Effects of Background Noise on Earphone Thresholds€¦ · Effects of Background Noise on Earphone Thresholds Tom Frank* Dennis L. Williams* Abstract Real-ear attenuation at threshold

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J Am Acad Audiol 4 : 201-212 (1993)

Effects of Background Noise on Earphone Thresholds Tom Frank* Dennis L. Williams*

Abstract

Real-ear attenuation at threshold (BEAT) and monaural pure-tone thresholds were deter-mined for normally hearing adults using a supra-aural, Audiocup, and insert earphone . The thresholds were obtained in quiet and in background noises approximating the maximum permissible ambient noise levels (MPANLs) allowed in an audiometric test room specified by ANSI S3.1-1991 and OSHA (1983) . The supra-aural BEAT was less than the Audiocup, which was less than the insert earphone . The mean threshold shifts were negligible regardless of earphone type in the ANSI noise and for the insert earphone in the OSHA noise. Thresholds can be obtained down to 0 dB HL in ANSI 1991 MPANLs using a supra-aural, Audiocup, or insert earphone and in OSHA MPANLs using an insert earphone . However, the OSHA MPANLs are too high and the BEAT is too low for a supra-aural earphone and Audiocup for testing down to 0 dB HL .

Key Words: Ambient noise, American National Standards Institute (ANSI), Audiocup, earphone, earphone attenuation, insert earphone, Occupational Safety and Health Adminis-tration (OSHA), supra-aural earphone, threshold

T

he American National Standards Insti-tute (ANSI) has specified maximum per-missible ambient noise levels (MPANLs)

allowed in an audiometric test room to ensure that testing down to 0 dB HL is not influenced by ambient noise masking. The ANSI MPANLs were originally specified in 1960 (ANSI S3 .1-1960) and revised in 1977 (ANSI S3.1-1977) and most recently in 1991 (ANSI S3.1-1991) . The 1991 revision specifies MPANLs in octave and 1/3 octave band intervals that would produce negligible masking (<_ 2 dB) of test signals pre-sented at 0 dB HL (re: ANSI S3.6-1989) for two test conditions and three test frequency ranges normally employed for audiometry . One test condition, called ears not covered, applies when either one or both ears are not covered with a typical supra-aural earphone . Testing in this condition would typically occur during bone-conduction (e .g., mastoid placement of a bone vibrator with an earphone placed on the other

*Department of Communication Disorders, Penn State University, University Park, Pennsylvania

Reprint requests : Tom Frank, 5-A Moore Building, Department of Communication Disorders, Penn State University, University Park, PA 16802

ear for the purposes ofmasking) and sound-field audiometry . The other test condition, called ears covered, applies when both ears are cov-ered simultaneously with supra-aural ear-phones. Testing in this condition would typi-cally occur during air-conduction audiometry . The three test frequency ranges are 125 to 8000 Hz, 250 to 8000 Hz, and 500 to 8000 Hz. The test frequency ranges 125 to 8000 Hz and 250 to 8000 Hz are typically employed for diagnostic audiometry while the test frequency range 500 to 8000 Hz is typically employed for hearing conservation (Appendix) .

Like the MPANLs specified in 1977, the 1991 ears covered MPANLs were obtained by adding the amount of attenuation provided by a supra-aural earphone to the ears not covered MPANLs. However, the supra-aural earphone attenuation values used in the 1991 revision were determined with test signals more reflec-tive of background noises occurring in audio-metric situations than those used in the 1977 version. The 1977 values were determined, for the most part, using pure tones presented in a free field and listener fitted supra-aural ear-phones to obtain maximum attenuation. The 1991 values were derived from several studies (Arlinger,1986 ; Berger and Killion,1989; Frank

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Journal of the American Academy of Audiology/Volume 4, Number 3, May 1993

Table 1 Supra-aural Ea rphone Attenuation Values

One -Third Octave Band Center Frequency

Standard 125 250 500 1000 2000 3150 4000 6300 8000

ANSI S3.1-1977 6 .5 4 .5 7 .0 15 .5 26 .0 30 .5 33 .0 27 .0 24 .5 ANSI S3.1-1991 6 .0 4 .0 5 .0 12 .5 19 .5 25 .0 25 .5 24 .0 23 .0 Difference 1991-1977 -0 .5 -0 .5 -2 .0 -3 .0 -6 .5 -5 .5 -7 .5 -3 .0 -1 .5

and Wright, 1990) that used '/a octave band noise test signals presented in a diffuse sound field (re: ANSI S12.6-1984 or ISO 4869-1981) and experimenter-fitted supra-aural earphones. Table 1 shows the ANSI 1977 and 1991 supra-aural earphone attenuation values. The 1991 attenuation values are lower (less attenuation) than those used in 1977, especially from 1000 to 6000/6300 Hz . As a result, the ANSI 1991 ears covered MPANLs are lower than those specified in 1977 (see Appendix).

MPANLs have also been specified by the Occupational Safety and Health Administra-tion (OSHA) in octave band intervals from 500 to 8000 Hz (OSHA, 1983, p. 9767-9768) and apply for hearing conservation testing purposes (e.g ., ears covered testing from 500 to 6000/8000 Hz). The OSHA MPANLs are considerably higher (more lenient) than the ANSI MPANLs (see Appendix). This occurs because the OSHA MPANLs were adopted from those specified in ANSI S3 .1-1960, which were based on reference threshold levels specified in the American Stand-ards Association (ASA) z24.5-1951. The ASA z24.5 reference thresholds were about 10 dB higher than those currently specified in ANSI S3.6-1989. As a result, OSHA allows higher levels of ambient noise in an audiometric test room compared with ANSI S3.1-1991 for ears covered testing from 500 to 8000 Hz . OSHNs rationale for not adopting more stringent MPANLs, such as the ANSI 1977 MPANLs, was the belief that lower MPANLs might not be obtainable in industrial settings and that thresh-olds could still be tested within the normal range, even though some thresholds might be elevated due to ambient noise masking (OSHA, 1983, p. 9767-9768; Franks et al, 1992).

