiling effect for each study separately. For both studies, Wilcoxon tests showed improvements of correctly understood words up to 79 dB(A) ( p , 0.001). The difference in speech intelligibility was not significant between 79 and 82 dB(A) (front noise: p ¼ 0.069; rear noise: p ¼ 0.399), but again between 82 and 85 dB(A) (front noise: p ¼ 0.033; rear noise: p ¼ 0.003).

2.2.2 Subjective speech assessments For the statistical analysis the subjective assessments of speech intelligibility and quality from studies 1 and 2 were pooled. High correlations between the single subjective ratings coping, concentration, subjective speech intelligibility, subjective speech quality and annoyance were found (Spearman correlations r ¼ 0.75– 0.89). Thus, data from the questionnaire were reduced by a principal component analysis, and the final analysis (GEE) was based on an overall dimension derived from this analysis. A single factor was extracted with high factor loadings ranging between 0.89 (annoyance) and 0.96 (subjective speech intelligibility). This subjective speech assessment dimension correlated highly with the percentage score of correctly ¨ SA (r ¼ 0.80). Regarding the single items of the questionnaire the highest understood words measured by the GO ¨ correlations with the GOSA results were found for subjective speech intelligibility (r ¼ 0.78) and the lowest for the assessment of annoyance (r ¼ – 0.68). The GEE analysis with the repeated factor SPL and the between-subjects factor study (cabin noise) was performed ¨ SA. according to the procedure used for the analysis of the sentence test. The results confirmed the effects found for the GO With rising SPL of the announcements the overall subjective speech assessment significantly improved (Table 2). Consequently, subjects reported that they were better able to cope with their task of reproducing the sentences correctly, they had to concentrate less, subjective speech intelligibility and quality ratings increased, and the communication situation ¨ SA, participants reported better subjective was evaluated as being less disturbing. Corresponding to the results of the GO speech assessments in study 2 with noise from the rear section of the cabin in the background than in study 1 with noise from the front section (Table 2). As an example Figure 3 shows the mean values from the single-item subjective speech intelligibility referring to the statement ‘I understood the sentences . . . ’ with possible answers ranging from 1 (‘bad’) to 5 (‘very good’). Subjective

Figure 3. Subjective speech intelligibility (scale values: 1 ¼ “bad”, 2 ¼ “poor”, 3 ¼ “fair”, 4 ¼ “good” and 5 ¼ “very good”) as a function of loudspeaker announcements’ SPL at a constant background noise of 74 dB(A) for study 1: seat position in the front, and study 2: seat position in the rear (mean values ^ standard error).

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speech intelligibility assessments varied between ‘bad’ and ‘poor’ at 70 dB(A) (SNR ¼ –4 dB(A)), increased at 76 dB(A) (SNR ¼ 2 dB(A)) to ‘fair” speech intelligibility, at 79 dB(A) (SNR ¼ 5 dB(A)) to evaluations between ‘fair’ and ‘good’ and reached ‘good’ to ‘very good’ subjective ratings not until an SPL of 85 dB(A) (SNR ¼ 11 dB(A)). Assessments of this rating scale were better in the rear noise than in the front noise. 3.

Variation of loudspeaker announcements with respect to frequencies: studies 3 and 4

3.1 Methods 3.1.1 Participants Altogether, 79 subjects (42 men, 37 women) participated in study 3. Age ranged from 18 to 60 years (M ¼ 31, SD ¼ 9). Study 4 was performed with 84 participants (43 men, 41 women). They were aged between 18 and 71 years (M ¼ 33, SD ¼ 13). Participants reported normal hearing and very good German language skills. 3.1.2

