Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Effect of bone-conduction harmonic distortions on hearing thresholds Anja Eichenauer Institute of Hearing Technology and Audiology, University of Applied Sciences, Oldenburg, Germany [email protected]
Harvey Dillon, Barry Clinch, and Teck Loi National Acoustic Laboratories, Sydney, Australia [email protected], [email protected], [email protected]
Abstract: This study compares bone conduction (BC) thresholds obtained with two bone vibrators (BV): The Radioear B71 and an advanced BV, the Radioear B81. B81 has less distortion and is capable of higher output levels. Two tests with 20 normal hearing subjects were conducted to determine if the low distortion of B81 offers any advantages for audiometry. Hearing threshold differences between B71 and B81 are only significant at 250 Hz for levels between 20 dB hearing level (HL) and 30 dB HL. At other frequencies and higher levels, where B81 also performs with less distortion than B71, tactile responses interfere before B81 can be advantageous. C 2014 Acoustical Society of America V
PACS numbers: 43.80.Qf, 43.80.Vj, 43.80.Gx [DO] Date Received: January 30, 2014 Date Accepted: June 17, 2014
1. Introduction Precise and reliable hearing thresholds are necessary for the successful treatment of people with hearing loss (Goldstein et al., 1962). Bone conduction (BC) thresholds are compared against air conduction (AC) thresholds to determine whether the hearing loss is primarily sensorineural, conductive, or a combination of both. Billings and Winter (1977) tested two different bone vibrator (BV) types: B71 and B72. They determined that the BC threshold varies up to 13 dB when a different BV is used. However, the main problem of B72 was an application problem. It was too heavy and, therefore, a stable placement on the mastoid was hardly possible. As a result, they recommended setting the clinical focus on B71, even though it showed significant harmonic energy when a 250 Hz signal was used. Frank et al. (1988) tested the influence of different vibrator types on BC thresholds. They compared the thresholds of Radioear B71, B72 as well as Pracitronic KH 70. According to Frank et al. (1988), harmonic distortions and resonance characteristics of the different BVs influence the hearing thresholds. At an input signal of 250 Hz, an analysis of harmonic distortions as well as a frequency-response diagram of B71 revealed resonances at 500 and 1250 Hz. Due to this harmonic energy, the apparent hearing threshold was found to be lower than the actual threshold. Consequently, a wrong air-bone gap can be diagnosed that makes BC audiometry unreliable (Hood, 1979). Ginter and Margolis (2013) conducted a study that compared the harmonic distortion of B71 against B81. They found out that B81 generates significantly lower distortions than B71. However, the results of harmonic distortion of the B71, determined at the signal levels 20, 50, and 60 dB hearing level (HL) at 250, 500, and 1000 Hz were still in the acceptable range according to ANSI (1996) and IEC (2012) standards (5.5% and 6%). Numerous experiments have established that at low frequencies senses other than pure auditory sensation can be triggered. Especially in bone conduction audiometry,
EL96 J. Acoust. Soc. Am. 136 (2), August 2014
C 2014 Acoustical Society of America V
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
mechanical vibrations that are transmitted to the mastoid can result in a perception that is non-auditory. Bocca and Perani (1960) suggested that those responses are a perception of the vestibular system caused by nerve endings in the saccule and cochlea. Contrarily to this hypothesis, Von Bekesy (1959) previously pointed out that a connection between the sensation of skin and hearing exists. Additionally, Nober (1964) as well as Boothroyd and Cawkwell (1970) argued that thresholds at low frequencies are related to tactile responses. Nober (1964) tested BC thresholds in non-auditory areas and, therefore, supported his suggestion of the existence of a vibrotactile sensation. In 1970, Nober applied a skin-injection to prevent tactile responses from influencing the threshold and showed that air thresholds of totally deaf people vanished (Nober, 1970). Consequently, it is important to be aware of the levels that cause vibrotactile sensations when hearing thresholds are determined. The present paper describes a study of a new BV, the Radioear B81. When compared against B71, it has less distortion and is capable of higher output levels. To determine whether these features offer any advantage in audiometric use, two tests were carried out. First, the level of tactile response was tested. According to Nober (1964), auditory thresholds shift in the presence of a masking noise, whereas nonauditory sensations are not affected by the masking noise. Under this assumption, the signal was masked with an AC noise to make it inaudible. By increasing the level of signal and noise, the signal stayed inaudible but stimulation of skin rose. Therefore the determined thresholds were in response to tactile stimulation. Second, the BC threshold was measured for a range of levels of narrow-band (NB)-AC masking noise. In contrast to the first test, only the BC-level was increased, from inaudible to audible, until the hearing threshold was found. By increasing the AC masker level, the corresponding BC-threshold should rise linearly. A deviation from this linear relationship is expected when distortion products in the bone-conducted sound become audible and contribute significantly to threshold. The purpose of this study is to see if the lower distortion and higher output levels of the B81 vibrator enable the measurement of higher bone conduction thresholds than is possible with the B71 vibrator. 