Noise-reducing earphones have been sug-gested as an . alternative to supra-aural ear-phones when hearing testing is conducted in high ambient noise levels (Stark and Barton, 1975; Killion, 1984; Murray and Waugh, 1988). This occurs because a supra-aural earphone provides minimal attenuation for the lower com-pared with higher frequencies and most envi-ronmental background noises have more low than high frequency energy . One type of a noise-

reducing earphone employs a domed enclosure housing a supra-aural earphone, which fits over and around the outer ear. Examples of this type would be an Audiocup, AudioMate, or Aural-dome . Another type is called an insert earphone and employs a shoulder mounted transducer coupled to the earcanal via tubing terminated by a disposable foam eartip . Examples of this type would be an Etymotic Research ER-3A or EARtone 3A. Several studies have reported that noise-reducing earphone enclosures provided more overall real-ear attenuation at threshold (REAT) than supra-aural earphones and insert earphones provided more REAT than either supra-aural earphones or noise-reducing ear-phone enclosures (Berger and Killion, 1989; Frank and Wright, 1990; Franks et al, 1992).

In theory, if testing is done in an audiomet-ric test room having ambient noise levels equal to the ANSI 1991 or OSHA MPANLs, negligible masking should occur for pure tones presented at 0 dB HL. However, limited research has been conducted to determine if this is true . Berger and Killion (1989) reported the amount of mask-ing for pure tones presented in quiet and in background noise approximating the ANSI 1977 and OSHA MPANLs using a supra-aural, Audiocup, and insert earphone . In the ANSI noise, pure-tone thresholds could be measured down to 0 dB HL using the Audiocup and insert but not with the supra-aural earphone . In the OSHA noise, thresholds could be measured down to 0 dB HL with the insert but not with the supra-aural earphone or Audiocup . They also estimated the amount of threshold shift for each earphone in each background noise using their REAT values and a computational formula based on power-law summation of masking principles (see Berger and Killion, 1989 for a detailed explanation) . Overall, there was very good agree-ment between the measured and estimated threshold shifts for each earphone in each back-ground noise.

Recently, Franks et al (1992) reported REAT for a supra-aural earphone and two noise-reducing earphone enclosures (Audiocup and AudioMate) . In addition, they estimated the amount of threshold shift if testing were con-

202

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Effects of Background Noise on Thresholds/Frank and Williams

ducted in ANSI 1977 and OSHA MPANLs using their REAT values and the Berger and Killion (1989) computational formula. Their results were consistent with Berger and Killion (1989) indicating that thresholds could not be obtained down to 0 dB HL with a supra-aural earphone in ANSI 1977 or OSHA MPANLs or with the two noise-reducing earphone enclosures in the OSHA MPANLs .

In part, the present study was an extension of and similar to the research reported by Berger and Killion (1989) and Franks et al (1992) . Recall, these studies reported the amount of masking by direct or estimated threshold shift measurements caused by background noise ap-proximating the ANSI 1977 and OSHA MPANLs on earphone thresholds . More masking occurred when listeners were fitted with an earphone having lower attenuation (supra-aural) in higher ambient noise levels (OSHA) than if the same listeners were fitted with an earphone having higher attenuation (noise-reducing) in lower ambient noise levels (ANSI 1977). More impor-tantly, these studies reported that hearing thresholds could not be measured down to 0 dB HL using a supra-aural earphone in ANSI 1977 or OSHA MPANLs . The ANSI 1991 MPANLs for ears covered testing are lower than those specified in 1977 because lower earphone at-tenuation values were used (see Table 1). In theory, earphone testing in an audiometric test room containing ambient noise equal to the ANSI 1991 MPANLs should produce negligible masking (<_ 2 dB) for pure tones presented at 0 dB HL. However, confirmation of the ANSI 1991 MPANLs has not yet been determined using a direct threshold measurement para-digm . Further, additional direct threshold meas-urements are needed to confirm that testing down to 0 dB HL cannot be done in OSHA MPANLs using a supra-aural earphone or Audiocup as reported by Berger and Killion (1989) and Franks et al (1992).

The purpose of this study was to obtain REAT values and to determine the amount of masking caused by background noise approxi-mating the ANSI 1991 and OSHA MPANLs on pure-tone thresholds using a supra-aural, Audiocup, and insert earphone .

Subjects

METHOD

Twenty-four subjects (12 male, 12 female) between 18 and 30 years old having a mean age of 22.2 years (SD of 3.3 years) voluntarily par-

ticipated. Each subject had pure-tone air-con-duction thresholds <_ 20 dB HL from 500 to 6000 Hz and normal tympanograms in each ear.

Instrumentation

Earphones

The earphones used were the following: (1) a TDH-49P in a Model 51 cushion held in place by a Telephonics headband (referred to as the supra-aural earphone), (2) a different TDH-49P in a Model 51 cushion mounted in an Audiocup noise-reducing enclosure held in place by the Audiocup headband (referred to as the Audiocup), and (3) an insert earphone (Etymotic Research ER-3A) terminated by a foam eartip (ER3-14A) (referred to as the insert earphone). Each earphone was fitted to the subject's test ear by the same experimenter . For the supra-aural earphone and Audiocup, the diaphragm was positioned over the entrance of the earcanal and then the headband was tightened. For the insert earphone, the foam eartip was rolled, compressed, and inserted into the earcanal so that the outer edge was flush or just inside the bowl of the concha creating a 0- to 1-mm inser-tion depth. Each subject's nontest ear was al-ways fitted with an E-A-R foam earplug having an insertion depth of at least 1 mm inside the concha bowl . Further, when testing was con-ducted with the supra-aural earphone or Audiocup, the nontest ear was covered with a nonworking earphone of the same type . When testing was conducted with the insert earphone, the nontest ear was covered with an earmuff (Silenta, Ergo II) and the earmuff normally covering the other ear was positioned behind the test ear. The static force of the supra-aural earphone and Audiocup headband was meas-ured according to the procedures reported by Frank and Wright (1990) . The static force was 4.9 N for the supra-aural earphone and 14.8 N for the Audiocup and remained stable (±0.5 N) relative to the initial measurement throughout data collection .