Experimental design and setting

Studies 3 and 4 focused on variation of the SPL in the 2 and 4 kHz octave bands of the loudspeaker announcements. This frequency range is particularly relevant for speech intelligibility (Bell, Dirks, and Trine 1992; Chari, Herman, and Danhauer 1977; DePaolis, Janota, and Frank 1996). Modifications of the 8 kHz frequency band were included in the experimental design to compare the effect of specific enhancement of frequencies adjacent to this central frequency range. The idea of this approach is to make use of selective energy reallocation. Energy from low-importance frequency regions is transferred to high-importance parts of the spectrum under the constraint of an unchanged overall energy (Tang and Cooke 2010). Each octave band was reduced by 6 dB and enhanced by 6 and 12 dB. This modification was realised by using the 10-band graphic equalizer (FIR filter) implemented in the software Adobe Audition 3.0. For each condition, the overall SPL of the announcements was kept constant to investigate only effects due to frequency modification. Thus, energy was transferred between the remaining parts of the spectrum and the modified octave bands. A 3 £ 3 within-subject design with the repeated factors octave band (2, 4, 8 kHz) and SPL (SPL modification – 6, þ 6, þ 12 dB of the altered octave band) was employed. The nine experimental conditions were presented following a balanced Latin square design (Jones and Kenward 2003). Identical cabin noise samples used in studies 1 and 2 were used again as background noise. In study 3 cabin noise from the aircraft’s front section and in study 4 from the rear section was played back at a constant background noise level of 74 dB(A). The SPL of the announcements was determined by the speech intelligibility curves found in the earlier studies 1 and 2 (Figure 2). A speech intelligibility of 70% seemed to offer an appropriate optimisation potential. Thus, announcements in study 3 were played back at 73 dB(A) and in study 4 at 71 dB(A). 3.1.3

Measures of speech intelligibility, speech quality and experimental procedure

¨ SA, subjective assessments, Speech intelligibility and quality were measured with the same tools used in studies 1 and 2 (GO see Section 2.1.3) and equal experimental procedures were applied (see Section 2.1.4).

¨ SA as a function of loudspeaker announcements’ modification of Figure 4. Percentage of correctly understood words in the GO frequencies (enhancement and reduction of SPL in octave bands 2, 4, 8 kHz) at a constant background noise of 74 dB(A) for study 3: noise recorded at seat positions in the front (mean values ^ standard error).

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3.2 Results 3.2.1 Go¨ttinger sentence test ¨ SA, the percentage of correctly understood words was determined using the written To analyse the results of the GO responses for each experimental condition. In study 3, the percentage of understood words ranged from 57% at – 6 dB and 4 kHz to 84% at þ 12 dB and 4 kHz (Figure 4). The speech intelligibility in study 4 increased from 59% at –6 dB and 2 kHz to 88 % at þ 12 dB and 2 kHz (Figure 5). Modifications in the 8 kHz octave band caused the least enhancements in speech intelligibility. This is emphasised by an interaction shown in Figures 4 and 5. The reduction of octave band 8 kHz caused less impairment of speech intelligibility than diminishing octave bands 2 and 4 kHz, but the enhancement of the 2 and 4 kHz bands contributed to more improvement of speech intelligibility than increasing the 8 kHz band. The effect of the frequency modifications on the percentage of correctly understood words were investigated in a GEE model separately for studies 3 and 4 with the repeated measurement factors SPL (–6, þ6, þ12 dB) and octave band (2, 4, 8 kHz) as well as their interaction (Table 3). SPL and octave band were treated as categorical and consequently, the estimates in Table 3

¨ SA as a function of loudspeaker announcements’ modification of Figure 5. Percentage of correctly understood words in the GO frequencies (enhancement and reduction of SPL in octave bands 2, 4, 8 kHz) at a constant background noise of 74 dB(A) for study 4: noise recorded at seat positions in the rear (mean values ^ standard error). ¨ SA as a function of SPL, Table 3. Summary of generalised estimating equation models for the percentage of understood words in the GO octave band and the interaction between SPL and octave band for study 3 (front noise) and study 4 (rear noise).

Front noise Intercept SPL 2 6 SPL 6 Octave band 2 Octave band 4 SPL 2 6 £ Octave band 2 SPL 2 6 £ Octave band 4 SPL 6 £ Octave band 2 SPL 6 £ Octave band 4 Rear noise Intercept SPL 2 6 SPL 6 Octave band 2 Octave band 4 SPL 2 6 £ Octave band 2 SPL 2 6 £ Octave band 4 SPL 6 £ Octave band 2 SPL 6 £ Octave band 4

b

SE

x 2 (1 df)

p

75.85 2 11.03 2 3.86 8.07 8.42 2 11.36 2 16.37 2 4.29 2 2.27

1.69 1.63 1.58 1.54 1.32 2.26 1.19 2.19 2.19

2024.73 45.89 5.95 27.57 40.83 25.36 70.88 3.85 1.08

,0.001 ,0.001 0.02 ,0.001 ,0.001 ,0.001 ,0.001 0.05 0.30

81.22 2 7.01 2 4.19 6.79 6.29 2 21.68 2 19.91 2 2.79 2 1.85

1.41 1.42 1.06 1.29 1.28 2.36 2.08 1.50 1.46

3296.37 24.33 15.64 27.82 24.25 84.19 91.79 3.47 1.62

,0.001 ,0.001 ,0.001 ,0.001 ,0.001 ,0.001 ,0.001 0.06 0.20

Note: Factors were treated as categorical variables. Thus, results have to be interpreted in relation to a reference category: SPL ¼ 12 dB; octave band ¼ 8 kHz. Estimates of the reference categories ¼ 0.