2. Methods 2.1 Subjects Twenty normal hearing subjects (10 male, 10 female) between 19 and 63, with a mean age of 26.25 yr voluntarily participated in this study. All subjects were tested with both the B71 and the B81 BV in one appointment that took approximately 1 h. 2.2 Apparatus All tests were conducted with the Radioear B71 and Radioear B81. Both BVs are similar in their housing dimensions as shown in Fig. 1, which displays both vibrators from the front, side and back. The Madsen OB 822 clinical audiometer was used to generate the BC-input as well as the AC masking noise. The B71 and the B81 BV were calibrated with a Bruel and Kjaer artificial mastoid type 4930 and a sound level meter Bruel and Kjaer 2231. The BVs were coupled to the head with the headband Radioear P-3333. The masking narrow band noise, which had a bandwidth of 1/3 octave and spectral shape conforming to IEC 60645, was conducted by Eartone 3 A insert phones. The audiometer was connected to an amplifier of the type Marantz integrated stereo amplifier PM-43 as well as a volt-meter Fluke 8600A, which supplied a constant overview of the output voltage. The distortion tests were performed with a HP 339 A distortion measurement set and a HP3582A spectrum analyzer. 2.3 Procedure The current investigation involved two tests at the frequencies 250, 500, 1000, and 2000 Hz that were carried out with the BVs B71 and B81. The placement on the left or right mastoid was chosen randomly. The subjects did not know whether B71 or B81 was tested first. Half of the subjects started with B71, the other half with B81.
J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions EL97
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Fig. 1. (Color online) Pictures of the B71 and B81 bone conductors from the front, side and back.
For the first test, both ears were constantly masked with an AC-NB noise that made the tone inaudible. The amplification of the noise and the test tone was increased simultaneously. The subjects were asked to respond when they could hear the tone, although the signal stayed inaudible at all times. At the level of response, it is assumed that tactile sensation was confounded with auditory perception. To confirm that the subjects were perceiving a tactile sensation, they were asked whether they could feel or hear the output. The result was noted when the subject responded twice at the same level. For the second test, the threshold of the BC level was measured as a function of AC narrow band masker level. First, the tone was presented clearly above the hearing threshold so that the subject could familiarize with the frequency. Both ears were then masked with the AC-NB noise centered at the test frequency. This caused the BC tone to be inaudible. Using the ascending method, the BC-tone was raised in 5 dB steps, from inaudible levels to audible levels, until the subject could hear the tone. A few trials were used as training intervals to familiarize the subject with the task. In the test phase, the procedure was repeated for a variety of increasing masker levels. Again the response was noted when the subject responded twice at the same level. To determine the audibility of harmonic distortion objectively, the BV force level was measured with the artificial mastoid. For each input level (in dB HL), the voltage output from the artificial mastoid for each distortion component was referenced to
EL98 J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Table 1. Median level of tactile response in dB HL and standard deviations (r) of B81 and B71 for the frequencies 250 Hz, 500 Hz and 1000 Hz. f (Hz)
B81 (dB HL)
r (dB HL)
B71 (dB HL)
r (dB HL)
250 500 1000
35 55 75
5 3 3
35 55 75
5 4 2
the voltage corresponding to the reference equivalent threshold force level (RETFL) applicable to the frequency of that distortion component. This enabled the level of the distortion component to be expressed in dB HL. It is assumed that distortion products are audible when their level is above 0 dB HL. 3. Results Table 1 shows the median of tactile responses for 250, 500, and 1000 Hz for both BVs. The results are 35 dB HL at 250 Hz, 55 dB HL at 500 Hz, and 75 dB HL at 1000 Hz. It can be seen that both BVs trigger the tactile responses at the same levels. Nevertheless it has to be noted that one subject could not feel any tactile response at all. At 1000 Hz, 13 subjects responded to a tactile response of B71, and 14 subjects could feel the vibration of B81. Distortion-measurements of the B71 BV are displayed in Fig. 2. For 250, 500, and 1000 Hz, it can be seen that the amount of distortion rises exponentially once the BV starts to distort the pure tones. At 20 dB HL (audiometer level) and 250 Hz, the B71 generates a distortion of 0.08%, which increases rapidly to 78% at 45 dB HL. At 500 Hz, the distortion increases from 0.1% at 50 dB HL to 4.3% at 60 dB HL and up to 14% at 70 dB HL. Figure 3 illustrates the distortion-measurements of B81. At 250 Hz, the distortion stays below 1% for all measured output levels with a maximum of 0.85% at 45 dB HL. Also at 500 and 1000 Hz, the B81 shows markedly less distortion than the B71 with a maximum of 1.55% (500 Hz at 70 dB HL) and 1.35% (1000 Hz at 75 dB HL). The hearing threshold level as a function of masker level for the B81 as well as the B71 at the frequencies 250, 500, 1000, and 2000 Hz are displayed in Fig. 4. An additional linear reference line is also plotted in all figures. It can be seen that at 250 Hz the thresholds start to differ at a level of 19 dB HL and converge again for levels higher than 30 dB HL. After a threshold of 32 dB, the graph of B81 starts to deviate from the reference line. For the B71, this already happens at 16 dB HL. Using the Wilcoxon signed-rank test, the 250 Hz threshold differences at the masking levels 81, 86, and 91 dB HL are significant (p 0.05). At all other frequencies and levels, the threshold differences between the B71 and B81 are not significant. At 500 Hz, there are
Fig. 2. (Color online) B71 distortion (%) as a function of the audiometer output level (dB HL) for the frequencies 250, 500, and 1000 Hz as well as all other frequencies.