Test Room

All testing was conducted in a sound-treated room modified to have a diffuse sound field (ANSI 512.6-1984) and having ambient noise levels not exceeding those specified in ANSI S3.1-1991 for ears not covered testing for a 125 to 8000 Hz range (see Appendix).

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Real-Ear Attenuation at Threshold

The test signals for the REAT measure-ments were 1/3 octave bands of noise (125, 250, 500, 1000, 3150, 4000, 6300, and 8000 Hz) originating from a pulsed (250 msec on-off; 50 msec rise-decay) signal generator (custom-built) routed to a stereo frequency equalizer (Realis-tic, 31-200A) to a 40-dB preamplifier (custom-built) and then to a recording attenuator (Grason-Stadler, E3262A ; attenuation rate of 2.5 dB/sec in 0.25-dB steps) . The attenuator output was directed to an impedance transformer (United Transformer Co., LS-34), a 10-dB step attenuator (Daven, 2513) and then to a cross-over network. The cross-over network output was directed to a two-channel amplifier (NAD, 2200), to two 20-dB attenuation pads (custom-built) and finally to three loudspeaker arrays . Each loudspeaker array was located in a corner of the test room and contained four 3-inch tweeters (CTS, 2T8) and one 15-inch woofer (CTS, 15G54L) mounted in a 0.8 by 0.5 m sheet of 2-cm plywood. The subjects used a noiseless response switch to control the output of the recording attenuator . The recording attenuator also provided hard copy of the subject's threshold tracings .

Background Noises

Two background noises were develdped to closely approximate (±1.5 dB) the MPANLs speci-fied by ANSI S3.1-1991 for ears covered testing in the 500 to 8000 Hz range and the MPANLs

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Figure 1 The 1/3 octave band interval MPANLs speci-fied by ANSI (ANSI S3.1-1991) for ears covered testing and 500 to 8000 Hz range and by OSHA (OSHA, 1983) and the ANSI and OSHA background noise levels used in the present study. The OSHA MPANLs were converted from octave to 1/3 octave band equivalents.

specified by OSHA (1983). Figure 1 illustrates the ANSI and OSHA 1/3 octave band MPANLs and corresponding background noise levels used in this study. The MPANLs in the OSHA regu-lation are specified in octave band intervals at 500, 1000, 2000, 4000, and 8000 Hz . For this study, the OSHA MPANLs were converted from octave to 1/3 octave band equivalents. Further, since the OSHA MPANLs were taken from the 1960 ANSI levels, the OSHA background noise levels were set to approximate the ANSI 1960 1/3 octave band MPANLs at 125, 250, 800, 1600, 3150, and 6300 Hz (see Fig. 1) .

The ANSI and OSHA background noises were developed using white noise (Grason-Stadler, 901), filtered by a'/3 octave band graphic spectrum equalizer (B&K, 124) and directed to the line input of a reel-to-reel tape recorder (Ampex, AG500) . The line output of the recorder was routed to a switcher (custom-built), a 10-dB step attenuator (Daven, 2513) and then to a cross-over network (custom-built) . The cross-over network output was directed to a two-channel amplifier (NAD, 2200), to two 20-dB attenuation pads (custom-built) and finally to the three loudspeaker arrays used for the REAT measurements . Due to the low-frequency out-put limitations of the instrumentation, an addi-tional white noise generator (custom-built) was directed to a low-pass filter (Krohn-Hite, 3750; 100 Hz cutoff, 24 dB/octave slope), to a 1/10-dB step attenuator (custom-built), and then to an amplifier/loudspeaker (Ampex, AA620) located in the test room .

The ANSI and OSHA background noises were created separately by adjusting each 1/ octave band from 125 to 8000 Hz on the equal-izer and the attenuator of the additional noise generator to target levels while the output of the loudspeakers was monitored by a 1-inch micro-phone (B&K, 4144) connected to an audio fre-quency spectrometer (B&K, 2113). The micro-phone was located in the middle of the test room at a marked position normally occupied by the theoretical midpoint of a subject's head and the spectrometer was located in an adjacent control room . When target levels were reached, each noise was recorded on the reel-to-reel tape re-corder . Then each recorded noise plus the out-put of the additional noise generator was played back through the instrumentation system and the SPL in each 1/3 octave band interval from 125 to 8000 Hz was measured using the moni-toring instrumentation. This procedure was repeated several times until the recorded noises plus the output of the additional noise generator

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was within ±1 .5 dB of the MPANLs specified by ANSI and OSHA (see Fig. 1) . The 1/3 octave band SPLs of the recorded noises plus the output of the additional noise generator remained stable (±1 dB) throughout data collection .

Test Signals for Earphone Thresholds

The test signals for the earphone threshold measurements were pure tones (500,1000, 2000, 3000, 4000, and 6000 Hz) generated by an audi-ometer (Maico, MA-24), directed to a 2-dB step attenuator and then to one of the three ear-phone types. The audiometer was calibrated (SPL output, attenuator linearity, frequency accuracy, and distortion re : ANSI S3.6-1989) for each test signal and earphone prior to, midway through, and at the end of data collection and remained stable (±1 dB). This was done by mounting the supra-aural earphone and the supra-aural earphone normally housed in the Audiocup on a National Bureau of Standards (NBS) 9A coupler (B&K, 4152), containing a f-inch microphone (B&K, 4144) connected to an audio frequency spectrometer (B&K, 2113) set to 1/3 octave band analysis . The same instru-mentation was used for the insert earphone except the NBS 9A was replaced by an HA-2 coupler with entrance through a rigid tube (B&K, DB-0138) as described by Frank and Richards (1991) .

