Most Comfortable Listening Level and Speech Attenuation by Hearing Protectors

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1 INTERNATIONAL JOURNAL OF OCCUPATIONAL SAFETY AND ERGONOMICS 1995, VOL. 1, NO. 2, Most Comfortable Listening Level and Speech Attenuation by Hearing Protectors Tomasz Letowski Donna M. Magistro Amy C. Ritter Pennsylvania State University Hearing protectors attenuate both the background noise and the useful sounds em bedded in noise such as the sounds of speech and w arning signals. An effective hearing protector is one that attenuates background noise w hile leaving sufficient energy of speech and w arning signals to reach the ear of the worker. A t present, how ever, there are no established criteria for assessing effective change in speech-to-noise ratio caused by hearing protection devices (HPDs). One such criterion could be a change in m ost com fortable (listening) level (MCL) for speech caused by the presence of HPDs. In this study the HPD-related shift in M CL for speech presented in quiet w as m easured and com pared w ith tw o measures of noise attenuation: Noise Reduction Rating (NRR) and h ig h -m e d iu m -lo w (H -M -L ). The results indicate that the M CL shift m ay be a sensitive m easure of speech attenuation by HPDs, w hich together w ith the appropriate H -M -L m ay describe technical properties of HPDs. hearing protection most com fortable level speech perception 1. INTRODUCTION An important issue in hearing conservation is to determine the effects of hearing protection on the wearers ability to communicate in noise, listen to operating machinery, and respond to warning signals. Hearing protection devices (HPDs) attenuate both the background noise and the signals that need to be heard at the workplace. Therefore, it is reasonable to assume that when speech and noise have similar spectral properties, both of them should be attenuated to the same degree. That means that the speech-to-noise (S/N) ratio outside and inside the protector should be the same. This does not mean, however, that speech recognition in noise will remain unaffected by HPDs because it depends on both the S/N ratio and the absolute level of the noise (Pekkarinen, Viljanen, Salmivalli, & Suonpaa, 1990). In many practical situations, the effect of HPDs on speech perception in noise is complicated by the fact that speech and noise have different spectral properties. In such cases the presence of HPDs may affect the S/N ratio in the ear of the wearer. This effect can be either positive or negative depending on the spectral properties of speech and noise as well as on the attenuation characteristics of the HPD. The primary method of assessing noise attenuation of HPDs is the real-ear attenuation at threshold (REAT) method (American National Standard Institute [ANSI], 1984; International Organization for Standardization [ISO], 1990a). In this method, the thresholds of hearing, or one-third octave bands of noise centered at 125,250,500,1,000,2,000,4,000, and 8,000 Hz, are measured with and without HPDs. Noise attenuation of an HPD at a given frequency is calculated as the shift in the threshold of hearing caused by the presence of the HPD. In the United States, commercially available HPDs have to have assigned a noise reduction rating (N RR) Correspondence and requests for reprints should be sent to Tomasz Letowski, Departm ent o f Com munication Disorders, Pennsylvania State University, University Park, PA