J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions EL99
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Fig. 3. (Color online) B81 distortion (%) as a function of the audiometer output level (dB HL) for the frequencies 250, 500, and 1000 Hz as well as all other frequencies.
no significant threshold differences, but it can be seen that both graphs start to deviate from the reference line from 55 dB HL onward. This threshold is equal to the threshold of tactile response at 500 Hz as illustrated in Table 1. At 1000 and 2000 Hz, the threshold levels are almost equal to the reference line. However, it has to be mentioned that using the B71 at 2000 Hz, only eight subjects could sense the tone at a masker level of 111 dB; in 12 cases, either the maximal output of the audiometer or the uncomfortable level was reached. The level of tactile response at 1000 Hz is at 75 dB HL for both BVs; for 2000 Hz, there is no measurable level of tactile response. The results of the audibility of harmonic distortions of both bone conductors (Table 2) show that for the B71 the output-level of the second and third harmonic are below 0 dB HL at input levels of 10 and 15 dB HL. At an input level of 20 dB HL, the second harmonic is transmitted with a force of 9.5 dB HL. The third harmonic is below threshold for input levels up to 25 dB HL. At an input level of 30 dB HL, the
Fig. 4. (Color online) Threshold level (dB HL) as a function of masker level [dB sound pressure level (SPL)] at the frequencies 250, 500, 1000, and 2000 Hz.
EL100 J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Table 2. Output level (dB RETFL) of the second harmonic (500 Hz) and third harmonic (750 Hz) at several input levels (dB HL) of the fundamental frequency 250 Hz with the Radioear B71 and Radioear B81. The effect of spreading of NB-masker noise is not included in the calculation. B71 Input (dB HL) 10 15 20 25 30 35 40 45
Second (dB RETFL) 10.1 0.4 9.5 19.3 29.6 39.8 48.4 55.9
B81 Third (dB RETFL) 30.2 25.2 18.5 6.6 4.9 15.1 33.2 53
Second (dB RETFL) 27.3 23.1 21.2 14.3 7.5 1.7 11.7 26.2
Third (dB RETFL) 24.5 18.2 10.9 0.2 9.6 16.9 24.9 33.6
third harmonic has a level of 4.9 dB HL. For the B81, the second harmonic does not rise above 0 dB HL until the input is 30 dB HL, and the third harmonic does not rise above 0 dB HL until the input level is 25 dB HL. 4. Discussion Prior work has documented harmonic distortion and resonances of B71 and assumed that this influences the hearing-threshold (Billings and Winter, 1977; Frank et al., 1988). However, these papers have only gathered thresholds of normal hearing subjects; this means only thresholds at low output levels were tested. This study was intended to test how thresholds at different BC output levels are influenced by distortion. Furthermore, it was compared if thresholds measured with the advanced BV, B81, are more precise and if B81 enables measurements at higher intensities. Therefore the levels of tactile response were tested to indicate the boundaries of auditory sensation. To determine the behavior of the hearing threshold as a function of BC output level, the fundamental frequency was masked with an AC-NB noise at different intensities. For each noise intensity, the test tone (BC) was raised in its level, from inaudible to audible, to detect the hearing threshold. If distortion products become audible, they cause the threshold to decrease. By comparing the threshold levels of B81 and B71 to a linear reference line, the influence of distortion products can be determined. The outcomes show that at 250 Hz, the differences between thresholds using B81 and B71 start to be significant at output levels of approximately 20 dB HL. Therefore it can be assumed that distortion products become audible at 20 dB HL, at 250 Hz. Furthermore the measurement of force levels of harmonic distortions shows audible distortion products for the B71 at 250 Hz from 20 dB HL onward (9.5 dB RETFL at 20 dB HL). This confirms the assumption that the distortion and resonance frequencies of B71 stimulate the cochlea and lead to lower thresholds. The first clearly audible harmonic distortion at 250 Hz of B81 is at 30 dB HL (9.6 dB RETFL). Consequently, the threshold results show that when BC output levels higher than approximately 30 dB HL are tested, the differences between the thresholds of the two devices decrease. Therefore the threshold levels of B81 also start to deviate from the linear reference line. In addition to the influence of distortion, it is likely this is also a result of tactile perception, for which the median threshold at 250 Hz was 35 dB HL for both BVs. Thresholds for the two devices were equivalent at 500, 1000, and 2000 Hz. At 500 Hz and 55 dB HL, the distortion of B71 starts to increase with a steep slope, whereas the distortion of B81 only increases slightly (Figs. 2 and 3). It can only be speculated that the results would have distinguished at thresholds higher than 55 dB HL if tactile responses did not occur at that level. Figure 4 shows that at 500 Hz, both BVs cause a deviation from the linear reference from 55 dB HL onward. At those levels, it is assumed that the threshold decreases due to tactile sensation.