Procedure Each subject participated in two 1'/2-hour

sessions and received every condition according to a counter-balanced schedule listing test ear (right, left), earphone thresholds (quiet, in ANSI noise, in OSHA noise), earphone type (supra-aural, Audiocup, insert), test frequency (N = 6), and REAT conditions (open ear, closed ear) . Two earphones were tested in the first session. The remaining earphone was tested in the second session along with the first earphone tested in session one for the purpose of obtaining retest data . Overall, each subject had the following five conditions for each earphone : (1) earphone threshold in quiet, (2) earphone threshold in ANSI background noise, (3) earphone threshold in OSHA background noise, (4) open ear (no earphone on the test ear) threshold for REAT measurement, and (5) closed ear (earphone fit-ted to the test ear but no pure-tone test signal presented through the earphone) threshold for REAT measurement. Further, each subject was tested so that the ear open REAT measure either preceded or followed the conditions in-

volving earphone testing. This was done so that once an earphone was fitted it remained in place for all of the earphone testing (conditions 1, 2, 3, and 5 above) .

Each subject was seated in the center of the test room and was visually observed to make certain their head position remained constant. For the REAT measurements, the test signals were always presented from 125 to 8000 Hz with a repeat of 1000 Hz . The subjects were in-structed to press the response switch as long as they heard and release the switch when they no longer heard the test signal . Recall that the response switch was connected to a recording attenuator, which not only controlled the test signal level but also provided hard copy of the threshold tracings. Each tracing met the crite-ria specified in ANSI S12 .6-1984. Each subject's open and closed ear thresholds were determined by ignoring the first reversal, calculating the midpoint of each excursion width for the next six reversals, averaging the six midpoints and rounding the average to the nearest whole number in decibels . The amount of attenuation was determined by subtracting the open from the closed ear threshold for each earphone at each frequency.

For the earphone threshold measurements (in quiet and in the ANSI or OSHA background noise played through the loudspeakers), the subjects were instructed to respond even if they just thought they heard a tone by pressing a response button. Initially, a tone was presented at a level loud enough to elicit a response, then decreased in 10-dB steps until no response oc-curred, and increased in 2-dB steps until a response occurred . This procedure (10 down, 2 up) continued until four ascending thresholds were obtained . Threshold was defined as the mean of the four ascending thresholds . The amount of threshold shift caused by the ANSI and OSHA background noise was determined by subtracting each subject's pure-tone thresh-old in quiet from their threshold in the ANSI or OSHA background noise for each earphone at each frequency.

RESULTS AND DISCUSSION

Real-Ear Attenuation at Threshold Measurements

One purpose of this study was to obtain REAT values for a supra-aural, Audiocup, and insert earphone . Table 2 shows the mean and median REAT for each earphone . The supra-


Table 2 Real-Ear Attenuation at Threshold Results Audiocup, and insert Earphones

for the Supra-aural,

One-Third Octave Band Center Frequency Earphone Type Measure 125 250 500 1000 2000 3150 4000 6300 8000 Supra-aural Mean 6.0 2 .7 5 .1 13 .9 21 .6 25 .1 28 .1 21 .4 20 .3

Median 5 .5 2 .0 5 .8 13 .2 20.4 25.2 27.6 21 .1 20 .2 SD 6.3 5 .3 7 .1 7 .7 6 .5 7 .2 7 .8 8 .8 7 .0

Audiocup Mean 5 .0 1 .1 19 .3 30.0 32.5 38.4 39.4 35 .7 34 .9 Median 5.4 1 .6 18 .5 30.5 31 .9 38.4 38.9 36 .2 35 .9 SD 5.5 7 .0 8 .1 7 .7 6 .8 6 .4 5 .7 7 .7 8 .1 Insert Mean 26 .5 29.2 32.0 33.3 33 .1 37 .7 40 .5 41 .5 40 .2

Median 27.2 28.8 32.9 34.3 33 .2 39 .2 41 .4 41 .3 40 .1 SD 6.1 6 .2 6 .3 5 .0 5 .9 4 .8 3 .8 5 .0 7 .9

aural provided the least and the insert ear-phone provided the most attenuation. The Audiocup attenuation values approximated the supra-aural for the lower (125 and 250 Hz) and the insert earphone values for the higher (1000 to 8000 Hz) frequencies . The mean and median values were very similar for each earphone at each frequency supporting the assumption that the attenuation values were normally distrib-uted . The standard deviations (see Table 2) ranged from about 4 to 8 dB across earphone type and frequency. A paired t-test for depend-ent observations was conducted for each ear-phone at each frequency using the test and retest values. The t-tests were not significant (p > .05) indicating reliable REAT measures .

Figure 2 illustrates the mean supra-aural earphone attenuation (see Table 2) and mean attenuation values reported by Berger and Killion (1989), ANSI S3 .1-1991 (see Table 1), and Franks et al (1992) . The attenuation values were fairly consistent indicating that the at-tenuation increased from about 5 dB at 125 Hz

to about 26 dB at 4000 Hz and then decreased to about 22 dB at 8000 Hz . The attenuation values obtained in the present study were in very good agreement with the ANSI values, except that 4 dB less attenuation occurred at 6000 and 8000 Hz .

Figure 3 illustrates the mean Audiocup attenuation (see Table 2) and the mean attenu-ation values reported by Murray and Waugh (1988), Berger and Killion (1989), Frank and Wright (1990), and Franks et al (1992) . Across studies, the attenuation was extremely variable for the lower (125 to 1000 Hz) and more consist-ent for the higher (2000 to 8000 Hz) frequencies. Frank and Wright (1990) reported that the variability in low frequency attenuation was related to the headband force and fitting proce-dures. However, Franks et al (1992) reported that fitting variables (placement and failure to obtain an adequate circumaural seal on some subjects) are probably more important than headband force. In this regard, the Audiocup attenuation values in the present study and

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Figure 2 Supra-aural earphone REAT values obtained in the present study and values reported by Berger and Killion (1989), ANSI S3.1-1991, and Franks et al (1992) .

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Figure 3 Audiocup REAT values obtained in the present study and values reported by Murray and Waugh (1988), Berger and Killion (1989), Frank and Wright (1990), and Franks et al (1992).