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Table 4. Summary of generalised estimation equation models for subjective speech intelligibility as a function of SPL (reference category ¼ 12) and octave band (reference category ¼ 8 kHz).

Front noise Intercept SPL -6 SPL 6 Octave band 2 Octave band 4 SPL 2 6 £ Octave band 2 SPL 2 6 £ Octave band 4 SPL 6 £ Octave band 2 SPL 6 £ Octave band 4 Rear noise Intercept SPL 2 6 SPL 6 Octave band 2 Octave band 4 SPL 2 6 £ Octave band 2 SPL 2 6 £ Octave band 4 SPL 6 £ Octave band 2 SPL 6 £ Octave band 4

b

SE

x 2 (1 df)

p

0.16 2 0.67 2 0.15 0.43 0.43 2 0.41 2 0.61 2 0.39 2 0.10

0.11 0.11 0.11 0.12 0.09 0.16 0.13 0.16 0.15

2.05 36.66 1.89 12.67 22.51 6.32 22.59 5.74 0.35

0.15 ,0.001 0.17 ,0.001 ,0.001 0.01 ,0.001 0.02 0.55

0.18 2 0.45 2 0.17 0.39 0.33 2 0.80 2 0.78 2 0.23 2 0.13

0.10 0.08 0.08 0.10 0.10 0.12 0.13 0.14 0.14

3.19 33.79 4.24 19.49 12.30 40.90 34.78 2.84 0.94

0.07 ,0.001 0.04 ,0.001 ,0.001 ,0.001 ,0.001 0.09 0.33

Note: Factors were treated as categorical variables. Thus, results have to be interpreted in relation to a reference category: SPL ¼ 12 dB; octave band ¼ 8 kHz. Estimates of the reference categories ¼ 0.

refer to a reference category with the highest value of the factor. The GEE analysis showed a significant improvement in the ¨ SA in the higher SPL categories in the octave bands. This is indicated in Table 3 by a percentage of correct words in the GO difference between 12 dB and the other modifications. Pairwise comparisons corrected according to Bonferroni also showed differences between the –6 dB and the 6 dB categories ( p , 0.001). The significant interaction indicated that the specific effect of SPL depends on the particular octave band. For example, pairwise comparisons corrected according to Bonferroni showed that the SPL enhancement of 12 dB in the octave bands 2 and 4 kHz induced a higher proportion of recognised words than in the 8 kHz band ( p , 0.001). 3.2.2 Subjective assessments The subjective assessments were summarised again by a factor analysis to an overall dimension in each study. The results of ¨ SA (Table 4). The modifications in all the GEE analysis based on this factor were in accordance with the results of the GO three octave bands (2, 4 and 8 kHz) had a measurable effect in both studies. As an example, the item subjective speech intelligibility was generally rated ‘poor’ when SPL was decreased by –6 dB and ratings enhanced to ‘fair’ when SPL was amplified by þ 12 dB in the examined octave bands. The weakest impact was found for the 8 kHz octave band. Spearman correlation coefficients showed moderate relations between the overall speech assessments and the percentage of correctly ¨ SA (Spearman correlation front noise: r ¼ 0.56; rear noise: r ¼ 0.67). understood words in the GO 4. Discussion In these laboratory studies human perception of the speech intelligibility of loudspeaker announcements in an aircraft cabin was tested. The sound pressure level (SPL) and frequencies of the announcements were manipulated. To examine the effects on speech intelligibility subjects were examined using psychological test methods instead of indices based on acoustic measurements. Results show the optimisation potential of both SPL and enhancement of particular frequency bands that are relevant for speech intelligibility. 4.1

Effects of varying announcements’ sound pressure level on speech intelligibility

As expected, with rising SPL of the announcements during constant cabin noise speech intelligibility improved ¨ SA sentence test increased at the highest SNR significantly. The percentage of understood words identified by the GO between cabin noise and announcements (11 dB(A)) up to 98% for the front noise and up to nearly 100% for the rear noise.