J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions EL101
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Similarly, at 1000 Hz, the distortion differences start to increase above 70 dB HL, which is also close to the tactile threshold. Consequently, it can be said that at higher intensities, tactile responses interfere with the advantages of B81. At 2000 Hz, distortion products are minor for both BVs. Therefore no significant threshold differences were determined. Previous studies tested tactile responses of partially or totally deaf subjects on non-auditory areas (Nober, 1964), unilateral deaf subjects on the mastoid (Boothroyd and Cawkwell, 1970), or subjects with profoundly hearing losses (Dean and Martin, 1997), whereas in this study, normal hearing subjects were tested and masking was used to increase the hearing threshold. The determined tactile thresholds are in good agreement with the generally accepted tactile levels for low frequencies at 35 and 55 dB HL for 250 and 500 Hz, respectively (Roeser and Valente, 2007). Conclusions in this paper are restricted to the effects of distortion and tactile sensation on threshold levels. The markedly lower distortion of B81 will be most noticeable at high presentation levels where the BVs are causing both auditory and tactile perception. This reduces the advantage in audiometric use. 5. Conclusion The B81 has markedly lower distortion than the B71. However, from 500 to 2000 Hz, the upper limit of bone-conduction audiometry is determined by the level at which tactile sensations occur rather than the level at which distortion components become audible. Consequently, as the B71 and B81 vibrators produce the same tactile thresholds, they enable the same upper limit of presentation level for bone conduction audiometry. At 250 Hz, distortion components are audible at and above 20 dB HL for the B71, but the same degree of audibility of the distortion does not occur until 30 dB HL for the B81. As the tactile sensation level is around 35 dB HL, the B81 enables testing to be carried out over a larger dynamic test range than for the B71, without concern about distortion components affecting the observed threshold. Future research is needed to determine if alternative methods of coupling the vibrator to the head can enable the higher quality, higher amplitude output of the B81 to result in an extended range over which bone conduction testing can be carried out for higher frequencies. References and links ANSI (1996). ANSI S3.6, Specification for Audiometers (Acoustical Society of America, New York). Billings, B. L., and Winter, M. (1977). “Calibration force levels for bone conduction vibrators,” J. Speech Lang. Hear. Res. 20, 653–660. Bocca, E., and Perani, G. (1960). “Further contributions to the knowledge of vestibular hearing,” Acta Otolaryngol. 51, 260–267. Boothroyd, A., and Cawkwell, S. (1970). “Vibrotactile thresholds in pure tone audiometry,” Acta Otolaryngol. 69, 381–387. Dean, M., and Martin, F. N. (1997). “Auditory and tactile bone-conduction thresholds using three different oscillators,” J. Am. Acad. Audiol. 8, 227–232. Frank, T., Byrne, D. C., and Richards, L. A. (1988). “Bone conduction threshold levels for different bone vibrator types,” J. Speech Hear. Disorders 53, 295–301. Ginter, S. M., and Margolis, R. H. (2013). “Acoustic method for calibration of audiometric bone vibrators. II. harmonic distortion” J. Acoust. Soc. Am. 134, EL33–EL37. Goldstein, D. P., Hayes, C. S., and Peterson, J. L. (1962). “A comparison of bone-conduction thresholds by conventional and Rainville methods,” J. Speech Lang. Hear. Res. 5, 244–255. Hood, J. (1979). “Clinical implications in calibration requirements in bone conduction standardisation,” Int. J. Audiol. 18, 36–42. IEC (2012). 60645-1, Electroacoustics—Audiometric Equipment. I. Equipment for Pure-Tone Audiometry (International Electrotechnical Commission, Geneva, Switzerland). Nober, E. H. (1964). “Pseudoauditory bone-conduction thresholds,” J. Speech Hear. Disorders 29, 469–476. Nober, E. H. (1970). “Cutile air and bone conduction thresholds of the deaf,” Exceptional Children 36, 571–579. Roeser, R., and Valente, M. (2007). Audiology Diagnosis (Thieme, New York). Von Bekesy, G. (1959). “Similarities between hearing and skin sensations,” Psychol. Rev. 66, 1–22.