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Figure 4 Insert earphone REAT values obtained in the present study and values reported by Berger and Killion (1989) and Frank and Wright (1990) .

those reported by Frank and Wright (1990) were very similar, except at 500 Hz where 6.5 dB more attenuation occurred in the present study. Both sets of attenuation values were obtained using the same test room and instru-mentation except the Audiocup headband force in the present study was 14.8 N compared with 9 .7 N used by Frank and Wright (1990) . If headband force was a primary variable influ-encing low frequency attenuation, more attenu-ation should have occurred for all of the lower frequencies in the present study compared with the attenuation reported by Frank and Wright (1990) . Since this did not occur, it would seem that fitting variables, as reported by Franks et al (1992) are the primary cause for the variabil-ity in low frequency attenuation provided by an Audiocup .

Figure 4 illustrates the mean insert ear-phone attenuation (see Table 2) and the mean attenuation values reported by Berger and Killion (1989) and Frank and Wright (1990) . The attenuation values were in good agreement with Frank and Wright (1990) and were less than those reported by Berger and Killion (1989)

from 125 to 1000 Hz . The differences in attenu-ation can be related to insertion depth. As a general rule, insert earphone low frequency attenuation increases as insertion depth in-creases (Lindgren and Berger,1989). The present study and that of Frank and Wright (1990) used an insertion depth of 0 to 1 mm while Berger and Killion (1989) used a 2- to 3-mm insertion depth.

Amount of Masking

Another purpose of this study was to deter-mine the amount of masking caused by back-ground noise approximating the ANSI 1991 and OSHA MPANLs on pure-tone thresholds using a supra-aural, Audiocup, and insert earphone .

Thresholds in Quiet

Prior to a presentation and discussion ofthe amount of masking, information is provided concerning the thresholds in quiet. Table 3 shows the mean SPL and HL thresholds in quiet and standard deviations for each earphone . The reference equivalent threshold sound pressure levels (RETSPLs) listed in ANSI S3.6-1989 were used to convert the mean SPL thresholds to HL for the supra-aural earphone and the Audiocup . The interim RETSPLs listed in ANSI S3 .6-1989 Appendix G, corrected for use with a B&K DB-0138 coupler (Wilber et al, 1988 ; Frank and Richards, 1991), were used to convert the mean SPL thresholds to HL for the insert earphone. The mean HLs closely approximated 0 dB HL for the supra-aural and insert earphone and to a lesser degree for the Audiocup . Averaged over frequency, the mean HL was 0.3 dB for the supra-aural, -1.5 dB for the Audiocup, and 0.1 dB for the insert earphone . The standard deviations were fairly consistent at each fre-quency across earphone type ranging from about 3 to 7 dB .

Table 3 Thresholds in Quiet for the Supra-aural, Audiocup, and Insert Earphones

Frequency (Hz) Earphone Measure 500 1000 2000 3000 4000 6000 Supra-aural SPL 11 .5 7 .7 7 .8 9 .9 10 .7 15 .8

HL -2.0 0 .2 -3 .0 0 .4 0 .2 2 .3 SD 5.3 4.7 4 .3 5 .7 6 .4 7 .2 Audiocup SPL 10 .4 6 .5 5 .5 9 .0 11 .0 14 .4 HL -3 .1 -1 .0 -5 .5 -0 .5 0 .5 0 .9 SD 5.3 3 .4 3 .9 5 .8 6 .1 7 .3 Insert SPL 6 .3 2 .6 5 .5 6 .7 5 .4 6 .0 HL 0 .3 -1 .1 2 .1 0 .2 -1 .8 0 .7 SD 6 .4 5 .5 4 .4 5 .8 5 .3 5 .0

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Table 4 Threshold Shifts in Background Noise for the Supra-aural, Audiocup, and Insert Earphones

Frequency (Hz)

Noise Earphone Measure 500 1000

ANSI Supra-aural Mean 2.0 SD 2.5

Audiocup Mean 1 .8 SD 1 .7

Insert Mean 0.3 SD 1 .3

OSHA Supra-aural Mean 13 .8 SD 5.5

Audiocup Mean 6.3 SD 5.4

Insert Mean 0.7 SD 1 .3

The insert earphone HLs were in very good agreement with those reported by Frank and Vavrek (1992) and support the ANSI interim RETSPLs. It should be noted that ANSI 53 .6-1989 does not list RETSPLs for a supra-aural earphone mounted in an Audiocup . It is com-monly assumed that RETSPLs for a supra-aural earphone will not be influenced by mount-ing the earphone in an Audiocup enclosure. Stated another way, it is assumed that thresh-olds will be similar whether a supra-aural ear-phone is located outside or inside an Audiocup enclosure. As such, we expected that the supra-aural earphone and Audiocup thresholds in quiet would be the same . However, the Audiocup thresholds were always lower than the supra-aural earphone thresholds at each frequency, except 4000 Hz (see Table 3). These findings would not support the assumption that supra-aural earphone thresholds are the same whether the earphone is located inside or outside the Audiocup . It could be that RETSPLs for an Audiocup are slightly lower than RETSPLs for a supra-aural earphone and warrants further investigation.

2000 3000 4000 6000

1 .3 1 .5 0.8 0.9 1 .8 2.4 1 .6 1 .7 0.9 0.7 0.2 0.4 1 .6 2.1 1 .6 1 .7 0.2 0.4 0.0 0.1 1 .4 1 .7 0.8 1 .2 9.8 9.8 10 .1 12 .1 6.5 7 .0 6 .8 7 .7 2.8 2.8 3.8 4.3 2.9 3.3 3.9 5.7 1 .3 1 .3 1 .5 0.8 1 .6 1 .4 1 .5 1 .6

1 .0 2.3 0.6 1 .8 0.4 1 .0 8 .1 5 .1 1 .9 2.5 0.8 1 .8

1000 Hz to 13.8 dB at 500 Hz and for the Audiocup from 1.9 dB at 1000 Hz to 6.3 dB at 500 Hz. Further, the mean threshold shifts were considerably higher for the supra-aural com-pared with the Audiocup at each frequency. The threshold standard deviations (see Table 4) in the ANSI noise ranged from 1.0 to 2.5 dB and in the OSHA noise from 1 .3 to 7.7 dB across ear-phone type and frequency. The conditions hav-ing the highest standard deviations also had the greatest threshold shift.