2 154 T. LETOWSKI, D. M. MAGISTRO, AND A. C. RITTER value indicating its weighted average noise attenuation in the 125- to 4,000-Hz range (Environmental Protection Agency, 1979; Occupational Safety and Health Administration, 1981). Within countries belonging to the ISO, two synthetic rating methods are recommended: the signal-tonoise ratio (SNR) method and the high-medium-low (H -M -L ) method (ISO, 1990b). According to Lundin (1992), the H -M -L measures are better predictors of the A-weighted sound pressure levels under the HPD than the NRR or the SNR measures. The H -M -L measure estimates noise reduction by a given HPD for three classes of noises: 1. Class H characterized by Lc - LA = - 2 db, 2. Class M characterized by Lc - LA = 2 db, 3. Class L characterized by Lc - LA = 10 db, where Lc and LA are C-weighted and A-weighted levels of noise, respectively. In other words, the H, M, and L values represent the mean attenuation of an HPD at high, medium, and low frequencies. In order to predict the effect of an HPD on speech perception in noise, it is necessary to know the spectral characteristics of the actual speech and noise signals as well as the frequency-dependent attenuation offered by the HPD. These data could be subsequently used in calculating an index of speech attenuation provided by the HPD. Some problems with using this m ethod are its complexity and time-consuming character. A faster and equally accurate approach might be to determine a most comfortable level (MCL) for speech in noise with and without HPDs and to consider the difference between these values as a measure of the effective speech attenuation (ESA) provided by a given HPD. A further simplification of this methodology could be the assessment of MCLs for speech in quiet both with and without HPDs. The difference between both MCLs could be subsequent used as a comprehensive measure of speech attenuation by individual HPDs. This measure, together with the H -M -L values or other comprehensive measures of noise attenuation, could be used as an indicator of how speech communication may be affected by a given HPD. The purpose of this study was to determine the sensitivity and reliability of the shift in MCLs for speech in quiet caused by the presence of an HPD. This information is needed for assessing the feasibility of ESA becoming a criterion for the assessment of speech attenuation by HPDs. 2. METHOD 2.1 Subjects Two matching groups of subjects (Group A and Group B) participated in the study; each group consisted of 10 subjects (22-34 years old) recruited from the Penn State student population. All subjects were otologically normal, had normal bilateral thresholds of hearing (HL < db HL), and had no recent history of ear pathology. 2.2 Hearing Protectors Three hearing protectors were compared in the study: (a) a formable foam earplug (FFE); (b) a three-flanged insert hearing protector (TFP); and (c) a hearing protection earmuff (MUFF). All protectors were fitted under the experimenters control. The experimenter-assisted fit was selected over the naive subject fit to ensure high repeatability of insertions and to compare collected data with the REA T data obtained under similar conditions by an independent certified laboratory. During fitting, an FFE earplug was compressed into a narrow cylindrical shape and inserted into the ear until flush with entrance to the ear canal. Similarly, a TFP was inserted flush with the entrance of the ear canal. That is, the plug stem was the only item exposed from the ear canal. Fitting the M UFF was done by positioning the muffs over the ears so that the entire

3 SPEECH ATTENUATION 155 Attenuation (db) Frequency (Hz) Figure 1. Noise attenuation measured for FFE, TFP and MUFF HPDs using REAT procedure (ANSI, 1984). external ears were surrounded by the muff cushions. The muffs were held in place by a standard headband fitted over the top of the subject s head. The force exerted by the earmuffs on the subject s head was about 13.7 N. The REAT characteristics of the three HPDs are shown in Figure Instrumentation All tests were conducted in an audiometric test booth with an ambient noise level suitable for ears open testing (ANSI, 1991). The subject was seated in the center of the booth with two matched custom-built loudspeaker systems placed symmetrically 1 m away from the subject s head. Both loudspeakers operated simultaneously and were adjusted to produce identical sound levels at the subject s location with no subject present. The subject was asked not to move during testing and to hold his or her head in the same position. A continuous speech signal (an Auditec recording of a story read by a male talker) was used as the test signal. The speech signal was played from a Fisher CRW-50 cassette recorder through a Beltone 2000 audiom eter and a McIntosh power amplifier and delivered to the loudspeakers. The long-term average spectrum (LTAS) of the speech signal used in this study is shown in Figure 2. The Lc L a value was 4 db. 2.4 Procedure The most comfortable level (MCL) tests were conducted nine times with HPDs (HPD) and nine times without H PD s (NOP). A single experim ental series consisting of six test conditions constituted an experimental series: N O P -H PD -N O P-H PD -N O P-H PD where HPD and NOP mean conditions with and without HPDs, respectively. The series was presented once for each of the HPDs resulting in 18 MCL determinations. Subjects in Group A participated in the testing of FFE HPDs whereas, subjects in Group B participated in the testing of TFP and M UFF HPDs. In each test condition, MCL determination began at 10 db HL (without hearing protectors) and 30 db H L (with hearing protectors); re: pure tone average threshold. The speech signal was increased in 2-dB steps until the subject indicated that the signal level was comfortable and the spoken story easy to follow. The MCL determination was repeated three times, and the average M CL was defined as the median value. The effective speech attenuation (ESA) level provided