EL102 J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions
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[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Effect of bone-conduction harmonic distortions on hearing thresholds Anja Eichenauer Institute of Hearing Technology and Audiology, University of Applied Sciences, Oldenburg, Germany [email protected]
Harvey Dillon, Barry Clinch, and Teck Loi National Acoustic Laboratories, Sydney, Australia [email protected], [email protected], [email protected]
Abstract: This study compares bone conduction (BC) thresholds obtained with two bone vibrators (BV): The Radioear B71 and an advanced BV, the Radioear B81. B81 has less distortion and is capable of higher output levels. Two tests with 20 normal hearing subjects were conducted to determine if the low distortion of B81 offers any advantages for audiometry. Hearing threshold differences between B71 and B81 are only significant at 250 Hz for levels between 20 dB hearing level (HL) and 30 dB HL. At other frequencies and higher levels, where B81 also performs with less distortion than B71, tactile responses interfere before B81 can be advantageous. C 2014 Acoustical Society of America V
PACS numbers: 43.80.Qf, 43.80.Vj, 43.80.Gx [DO] Date Received: January 30, 2014 Date Accepted: June 17, 2014
1. Introduction Precise and reliable hearing thresholds are necessary for the successful treatment of people with hearing loss (Goldstein et al., 1962). Bone conduction (BC) thresholds are compared against air conduction (AC) thresholds to determine whether the hearing loss is primarily sensorineural, conductive, or a combination of both. Billings and Winter (1977) tested two different bone vibrator (BV) types: B71 and B72. They determined that the BC threshold varies up to 13 dB when a different BV is used. However, the main problem of B72 was an application problem. It was too heavy and, therefore, a stable placement on the mastoid was hardly possible. As a result, they recommended setting the clinical focus on B71, even though it showed significant harmonic energy when a 250 Hz signal was used. Frank et al. (1988) tested the influence of different vibrator types on BC thresholds. They compared the thresholds of Radioear B71, B72 as well as Pracitronic KH 70. According to Frank et al. (1988), harmonic distortions and resonance characteristics of the different BVs influence the hearing thresholds. At an input signal of 250 Hz, an analysis of harmonic distortions as well as a frequency-response diagram of B71 revealed resonances at 500 and 1250 Hz. Due to this harmonic energy, the apparent hearing threshold was found to be lower than the actual threshold. Consequently, a wrong air-bone gap can be diagnosed that makes BC audiometry unreliable (Hood, 1979). Ginter and Margolis (2013) conducted a study that compared the harmonic distortion of B71 against B81. They found out that B81 generates significantly lower distortions than B71. However, the results of harmonic distortion of the B71, determined at the signal levels 20, 50, and 60 dB hearing level (HL) at 250, 500, and 1000 Hz were still in the acceptable range according to ANSI (1996) and IEC (2012) standards (5.5% and 6%). Numerous experiments have established that at low frequencies senses other than pure auditory sensation can be triggered. Especially in bone conduction audiometry,
EL96 J. Acoust. Soc. Am. 136 (2), August 2014
C 2014 Acoustical Society of America V
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
mechanical vibrations that are transmitted to the mastoid can result in a perception that is non-auditory. Bocca and Perani (1960) suggested that those responses are a perception of the vestibular system caused by nerve endings in the saccule and cochlea. Contrarily to this hypothesis, Von Bekesy (1959) previously pointed out that a connection between the sensation of skin and hearing exists. Additionally, Nober (1964) as well as Boothroyd and Cawkwell (1970) argued that thresholds at low frequencies are related to tactile responses. Nober (1964) tested BC thresholds in non-auditory areas and, therefore, supported his suggestion of the existence of a vibrotactile sensation. In 1970, Nober applied a skin-injection to prevent tactile responses from influencing the threshold and showed that air thresholds of totally deaf people vanished (Nober, 1970). Consequently, it is important to be aware of the levels that cause vibrotactile sensations when hearing thresholds are determined. The present paper describes a study of a new BV, the Radioear B81. When compared against B71, it has less distortion and is capable of higher output levels. To determine whether these features offer any advantage in audiometric use, two tests were carried out. First, the level of tactile response was tested. According to Nober (1964), auditory thresholds shift in the presence of a masking noise, whereas nonauditory sensations are not affected by the masking noise. Under this assumption, the signal was masked with an AC noise to make it inaudible. By increasing the level of signal and noise, the signal stayed inaudible but stimulation of skin rose. Therefore the determined thresholds were in response to tactile stimulation. Second, the BC threshold was measured for a range of levels of narrow-band (NB)-AC masking noise. In contrast to the first test, only the BC-level was increased, from inaudible to audible, until the hearing threshold was found. By increasing the AC masker level, the corresponding BC-threshold should rise linearly. A deviation from this linear relationship is expected when distortion products in the bone-conducted sound become audible and contribute significantly to threshold. The purpose of this study is to see if the lower distortion and higher output levels of the B81 vibrator enable the measurement of higher bone conduction thresholds than is possible with the B71 vibrator. 2. Methods 2.1 Subjects Twenty normal hearing subjects (10 male, 10 female) between 19 and 63, with a mean age of 26.25 yr voluntarily participated in this study. All subjects were tested with both the B71 and the B81 BV in one appointment that took approximately 1 h. 2.2 Apparatus All tests were conducted with the Radioear B71 and Radioear B81. Both BVs are similar in their housing dimensions as shown in Fig. 1, which displays both vibrators from the front, side and back. The Madsen OB 822 clinical audiometer was used to generate the BC-input as well as the AC masking noise. The B71 and the B81 BV were calibrated with a Bruel and Kjaer artificial mastoid type 4930 and a sound level meter Bruel and Kjaer 2231. The BVs were coupled to the head with the headband Radioear P-3333. The masking narrow band noise, which had a bandwidth of 1/3 octave and spectral shape conforming to IEC 60645, was conducted by Eartone 3 A insert phones. The audiometer was connected to an amplifier of the type Marantz integrated stereo amplifier PM-43 as well as a volt-meter Fluke 8600A, which supplied a constant overview of the output voltage. The distortion tests were performed with a HP 339 A distortion measurement set and a HP3582A spectrum analyzer. 2.3 Procedure The current investigation involved two tests at the frequencies 250, 500, 1000, and 2000 Hz that were carried out with the BVs B71 and B81. The placement on the left or right mastoid was chosen randomly. The subjects did not know whether B71 or B81 was tested first. Half of the subjects started with B71, the other half with B81.