The mean threshold shifts were in very good agreement with those reported by Berger and Killion (1989) for each earphone type in the OSHA noise and for the Audiocup and insert earphone in the ANSI noise. However, the mean threshold shifts were about 3.5 dB smaller than reported by Berger and Killion (1989) for the supra-aural earphone in the ANSI noise at

Threshold Shift

The mean threshold shift caused by the ANSI and OSHA background noise for each earphone is shown in Table 4 and illustrated in Figure 5. In the ANSI noise, the mean threshold shift was <_2 dB for each earphone at each frequency. In the O SHA noise, the mean thresh-old shift was >_2 dB for the supra-aural earphone and Audiocup, except for the Audiocup at 1000 Hz, and <_2 dB for the insert earphone . Specifi-cally, in the OSHA noise, the mean supra-aural earphone threshold shift ranged from 8.1 dB at

16

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=1 0

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OBB60ANSI, SUPRA-AURAL GMee7ANSl, AUDIOCUP e-mee ANSI, INSERT 00000

OSHA, SUPRA-AURAL ~ OSHA, AUDIOCUP AAA" OSHA, INSERT

s e ° 1000 1 3 4 3 - s 7 e

FREQUENCY (Hz)

Figure 5 Mean threshold shifts in the ANSI and OSHA background noise levels for the supra-aural, Audiocup, and insert earphone .

208

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1000, 2000, and 4000 Hz . Berger and Killion (1989) did not test at 3000 and 6000 Hz . This finding can be related to earphone attenuation and ANSI noise level differences between the studies. The present study found higher supra-aural earphone attenuation at 1000, 2000, and 4000 Hz (see Fig. 2) than reported by Berger and Killion (1989) and the ANSI noise levels were lower because they approximated the 1991 MPANLs not the 1977 MPANLs used by Berger and Killion (1989) (see Appendix). As such, a lower amount of masking noise was theoreti-cally present in the subjects' earcanals causing smaller thresholds shifts (3 .5 dB) for the supra-aural earphone at 1000, 2000, and 4000 Hz than reported by Berger and Killion (1989) .

Since the data were collected for only 24 subjects, the normality of the threshold shift distributions was tested for each background noise and earphone at each frequency. This was done by converting the threshold shift raw data (measured values) to the expected values of the distribution (NSCORES) and then testing the relation between the measured and expected values using a correlation test for normality (Minitab, 1989, Ch . 4, p. 7-8) . The results of each test revealed that the measured values were not significantly (p > .05) different than the expected normal distribution . Consequently, the threshold shift raw data were analyzed using parametric statistics . The test and retest threshold shift raw data were not significantly (p > .05) different using paired t-tests for depen-dent observations for each earphone at each fre-quency indicating good test-retest reliability .

The threshold shift raw data were subjected to a three-way analysis of variance with re-peated measures (ANOVR). The factors were background noise (ANSI and OSHA) by ear-phone type (supra-aural, Audiocup, and insert) by frequency (500, 1000, 2000, 3000, 4000, and 6000 Hz). Each main effect and interaction was significant (p < .0001) . Consequently, several planned orthogonal contrasts were performed using a multiple mean comparison . The amount ofthreshold shift for each earphone type at each frequency was not significantly (p > .9899) dif-ferent in the ANSI noise but was significantly (p < .0002) different in the OSHA noise. In the OSHA noise, the amount of threshold shift at each frequency was significantly (p < . 01) higher for the supra-aural compared with the Audiocup, the supra-aural compared with the insert, and the Audiocup compared with the insert ear-phone. These findings are evident upon inspec-tion of Figure 5.

The threshold shift raw data were also analyzed by determining percentage cumula-tive frequency distributions for each earphone in each background noise collapsed across the six test frequencies. The distributions are shown in Figure 6. Each distribution represents the cumulative number of threshold shifts divided by the total number (N = 144; 24, subjects x 6 frequencies). The cumulative distributions were

8

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0

8

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0

m

0

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Fi ure 6a SUPRA-AURAL

000" ANSI NOISE 0eeeo OSHA NOISE

9~J

THRESHOLD SHIFT (dB)

24 ' :22 ' 0 4 6 8 10 12 14 18 1 22 2 28 THRESHOLD SHIFT (dB)

Figure 6 Percentage cumulative frequency threshold shift distributions in the ANSI and OSHA background noise for the supra-aural (6a), the Audiocup (6b), and insert earphone (6c) . Each distribution represents the number of threshold shifts divided by the total number (N = 144, 6 frequencies x 24 subjects).

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collapsed across frequency because, in theory, a threshold shift >2 dB should not occur regard-less of frequency when testing is conducted in background noise equal to the ANSI 1991 MPANLs for normally hearing (0 dB HL) listen-ers. Averaged over frequency, the subject's thresholds in quiet (see Table 3) closely approxi-mated 0 dB HL for the supra-aural (0.3 dB HL) and insert earphone (0.1 dB HL) and were slightly less than normal for the Audiocup (-1 .5 dB HL). Since RETSPLs are not specified for an Audiocup in ANSI S3.6-1989, the Audiocup SPL thresholds were converted to HL using RETSPLs for a supra-aural earphone not mounted in an Audiocup . As described earlier, this may be in error and 0 dB HL for a supra-aural earphone inside an Audiocup might be slightly lower than 0 dB HL for a supra-aural earphone outside an Audiocup enclosure. In any event, it was assumed that the subjects in the present study were normally hearing listeners for each earphone type; as such, 0-dB threshold shift shown in Figure 6 corresponds to 0 dB HL, a 2-dB threshold shift corresponds to 2 dB HL, and so on.

