reference thresholds for the er-3a insert earphone

Reference thresholds for the ER-3A insert earphone LauraAnn Wilber

Program in.4udiology, Northwestern University, Evanston, Illinois 60208

Barbara Kruger Kruger.4ssociates, 37Somerset Drive, Cornmack, New York 11725

Mead C. Killion

Etymotic Research, 61 Martin Lane, Elk Grove Village, Illinois 60007

(Received 16 July 1987; accepted for publication 7 October 1987)

Several recent studies have demonstrated that the ER-3A insert earphone may sometimes be directly substituted, without recalibrating, for a TDH-39/MX-41AR earphone. However, most available data have not been reduced to a form suitable for establishing a revised estimate of the reference threshold levels. This article reports such a data analysis performed on the results of five recent studies. The mean data from the five studies are typically within 1 dB of the provisional reference threshold SPLs given by the ER-3A manufacturer for calibration in a (HA-1 ) 2-cc coupler. After converting the mean data to equivalent Zwislocki-coupler-type ear simulator SPLs at each of the reported audiometric frequencies ( 125, 250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz), agreement within 1.5 dB was seen with the revised estimate of minimum audible pressures given by Killion [J. Acoust. Soc. Am. 63, 1501-1508 (1978) ]. Either the manufacturer's provisional SPLs or the average results from this study may be used with little noticeable difference for most purposes.

PACS numbers: 43.66.Cb, 43.66.Sr, 43.85.Vb, 43.88.Si [NFV]

INTRODUCTION

For some time it has been evident that an insert ear-

phone would help alleviate some common clinical problems, e.g., occlusion affect, interaural attenuation (which can re- suit in a masking dilemma), and problems encountered with a collapsing ear canal. In 1953, Zwislocki described an insert earphone that could be used to minimize these problems (especially the problem ofinteraural attenuation). Unfortu- nately, the earphone that he described was never manufac- tured commercially. Recently, Etymotic Research has developed an insert earphone that is commercially available. An initial study by Killion et al. (1985) indicated that this "ER-3A" earphone would be a useful addition for the clini- cian since it maximized the interaural attenuation. The man-

ufacturer's "provisional" calibration data were based on a prior estimate of eardrum sound pressure level at threshold (Killion, 1978) derived from estimated supra-aural and free-field threshold data. Since these provisional calibration data were not based on direct observation, it was unclear how thresholds obtained with these earphones might relate t ø thresholds obtained with supra-aural earphones. It was also unclear whether a direct study would' yield the same numbers as those derived indirectly. Our primary concern, thus, was to determine directly an estimate for provisional reference equivalent thresholds. Our secondary concern was whether direct and indirect measurements would yield the same provisional reference equivalent threshold data. As a first step to providing these data, we contacted the authors of five studies that had investigated the insert receivers for various purposes (O'Connor and Wilber, 1985; Wilber, 1986; Clemis et al., 1986; Clark and Roeser, 1986; and Larson et

al., 1986). Figure 1 shows the insert receiver used in this study along with a typical supra-aural earphone.

I. METHOD

These five studies had been conducted in various labora-

tories, among other reasons, to determine the thresholds that would be obtained for normal listeners using the ER-3A insert earphones. In each case, thresholds were also obtained using a supra-aural earphone. The averaged data for the insert earphone and the supra-aural earphone as well as the

FIG. 1. The TDH-39 supra-aural earphone and ER-3A insert earphone shown with eartips.

669 J. Acoust. Soc. Am. 83 (2), February 1988 0001-4966/88/020669-08500.80 ¸ 1988 Acoustical Society of America 669

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TABLE I. Reported threshold SPL (referenced to coupler).

Authors N Coupler Insertion

O'Connor and Wilber (17) HA- 1 full Wilber (28) HA- 1 full Clemis et al. ( 8 ) DB-0138 full Clark and Roeset (12) HA-1 shallow Larson et al. (60) HA- 1 shallow

125

31.1

25.0

44.0

Frequency 250 500 1000 2000 3000 4000 6000 8000

21.8 16.3 12.5 11.9 6.4 -- 7.7 17.0 8.2 1.9 3.2 6.4 -- 0.3 -- 5.1 19.5 11.7 6.9 11.7 13.0 5.0

19.8 14.5 7.3 6.5 6.4 5.0 3.5 4.3

25.2 13.1 5.2 7.5 5.0 2.5 -- 0.5 -- 5.3

calibration data were collected from each of the authors. In

four of these studies, the authors had measured the output of the ER-3A using the HA-1 version (used to measure "in- the-ear" hearing aids) of the 2-cc coupler, usually with the ER-3-14 foam eartip, (supplied with the earphones) sealed directly to the top surface of the coupler. In the fifth study, the author had used the B&K DB-0138 version of the HA-2

2-cc coupler. These results are shown in Table I along with the number of subjects used in each study. In order to compare the data, we corrected the latter using the differences between the two couplers as reported by the manufacturer of the ER-3A insert earphone and reproduced here at the bot- tom of Table II.