J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions EL97
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Fig. 1. (Color online) Pictures of the B71 and B81 bone conductors from the front, side and back.
For the first test, both ears were constantly masked with an AC-NB noise that made the tone inaudible. The amplification of the noise and the test tone was increased simultaneously. The subjects were asked to respond when they could hear the tone, although the signal stayed inaudible at all times. At the level of response, it is assumed that tactile sensation was confounded with auditory perception. To confirm that the subjects were perceiving a tactile sensation, they were asked whether they could feel or hear the output. The result was noted when the subject responded twice at the same level. For the second test, the threshold of the BC level was measured as a function of AC narrow band masker level. First, the tone was presented clearly above the hearing threshold so that the subject could familiarize with the frequency. Both ears were then masked with the AC-NB noise centered at the test frequency. This caused the BC tone to be inaudible. Using the ascending method, the BC-tone was raised in 5 dB steps, from inaudible levels to audible levels, until the subject could hear the tone. A few trials were used as training intervals to familiarize the subject with the task. In the test phase, the procedure was repeated for a variety of increasing masker levels. Again the response was noted when the subject responded twice at the same level. To determine the audibility of harmonic distortion objectively, the BV force level was measured with the artificial mastoid. For each input level (in dB HL), the voltage output from the artificial mastoid for each distortion component was referenced to
EL98 J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions
Eichenauer et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4885771]
Published Online 11 July 2014
Table 1. Median level of tactile response in dB HL and standard deviations (r) of B81 and B71 for the frequencies 250 Hz, 500 Hz and 1000 Hz. f (Hz)
B81 (dB HL)
r (dB HL)
B71 (dB HL)
r (dB HL)
250 500 1000
35 55 75
5 3 3
35 55 75
5 4 2
the voltage corresponding to the reference equivalent threshold force level (RETFL) applicable to the frequency of that distortion component. This enabled the level of the distortion component to be expressed in dB HL. It is assumed that distortion products are audible when their level is above 0 dB HL. 3. Results Table 1 shows the median of tactile responses for 250, 500, and 1000 Hz for both BVs. The results are 35 dB HL at 250 Hz, 55 dB HL at 500 Hz, and 75 dB HL at 1000 Hz. It can be seen that both BVs trigger the tactile responses at the same levels. Nevertheless it has to be noted that one subject could not feel any tactile response at all. At 1000 Hz, 13 subjects responded to a tactile response of B71, and 14 subjects could feel the vibration of B81. Distortion-measurements of the B71 BV are displayed in Fig. 2. For 250, 500, and 1000 Hz, it can be seen that the amount of distortion rises exponentially once the BV starts to distort the pure tones. At 20 dB HL (audiometer level) and 250 Hz, the B71 generates a distortion of 0.08%, which increases rapidly to 78% at 45 dB HL. At 500 Hz, the distortion increases from 0.1% at 50 dB HL to 4.3% at 60 dB HL and up to 14% at 70 dB HL. Figure 3 illustrates the distortion-measurements of B81. At 250 Hz, the distortion stays below 1% for all measured output levels with a maximum of 0.85% at 45 dB HL. Also at 500 and 1000 Hz, the B81 shows markedly less distortion than the B71 with a maximum of 1.55% (500 Hz at 70 dB HL) and 1.35% (1000 Hz at 75 dB HL). The hearing threshold level as a function of masker level for the B81 as well as the B71 at the frequencies 250, 500, 1000, and 2000 Hz are displayed in Fig. 4. An additional linear reference line is also plotted in all figures. It can be seen that at 250 Hz the thresholds start to differ at a level of 19 dB HL and converge again for levels higher than 30 dB HL. After a threshold of 32 dB, the graph of B81 starts to deviate from the reference line. For the B71, this already happens at 16 dB HL. Using the Wilcoxon signed-rank test, the 250 Hz threshold differences at the masking levels 81, 86, and 91 dB HL are significant (p 0.05). At all other frequencies and levels, the threshold differences between the B71 and B81 are not significant. At 500 Hz, there are
Fig. 2. (Color online) B71 distortion (%) as a function of the audiometer output level (dB HL) for the frequencies 250, 500, and 1000 Hz as well as all other frequencies.