4 156 T. LETOWSKI, D. Nl. MAGISTRO, AND A. C. RITTER Relative Level (db) Figure 2. Long-term average spectrum of the continuous speech signal used in the study. by a given HPD was calculated as the difference between the MCL level obtained with the HPD and the MCL level obtained with no hearing protector in a preceding trial. 3. RESULTS The average MCL level obtained for 10 subjects listening to speech without HPDs was 37 db HL (SD = 5.3 db) in Group A and 37 db HL (SD = 4.7 db) in Group B. These values correspond well to the 40 db HL that is used conventionally in audiological clinics as the presentation level in speech recognition testing. The average MCL levels measured with FFE, TFP, and MUFF HPDs were 67.0 (SD = 6.0 db), 56.9 (SD = 7.8 db), and 62.3 (SD = 9.1 db) db HL, respectively. The average ESA levels (db) obtained in each series of tests and for all three HPDs are listed in Table 1. A repeated-measures analysis of variance (ANOVA) on two variables, hearing protector (HPD) and repetition (series) with one nesting variable (group), showed significant differences F(2,18) = 11.21, p <.01, in the ESA levels among HPDs but no series or group effect at 0.05 level. Therefore, all ESA data have been collapsed and averaged across all three repetitions. The collapsed data are presented in Table 2. Table 3 TABLE 1. Means and Standard Deviations of Effective Speech Attenuation Levels (db) for Three Hearing Protection Devices (HPDs) Measured in Three Consecutive Series Series 1 Series 2 Series 3 HPD M SD M SD M SD FFE TFP MUFF Note. FFE = form able foam earlplug; TFP = three-flanged insert hearing protector; MUFF = hearing protection earmuff.

5 TABLE 2. Means and Standard Deviations of Effective Speech Attenuation Levels (db) for Three Hearing Protection Devices (HPDs) Tested in the Study Effective Speech Attenuation SPEECH ATTENUATION 157 HPD M SD FFE TFP MUFF Note. FFE = formable foam earlplug; TFP = three-flanged insert hearing protector; MUFF = hearing protection earmuff. TABLE 3. Noise Reduction Rating (NRR), L (Low), M (Medium) and H (High) Attenuation Values (in db) for Three Hearing Protection Devices (HPDs) Tested in the Study. HPD NRR H M L FFE TFP MUFF Note. FFE = formable foam earlplug; TFP = three-flanged insert hearing protector; MUFF = hearing protection earmuff. contains the NRR and L -M -H values (db) calculated from REAT data provided by an independent certified laboratory. The L -M -H values were calculated according to ISO (1990b) using a 2 X standard deviation correction factor to yield similar probability of protection as the NRR coefficient (EPA, 1979; ISO, 1990b, OSHA, 1981). 4. DISCUSSION Several authors who compared speech perception with and without HPDs reported better speech recognition and detection of warning signals with HPDs in noise levels exceeding db(a) (Kryter, 1946; Pekkarinen et al., 1990; Wilkins & Martin, 1979). Pollack (1957) and Michael (1983) reported little effect of HPDs on speech perception in noise up to 100 db(a) levels. Similarly, Letowski and Me Gee (1993) observed little effects of HPDs on the detection of warble-tone signals in 100 db(a) noise. There are also reports indicating negative effects of HPDs on speech perception at low S/N ratios (S/N < 0 db) occurring at high noise levels (Bauman & Marston, 1986; Chung & Gannon, 1979). The different effect of HPDs observed in the reported studies can, to some extent, be related to the differences in speech and noise spectra used in those studies. In addition, HPDs seem to have negative effects on speech perception in noise by subjects suffering from sensorineural hearing loss (Pekkarinen et al., 1990). Inspection of the data presented in Tables 2 and 3 indicates no clear relationship between the H -M -L values and the ESA values obtained in this study. However, an intriguing finding of this study was a close relationship between NRR and ESA values obtained in the