Figure 6A reveals that the supra-aural ear-phone cumulative shifts were remarkably dif-ferent in each background noise ranging from -4 to 8 dB in the ANSI and from -4 to 26 dB in the OSHA noise . A threshold shift of 2 dB corresponded to 80 percent of the cumulative shifts in the ANSI compared with only 15 per-cent in the OSHA noise. Stated differently, 20 percent of the cumulative shifts were greater than 2 dB in the ANSI compared with 85 percent in the OSHA noise. This finding would indicate that the ANSI negligible masking criteria or maximum threshold shift of <_2 dB cannot be met 100 percent of the time, especially if applied to the OSHA MPANLs . However, closer inspec-tion of Figure 6A reveals that a threshold shift of 4 and 6 dB corresponded to 93 and 99 percent of the cumulative shifts in the ANSI noise. Figure 6B reveals that the cumulative shifts for the Audiocup ranged from -4 to 6 dB in the ANSI and from -4 to 20 dB in the OSHA noise. A threshold shift of 2 dB corresponded to 90 percent ofthe cumulative shifts in the ANSI and only 58 percent in the OSHA noise. This finding would indicate that if Audiocups were used for industrial testing in an environment having background noise equal to the OSHA MPANLs, 42 percent of the thresholds for normally hear-ing listeners would be elevated >2 dB . Figure 6C shows that the cumulative shifts for the insert earphone ranged from only -4 to 4 dB and that

a 2-dB threshold shift corresponded to about 94 percent of the cumulative shifts regardless of background noise.

The ANSI 1991 MPANLs were derived us-ing mean values (ANSI S3.1-1991, Appendix) so that accurate supra-aural earphone thresholds could be obtained at 0 dB HL with no more than a 2-dB elevation due to ambient noise masking. Overall, the mean threshold shift results (see Table 4 and Fig. 5) confirmed that the ANSI 1991 MPANLs for ears covered testing over a 500 to 8000 Hz range will produce negligible masking (<_2 dB) for normally hearing listeners using a supra-aural earphone as well as an Audiocup and insert earphone . Hopefully, fur-ther research will confirm the ANSI 1991 MPANLs for ears covered testing over a 125 to 8000 Hz and 250 to 8000 Hz range and for ears not covered testing for all three frequency ranges .

Since mean values were used to compute the ANSI 1991 MPANLs and variance always exists around mean values, it was expected that the cumulative threshold shift results (see Fig. 6) would indicate that the ANSI negligible mask-ing criteria of <_2 dB could not be met 100 percent of the time for each earphone. Assuming that the supra-aural earphone cumulative frequency distribution in the ANSI noise is normally dis-tributed around 0 dB HL (see Fig. 6A), 84 percent of the threshold shifts would be covered by the mean plus one standard deviation. Since a 2-dB threshold shift covered about 80 percent of the cumulative shifts, one could infer that the ANSI 1991 negligible masking criteria of <_2 dB for a supra-aural earphone would cover about 84 percent of the population . Further, 90 per-cent and 98 percent of the cumulative threshold shifts occurred at 2 dB for the Audiocup (see Fig. 6B) and insert earphone (see Fig. 6C), respec-tively, in the ANSI noise. Also, 100 percent of the cumulative shifts corresponded to a thresh-old shift of 8 dB for the supra-aural, 6 dB for the Audiocup, and 4 dB for the insert earphone. Overall, we believe that the ANSI 1991 MPANLs (ears covered, 500 to 8000 Hz range) and the negligible masking criteria of 52 dB are appro-priate and provide sufficient audiometric accu-racy for testing down to 0 dB HL using a supra-aural, Audiocup, or insert earphone .

Based on the mean threshold shifts (see Table 4 and Fig. 5) and the cumulative thresh-old shifts (see Fig. 6), testing in background noise levels equal to the OSHA MPANLs using an insert earphone will produce negligible mask-ing (<- 2 dB) for pure tones presented at 0 dB HL. However, the use of an Audiocup and especially

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a supra-aural earphone will produce sufficient masking so that pure-tone testing cannot be done at 0 dB HL. This finding is important for industrial testing since supra-aural earphones and Audiocups are commonly used and it has implications regarding accurate baseline audio-grams and significant threshold shifts (Frank and Vavrek, 1992; Franks et al, 1992). Further, these findings are in agreement with previous direct and estimated threshold shift measure-ments (Berger and Killion, 1989 ; Franks et al, 1992) reporting that hearing testing cannot be done down to 0 dB HL in OSHA MPANLs.

It is important to remember that the back-ground noise levels used in this study closely approximated the ears covered MPANLs al-lowed by ANSI and OSHA for a 500 to 8000 Hz range . If testing is done in an audiometric test room having ambient noise levels lower than the ANSI 1991 or OSHA MPANLs, the chance of ambient noise elevating (masking) a nor-mally hearing listener's threshold will be re-duced. However, Siegenthaler (1981) found that only 18 of the 46 audiometric rooms used for clinical audiology met the ANSI 1977 ears cov-ered and only 5 of the 46 rooms met the ears not covered MPANLs at all octave bands (125 to 8000 Hz). One would suspect that the number of test rooms not meeting the ears covered ANSI 1991 MPANLs would increase since the ANSI 1991 MPANLs for ears covered testing from 125 to 8000 Hz are lower than the 1977 MPANLs (Appendix) . Thus, based on limited data, it would appear that real-world clinical audiology is being conducted in audiometric test rooms having ambient noise levels higher than the MPANLs specified in ANSI S3.1-1991. To our knowledge, there is no published report con-cerning ambient noise levels in test rooms or mobile vans used for industrial audiology.

CONCLUSION

M PANLs allowed in an audiometric test room are needed to ensure that hearing thresholds obtained at reference levels will not be elevated by ambient noise masking. The results of this study indicated that the MPANLs specified in ANSI S3.1-1991 for ears covered testing from 500 to 8000 Hz are appropriate for measuring hearing thresholds down to 0 dB HL using a supra-aural, Audiocup, or insert ear-phone. On the other hand, the results of this study and others (Berger and Killion, 1989; Franks et al, 1992) leave little doubt that the MPANLs specified by OSHA (1983) are too high

for testing down to 0 dB HL using a supra-aural earphone or Audiocup . We are of the opinion that audiometry is audiometry no matter the setting and that audiologists have a profes-sional obligation to ensure that their testing accurately reflects an individual's hearing. We would recommend that the OSHA MPANLs be replaced by the ANSI 1991 MPANLs for ears covered testing over a 500 to 8000 Hz range.