J. Acoust. Soc. Am. 136 (2), August 2014
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Fig. 3. (Color online) B81 distortion (%) as a function of the audiometer output level (dB HL) for the frequencies 250, 500, and 1000 Hz as well as all other frequencies.
no significant threshold differences, but it can be seen that both graphs start to deviate from the reference line from 55 dB HL onward. This threshold is equal to the threshold of tactile response at 500 Hz as illustrated in Table 1. At 1000 and 2000 Hz, the threshold levels are almost equal to the reference line. However, it has to be mentioned that using the B71 at 2000 Hz, only eight subjects could sense the tone at a masker level of 111 dB; in 12 cases, either the maximal output of the audiometer or the uncomfortable level was reached. The level of tactile response at 1000 Hz is at 75 dB HL for both BVs; for 2000 Hz, there is no measurable level of tactile response. The results of the audibility of harmonic distortions of both bone conductors (Table 2) show that for the B71 the output-level of the second and third harmonic are below 0 dB HL at input levels of 10 and 15 dB HL. At an input level of 20 dB HL, the second harmonic is transmitted with a force of 9.5 dB HL. The third harmonic is below threshold for input levels up to 25 dB HL. At an input level of 30 dB HL, the
Fig. 4. (Color online) Threshold level (dB HL) as a function of masker level [dB sound pressure level (SPL)] at the frequencies 250, 500, 1000, and 2000 Hz.
EL100 J. Acoust. Soc. Am. 136 (2), August 2014
Eichenauer et al.: Bone-conduction harmonic distortions
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Table 2. Output level (dB RETFL) of the second harmonic (500 Hz) and third harmonic (750 Hz) at several input levels (dB HL) of the fundamental frequency 250 Hz with the Radioear B71 and Radioear B81. The effect of spreading of NB-masker noise is not included in the calculation. B71 Input (dB HL) 10 15 20 25 30 35 40 45
Second (dB RETFL) 10.1 0.4 9.5 19.3 29.6 39.8 48.4 55.9
B81 Third (dB RETFL) 30.2 25.2 18.5 6.6 4.9 15.1 33.2 53
Second (dB RETFL) 27.3 23.1 21.2 14.3 7.5 1.7 11.7 26.2
Third (dB RETFL) 24.5 18.2 10.9 0.2 9.6 16.9 24.9 33.6
third harmonic has a level of 4.9 dB HL. For the B81, the second harmonic does not rise above 0 dB HL until the input is 30 dB HL, and the third harmonic does not rise above 0 dB HL until the input level is 25 dB HL. 4. Discussion Prior work has documented harmonic distortion and resonances of B71 and assumed that this influences the hearing-threshold (Billings and Winter, 1977; Frank et al., 1988). However, these papers have only gathered thresholds of normal hearing subjects; this means only thresholds at low output levels were tested. This study was intended to test how thresholds at different BC output levels are influenced by distortion. Furthermore, it was compared if thresholds measured with the advanced BV, B81, are more precise and if B81 enables measurements at higher intensities. Therefore the levels of tactile response were tested to indicate the boundaries of auditory sensation. To determine the behavior of the hearing threshold as a function of BC output level, the fundamental frequency was masked with an AC-NB noise at different intensities. For each noise intensity, the test tone (BC) was raised in its level, from inaudible to audible, to detect the hearing threshold. If distortion products become audible, they cause the threshold to decrease. By comparing the threshold levels of B81 and B71 to a linear reference line, the influence of distortion products can be determined. The outcomes show that at 250 Hz, the differences between thresholds using B81 and B71 start to be significant at output levels of approximately 20 dB HL. Therefore it can be assumed that distortion products become audible at 20 dB HL, at 250 Hz. Furthermore the measurement of force levels of harmonic distortions shows audible distortion products for the B71 at 250 Hz from 20 dB HL onward (9.5 dB RETFL at 20 dB HL). This confirms the assumption that the distortion and resonance frequencies of B71 stimulate the cochlea and lead to lower thresholds. The first clearly audible harmonic distortion at 250 Hz of B81 is at 30 dB HL (9.6 dB RETFL). Consequently, the threshold results show that when BC output levels higher than approximately 30 dB HL are tested, the differences between the thresholds of the two devices decrease. Therefore the threshold levels of B81 also start to deviate from the linear reference line. In addition to the influence of distortion, it is likely this is also a result of tactile perception, for which the median threshold at 250 Hz was 35 dB HL for both BVs. Thresholds for the two devices were equivalent at 500, 1000, and 2000 Hz. At 500 Hz and 55 dB HL, the distortion of B71 starts to increase with a steep slope, whereas the distortion of B81 only increases slightly (Figs. 2 and 3). It can only be speculated that the results would have distinguished at thresholds higher than 55 dB HL if tactile responses did not occur at that level. Figure 4 shows that at 500 Hz, both BVs cause a deviation from the linear reference from 55 dB HL onward. At those levels, it is assumed that the threshold decreases due to tactile sensation.