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The curve describing the relationship between SPL of the announcements and therefore SNR and the percentage of correctly recognised words flattens out at higher SNRs when a speech intelligibility of 90% is reached. The improvement by enhancing the announcement by 3 dB(A) ranged from nearly 30% (improvement from 70 to 73 dB(A), SNR ¼ 2 4 to 2 1 dB(A)) to approximately 1% when 79 dB(A) (SNR ¼ 5 dB(A)) is reached. The subjective ratings generally support the ¨ SA. Measurements by the sentence test showed good agreement with the subjective assessments, effects shown in the GO since high correlations between the measurements and similar differences between the test conditions were found. This result supports former studies analysing the relationship between sentence tests and rating scales (Kollmeier and Wesselkamp 1997; Sato, Bradley, and Morimoto 2005; Volberg et al. 2006). At higher levels of the SPL of announcements and therefore higher SNR a subjectively perceived differentiation can be observed. Speech intelligibility was rated at 79 dB (A) (SNR ¼ 5 dB(A)) as ‘fair’ to ‘good’, and at 82 dB(A) (SNR ¼ 8 dB(A)) as ‘good’. ‘Good’ to ‘very good’ speech intelligibility was not reported until announcements were presented at 85 dB(A) (SNR ¼ 11 dB(A)). At these high SNR ¨ SA results. This indicates that the rating scales are values the subjective assessments showed a steeper curve than the GO more sensitive as reported in the literature (Sato, Bradley, and Morimoto 2005; Sust et al. 2009; Volberg et al. 2006). Therefore, the inclusion of subjective speech assessment scales in addition to a speech test is advisable for the evaluation of speech intelligibility and quality of loudspeaker announcements in an aircraft cabin. The definition of SNR values and subjective speech intelligibility in the current studies is consistent with the results of Sust et al. (2009) who determined an SNR of 4 dB(A) for subjective ratings of ‘fair’ to ‘good’ and 10 dB(A) for assessments of ‘good’ to ‘very good’. Volberg et al. (2006) indicated only slightly higher SNR values of 6.6 and 12.5 dB(A) for the same quality steps. The general ‘rule of thumb’ reported by Mapp (2008) suggesting a minimum of 6 dB(A) and a preferred SNR of at least 10 dB(A) are in accordance with our findings as well. The present outcome supports a definition of a higher SNR in the recent ISO standard (ISO 9921 2003). In this standard ‘very good’ or ‘excellent’ speech intelligibility was determined already at an SNR of higher than 7.5 dB(A). An SNR higher than 10 dB(A) was proposed in other studies, for example in the school domain (15 dB(A); Bistafa and Bradley 2000), and for specific ‘very good’ speech quality aspects such as concentration (22 dB(A); Sust et al. 2009). Such conditions (SNR . 11 dB(A)) were not tested in the present experiments since in contrast to those background noise levels in these studies noise levels are already high, i.e. 74 dB(A) as an average for cruise condition. Thus, an unlimited increase of the SPL of announcements is not recommendable. It could lead to overloading effects and therefore a decrease in speech intelligibility (French and Steinberg 1947; Studebaker et al. 1999). The announcements in the front section have to be enhanced approximately by 3 dB(A) more than in the rear section for the same cabin noise levels to yield the same subjective evaluation of speech intelligibility. Obviously, the frequency spectrum found in the front section of a short-range aircraft interferes with speech more than found in the rear section. During real flights, however, the SPL in the front section is usually lower than in the rear section. Therefore, speech intelligibility should not be impaired when using the same SPL for the whole cabin. 4.2