Acknowledgment. Portions of this. paper were pre-sented as a Poster Session by T. Frank, D.L . Williams, and M.K . Schmitt at the American Academy ofAudiology Meeting (April, 1992) in Nashville, Tennessee and as a Technical Paper at the Acoustical Society of America Meeting (May, 1992) in Salt Lake City, Utah .

REFERENCES

American National Standards Institute . (1960) . Criteria for Background Noise in Audiometer Rooms . (ANSI 83.1-1960) . New York : ANSI .

American National Standards Institute. (1977) . Criteria for Permissible Ambient Noise during Audiometric Test-ing. (ANSI S3.1-1977 R 1986). New York : ANSI .

American National Standards Institute. (1984). Method for t1te Measurement of the Real-EarAttenuation of Hear-ing Protectors . (ANSI 512.6-1984) . New York: ANSI .

American National Standards Institute. (1989) . Specifi-cation for Audiometers. (ANSI S3.6-1989) . New York : ANSI.

American National Standards Institute. (1991) . Maxi-mum Permissible Ambient Noise Levels for Audiometric Test Rooms. (ANSI S3.1-1991) . New York : ANSI .

American Standards Association. (1951). Audiometers for General Diagnostic Procedures . (ASA 24.5-1951) . New York : ASA.

Arlinger SD . (1986) . Sound attenuation of TDH-39 ear-phones in a diffuse field of narrow-band noise. JAcoust Soc Am 79:189-191 .

Berger EH, Killion MC . (1989) . Comparison of the noise attenuation of three audiometric earphones with addi-tional data on masking near threshold. JAcoust Soc Am 86:1392-1403 .

Frank T, Richards WD . (1991) . HA coupler output level variability and coupler correction levels for insert ear-phones . Ear Hear 12:221-227 .

Frank T, Vavrek MJ. (1992) . Reference threshold levels for an ER-3A insert earphone . JAmAcadAudiol 3:51-59 .

Frank T, Wright DC . (1990) . Attenuation provided by four different audiometric earphone systems. Ear Hear 11:70-78 .

Franks JR, Engle DP, Themann CL . (1992) . Real ear attenuation at threshold for three audiometric headphone devices: implications for maximum permissible ambient noise level standards. Ear Hear 13:2-10.

International Organization for Standardization. (1981) . Measurement of Sound Attenuation of Hearing Protec-tors--Subjective Method . (ISO 4869-1981) . Geneva : ISO.

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Killion MC . (1984). New insert earphone for audiometry. Hear Instr 35:45-46 .

Lindgren F, Berger EH. (1989) . Influence of insertion depth on model 3A insert earphones on hearing threshold levels and real-ear attenuation values. JAcoust Soc Am 86(Suppl 1):544 .

Minitab. (1989). Minitab Reference Manual Release 7. Minitab Statistical Software, State College PA : Minitab Inc.

Murray NM, Waugh D. (1988). Estimated maximum acceptable background noise levels for audiometric test-ing when using sound-excluding enclosures. Aust JAudiol 10:7-10.

Occupational Safety and Health Administration . (1983 ) . Occupational noise exposure : hearing conservation amendment. Fed Reg 48(46):9738-9785 .

Siegenthaler BM . (1981) . Discussion paper: a survey of hearing test rooms. Ear Hear 2:122-126.

Stark E, Barton T. (1975) . Noise excluding enclosure for audiometry . Audiology 14:232-237 .

Wilber LA, Kruger B, Killion MC . (1988). Reference thresholds for the ER-3A insert earphone . JAcoust Soc Am 83:669-676 .

APPENDIX

Maximum Permissible Ambient Noise Levels Ears Covered One-Third Octave Band Center Frequency (Hz) Standard 125 250 500 800 1000 1600 2000 3150 4000 6300 8000

OSHA 1983 - - 35.0 - 35.0 - 42.0 - 52 .0 - 57 .0 ANSI S3 .1-1977 29 .5 18 .0 16 .5 17 .5 24 .5 24 .0 29 .5 34 .0 37 .0 36 .0 40 .0 ANSI S3.1-1991

125 to 8000 Hz 29 .0 17 .5 14 .5 16 .5 21 .5 21 .5 23 .0 28 .5 29 .5 33 .0 38 .5 250 to 8000 Hz 31 .5 17 .5 14 .5 16 .5 21 .5 21 .5 23 .0 28 .5 29 .5 33.0 38 .5 500 to 8000 Hz 42.5 28 .5 14 .5 16 .5 21 .5 21 .5 23 .0 28 .5 29 .5 33.0 38 .5

Ears Not Covered One-Third Octave Band Center Frequency (Hz) Standard 125 250 500 800 1000 1600 2000 3150 4000 6300 8000

ANSI S3.1-1977 23 .0 13 .5 9 .5 7 .5 9 .0 5 .5 3 .5 3 .5 4 .0 9 .0 15 .5 ANSI S3 .1-1991

125 to 8000 Hz 23 .0 13 .5 9 .5 7 .5 9 .0 5 .5 3 .5 3 .5 4 .0 9 .0 15 .5 250 to 8000 Hz 27 .5 13 .5 9 .5 7 .3 9 .0 5.5 3 .5 3 .5 4.0 9 .0 15 .5 500 to 8000 Hz 37.5 23 .5 9 .5 7 .5 9 .0 5 .5 3 .5 3 .5 4 .0 9 .0 15 .5

Maximum permissible ambient noise levels (MPANLs) specified by OSHA (1983), ANSI (S3 .1-1977), and ANSI (S3 .1-1991) . Tabled values are 1/3 octave band interval sound pressure levels in dB re : 20 wPa . The OSHA MPANLs were converted from octave to '/3 octave bands .

Documents

Effects of Background Noise on Earphone Thresholds€¦ · Effects of Background Noise on Earphone Thresholds Tom Frank* Dennis L. Williams* Abstract Real-ear attenuation at threshold