J. Acoust. Soc. Am. 136 (2), August 2014
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Similarly, at 1000 Hz, the distortion differences start to increase above 70 dB HL, which is also close to the tactile threshold. Consequently, it can be said that at higher intensities, tactile responses interfere with the advantages of B81. At 2000 Hz, distortion products are minor for both BVs. Therefore no significant threshold differences were determined. Previous studies tested tactile responses of partially or totally deaf subjects on non-auditory areas (Nober, 1964), unilateral deaf subjects on the mastoid (Boothroyd and Cawkwell, 1970), or subjects with profoundly hearing losses (Dean and Martin, 1997), whereas in this study, normal hearing subjects were tested and masking was used to increase the hearing threshold. The determined tactile thresholds are in good agreement with the generally accepted tactile levels for low frequencies at 35 and 55 dB HL for 250 and 500 Hz, respectively (Roeser and Valente, 2007). Conclusions in this paper are restricted to the effects of distortion and tactile sensation on threshold levels. The markedly lower distortion of B81 will be most noticeable at high presentation levels where the BVs are causing both auditory and tactile perception. This reduces the advantage in audiometric use. 5. Conclusion The B81 has markedly lower distortion than the B71. However, from 500 to 2000 Hz, the upper limit of bone-conduction audiometry is determined by the level at which tactile sensations occur rather than the level at which distortion components become audible. Consequently, as the B71 and B81 vibrators produce the same tactile thresholds, they enable the same upper limit of presentation level for bone conduction audiometry. At 250 Hz, distortion components are audible at and above 20 dB HL for the B71, but the same degree of audibility of the distortion does not occur until 30 dB HL for the B81. As the tactile sensation level is around 35 dB HL, the B81 enables testing to be carried out over a larger dynamic test range than for the B71, without concern about distortion components affecting the observed threshold. Future research is needed to determine if alternative methods of coupling the vibrator to the head can enable the higher quality, higher amplitude output of the B81 to result in an extended range over which bone conduction testing can be carried out for higher frequencies. References and links ANSI (1996). ANSI S3.6, Specification for Audiometers (Acoustical Society of America, New York). Billings, B. L., and Winter, M. (1977). “Calibration force levels for bone conduction vibrators,” J. Speech Lang. Hear. Res. 20, 653–660. Bocca, E., and Perani, G. (1960). “Further contributions to the knowledge of vestibular hearing,” Acta Otolaryngol. 51, 260–267. Boothroyd, A., and Cawkwell, S. (1970). “Vibrotactile thresholds in pure tone audiometry,” Acta Otolaryngol. 69, 381–387. Dean, M., and Martin, F. N. (1997). “Auditory and tactile bone-conduction thresholds using three different oscillators,” J. Am. Acad. Audiol. 8, 227–232. Frank, T., Byrne, D. C., and Richards, L. A. (1988). “Bone conduction threshold levels for different bone vibrator types,” J. Speech Hear. Disorders 53, 295–301. Ginter, S. M., and Margolis, R. H. (2013). “Acoustic method for calibration of audiometric bone vibrators. II. harmonic distortion” J. Acoust. Soc. Am. 134, EL33–EL37. Goldstein, D. P., Hayes, C. S., and Peterson, J. L. (1962). “A comparison of bone-conduction thresholds by conventional and Rainville methods,” J. Speech Lang. Hear. Res. 5, 244–255. Hood, J. (1979). “Clinical implications in calibration requirements in bone conduction standardisation,” Int. J. Audiol. 18, 36–42. IEC (2012). 60645-1, Electroacoustics—Audiometric Equipment. I. Equipment for Pure-Tone Audiometry (International Electrotechnical Commission, Geneva, Switzerland). Nober, E. H. (1964). “Pseudoauditory bone-conduction thresholds,” J. Speech Hear. Disorders 29, 469–476. Nober, E. H. (1970). “Cutile air and bone conduction thresholds of the deaf,” Exceptional Children 36, 571–579. Roeser, R., and Valente, M. (2007). Audiology Diagnosis (Thieme, New York). Von Bekesy, G. (1959). “Similarities between hearing and skin sensations,” Psychol. Rev. 66, 1–22.
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