Effects of varying announcements’ frequencies on speech intelligibility

The findings of the two studies examining the impact of frequency manipulation on speech intelligibility support the effectiveness of using filter settings. Thus, an overall raise in SPL of loudspeaker announcements in the aircraft cabin can be avoided. A significant impact on speech intelligibility could be shown in all varied octave bands (2, 4, 8 kHz). A decrease of 6 dB(A) diminished the percentage of understood words by approximately 10% with respect to the unmodified announcement signal. An increase of 6 dB(A), however, resulted in an improvement of speech intelligibility up to 11% and an increase of 12 dB(A) in an improvement up to 18%. Similar to the enhancement of the SPL in general, the positive effect of a modification of specific frequencies and their combinations was limited at high presentation levels (Goshorn and Studebaker 1994). In line with previous research (Bell, Dirks, and Trine 1992; Chari, Herman, and Danhauer 1977; DePaolis, Janota, and Frank 1996) the octave bands with the centre frequencies 2 and 4 kHz contributed particularly to an improvement of speech intelligibility. The same modifications tested for the adjacent 8 kHz band were less effective (on average, 6% less improvement with respect to correctly understood words). Therefore, an amplification of specific frequencies via appropriate filter settings could be verified as a useful approach for cabin design. In general, the current results extend the earlier findings since they refer specifically to an application in the aircraft cabin during cruise condition. Furthermore, the results were supported in both cabin noise samples (front and rear section). Hence, it can be assumed that it is not necessary to apply different filter settings in the cabin with respect to frequency manipulation. A combination of the 2 and 4 kHz octave bands could be useful for an effective regulation of communicating from the cockpit to passengers via loudspeakers. 4.3

Limitations and future research

Due to the laboratory setting a limitation of ecological validity has to be considered, since the subjects’ attention was focused on the loudspeaker announcements. During real flights, passengers are usually engaged in activities like reading or

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talking to other passengers and are not prepared to listen to loudspeaker announcements. In other laboratory studies examining speech intelligibility and speech quality, however, subjects performing at the same time an additional task using up attentional resources reported higher levels of concentration and annoyance, but their speech intelligibility performance remained unaffected (Lazarus-Mainka, Arnold, and Tkocz 1989; Sust et al. 2009; Volberg et al. 2006). This suggests that a higher SNR in a real aircraft cabin may not be necessary to maintain speech intelligibility, but to minimise annoyance. Nevertheless, the effectiveness of the proposed modifications of the loudspeaker announcements has to be confirmed during real flights. Furthermore, the speech intelligibility optimisations found in the studies are limited to the loudspeaker system implemented in the acoustic laboratory (original announcement loudspeakers from an aircraft cabin for short haul flights). Other loudspeaker systems could lead to other outcomes and therefore an examination of these effects should be addressed in further experiments. In future announcement systems recently developed noise-dependent algorithms to promote speech intelligibility (Cooke et al. 2013) could be considered, as well. With regard to the speech material employed, announcements were spoken by a professional male speaker. It can be expected that articulation, speech rate and psycholinguistic factors (e.g. effects of context, vocabulary and native vs. nonnative speakers or listeners) are additional aspects influencing speech intelligibility and annoyance during cabin noise exposure (e.g. Bradlow and Pisoni 1999; Lazarus 1990; Volberg et al. 2006). To counter the impact of native listener effects thresholds can be defined on the basis of similar laboratory experiments using other test materials not depending on a particular language. Finally, future research may include the investigation of manipulated announcements during other flight phases like climb or descent. Due to different SPLs and spectral compositions of cabin noise other SNRs and filter settings based on frequency enhancements have to be defined for these periods. On this basis flight phase adjusted filter settings of the loudspeakers could be realised promoting the intelligibility of safety-relevant information during the whole flight. 5. Conclusions The results indicate that both the performed sentence test and the subjective evaluations of speech intelligibility and quality are suitable methods to assess speech intelligibility and quality in an aircraft cabin environment. Speech intelligibility of loudspeaker announcements can be optimised by increasing the SNR between cabin noise and announcement. For the front section of the cabin a slightly higher SNR is required than for the rear section. The resulting levels of speech intelligibility can provide orientation for aircraft cabin design considering the required percentage of speech intelligibility, subjectively perceived speech quality level, as well as technical specifications and costs. The enhancement of specific frequency bands in the announcement signal improved speech intelligibility up to 18%. The overall SPL of the announcement and therefore loudspeaker performance was kept constant. Therefore, the studies support the employment of specific filter settings with respect to SPL and frequencies adjusted for the different cabin sections. Acknowledgements This work was supported by LuFo IV (4th Aviation Research Program) of the German Ministry of Economics and Technology. We thank our colleagues at the Institute of Aerospace Medicine and the Institute of Aerodynamics and Flow Technology at the German Aerospace Center for their assistance in the experiments.

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