6 158 T. LETOWSKI, D. M. MAGISTRO, AND A. C. RITTER case of FFE and M UFF HPDs. This may indicate that NRR, which is a poor predictor of A-weighted sound pressure level under the HPD, may actually be a good predictor of ESA by the HPD. In contrast to the FFE and M UFF HPDs, the ESA value measured for the TFP was considerably less than the H -L -M and NRR value reported for this plug. This plug comes in three sizes intended for small, medium, and large ear canals. Despite the availability of all three sizes of the plug, 2 of our subjects were apparently misfitted because their ESA values were much lower than the rest of the group. It seemed that these subjects had ear sizes that fell between the sizes provided by the manufacturer, or were otherwise misfitted. Even a very skilled person cannot always determine ear sizes (Smith, Bordon, Patterson, Mozo, & Camp, 1980). This may be a partial explanation of the observed discrepancy between ESA and the attenuation values. When the 2 misfitted subjects were excluded from the analysis, the ESA value for the three-flanged plug increased slightly to 22 db (SD = 4.1 db), but it was still less than its NRR value. Therefore, it may be hypothesized that the observed low value of attenuation may be at least partially due to the specific relation between the frequency response of the plug (see Figure 1), on the one hand, and the spectrum of the speech signal (see Figure 2), on the other. This would mean that ESA and NRR values are not always in agreement. This issue requires further studies. Because the Lc - LA value for the speech signal spectrum used in this study was 4 db, the mean value of the medium and low attenuations listed in Table 3 should be a good estimate of the noise attenuation provided by an HPD. These mean values are 23,25, and 21 for FFE, TFP, and MUFF, respectively. Compared to the corresponding ESA values, they show - 7,3 (value for 8 subjects), and - 4 db change in the theoretical S/N ratio due to the wearing of HPDs, respectively. These are significant differences encouraging further studies of the ESA index. It is, however, necessary to stress the fact that both the sample of subjects and the sample of HPDs were relatively small in this pilot study. Therefore, these data should be treated only as preliminary. 5. CONCLUSIONS All three HPDs utilized in this study differed substantially in their ESA values, indicating the good sensitivity of this index. Intrasubject variability was small and insignificant, whereas intersubject variability was comparable to that observed in the REAT studies. Both larger and smaller speech attenuation, in comparison to noise attenuation by HPDs, was observed and encourages further investigation. These studies should include a greater variety of HPDs, larger groups of subjects, and an assessment of the HPD-based MCL shift for speech in noise. REFERENCES American National Standards Institute. (1984). Method for the measurement o f the real-ear attenuation o f hearing protectors (Tech. Rep. No. ANSI S ). Washington, DC: Author. American National Standards Institute. (1991). Methods for the maximum permissible ambient noise levels in audiometric rooms (Tech. Rep. No. ANSI S ). Washington, DC: Author. Bauman, K. S., & Marston, L. E. (1986). Effects of hearing protection on speech intelligibility in noise. Sound and Vibration, 20, Chung, D. Y., & Gannon, R. P. (1979). The effect of ear protectors on word discrimination in subjects with normal hearing and subjects with noise-induced hearing loss. Audiology, 15, Environmental Protection Agency. (1979, September). General provisions for product noise labeling and noise labeling requirements for hearing protectors, approval and promulgation. Federal Register, 40 CFR Part 211, Subpart B, International Organization for Standardization. (1990a). Acoustics Hearing protectors Part 1: Subjective method for the measurement o f sound attenuation (Tech. Rep. No. ISO ). Geneva: Author.

7 SPEECH ATTENUATION 159 International Organization for Standardization. (1990b). Acoustics Hearing protectors Part 2: Estimated noise reduction (Tech. Rep. No. ISO ). Geneva: Author. Kryter, K. D. (1946). Effects of ear protective devices on the intelligibility of speech in noise. Journal of the Acoustical Society o f America, 18, Letowski, T., & Me Gee, L. (1993). Detection of warble tones in wideband noise with and without hearing protection devices. Annals o f Occupational Hygiene, 37, Lundin, R. (1992). Properties of hearing protector rating methods. Proceedings of the 1992 Hearing Conservation Conference. Lexington, KY: National Hearing Conservation Association. Michael, L. E. (1983). Steel industry hearing protection study (Vol 1). Washington, DC: Iron and Steel Institute. Occupational Safety and Health Administration. (1981, August). Occupational noise exposure: Hearing conservation amendment. Federal Register, 29 CFR, Part Pekkarinen, E., Viljanen, V., Salmivalli, A., & Suonpaa, J. (1990). Speech recognition in a noisy and reverberant environment with and without earmuffs. Audiology, 29, Pollack, I. (1957). Speech communication at high noise levels: The roles of noise-operated automatic gain control system and hearing protection. Journal o f the Accoustical Society o f America, 29, Smith, C. R., Bordon, T. E., Patterson, L. B., Mozo, B. T., & Camp, R. T. (1980). Insert hearing protector effects. Ear & Hearing, 1, Wilkins, P. A. & Martin, A. M. (1979). The effect of hearing protectors on the masked thresholds of acoustic warning signals. Proceedings o f the ninth International Congress on Acoustics. Madrid, Spain: ICA.

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