Additional corrections were required for the depth of eartip insertion used by two authors. When the ER-3-14 foam eartip is inserted 2-3 mm past the canal entrance in order to obtain maximum interaural attenuation as recom-

mended by Killion et al. ( 1985 ), the result is a total insertion depth of approximately 16 mm measured from the plane of the coneha floor. Larson et al. (1986) reported a measured mean insertion depth of 10.1 mm (average of six males and five females) for the impedance-probe eartips they used. Clark and Roeser (1986) reported a shallow insertion of the standard ER-3-14 foam eartips, which we estimated also produced an average of 10-mm insertion depth using their measurements. Computer simulations and direct measurements in a Zwislocki-type occluded-ear simulator (Indus- trial Research Products, Inc. DB-100) of the difference between 10- and 16-mm insertion depth produced the results shown in Table II. The average was used to correct the data

of the above two authors. As seen in Table II, the corrections ranged from -- 1 to --2.2 dB except at 8 kHz where the estimated correction was negligible (0.1 dB).

In addition to the above correction, we also took into account the average hearing level for each group as measured using a TDH-xx supra-aural earphone. [ Not all authors used the same model of Telephonics Dynamic Head- phone, but there are tentative reference- equivalent-threshold data that allowed us to relate these earphones ( Wilber, 1985; Cox, 1986). ] Since we did not control the subject selection, we decided this was the fairest way to equate the studies. This also allowed us to compare the ER- 3A data to values using the standard supra-aural earphone (ANSI S3.6, 1969).

A final question involves how data from several inde- pendent studies, each involving a different number of subjects, are to be combined. In this year of the Constitution's 200th anniversary, one may use a House of Representatives ( Madisonian ) or Senate (Patterson) approach. We chose to use a Wm. Patterson approach in comparing the data--and thus each laboratory was treated equally rather than equat- ing for the difference in subject sample size. However, since it is the bicentennial year of the Constitution, we have also presented the Madison approach (weighted data averages by subject samples) in the Appendix tables. More practical- ly, in the face of unknown experimental bias across laboratories, treating each laboratory as an equally probable and in- dependent estimator of the true average is obtained by using groups of data (unweighted for sample size) rather than grouped data (weighted for sample size). This approach

TABLE II. Corrections for eartip insertion depth and coupler type.

Frequency 125 250 500 1000 2000 3000 4000 6000 8000

Insertion depth: shallow ( 10 ram) to full ( 16 ram) Calculated

Measured -- 1.0 -- 1.5 -- 1.6 -- 1.6 -- 2.1 -- 2.4 -- 2.5 -- 2.3 0.3 --0.9 -- 1.5 - 1.5 -- 1.4 -- 1.8 -- 1.9 -- 1.9 -- 1.2 --0.1

Average - 1.0 - 1.5 - 1.6 - 1.5 - 1.9 -2.2 -2.2 - 1.8 0.1

Coupler correction: DB-0138 to HA-1 Provisional reference equivalent threshold HA-1 Provisional reference equivalent threshold DB-0138

27.5 15.5 8.5 3.5 6.5 5.5 1.5 -- 1.5 -- 4.0 27.0 15.0 8.0 3.5 6.5 6.0 7.0 3.0 0.0

Difference 0.5 0.5 0.5 0.0 0.0 -- 0.5 -- 5.5 -- 4.5 -- 4.0

670 J. Acoust. Soc. Am., Vol. 83, No. 2, February 1988 Wilber otaL' Insert earphone thresholds 670


5O

4O

• 20

-lO i I I I i I

125 250 500 1000

FREQUENCY

O'CONNOR & WILBER

WILBER

CLEMIS ET AL.

CLARK & ROESER

X LARSON E'F AL.

I I I

2000 3000 4000

I

6000 80O0

FIG. 2. The ER-3A threshold SPL data as

reported from five studies, corrected for coupler type and eartip insertion depth.

seemed particularly called for in the present case because of reported differences in eartip placement in the ear canal. Although we felt our correction for the shallower receiver placement was a reasonable one, to avoid weighting onepro- cedure more (or less) than another, we chose to weight each procedure equally and thus the studies were averaged.

II. RESULTS

As mentioned above, Table I showed the uncorrected ER-3A threshold SPL data reported by each author, and Table II shows the corrections we used to refer all data to the

HA-1 2-cc coupler and fully inserted eartip. Figure 2 shows the threshold data in SPL from each of the five studies after

the above corrections were made. Note that there is general agreement among studies, with the greatest range of differences being at 125 Hz ( 19.0 dB).

Table III shows the average hearing level for each subject group based on the particular supra-aural earphone used. These data were either reported directly in HL by the authors, or derived from the earphone SPL data that they presented to us. Table IV shows the resulting ER-3A threshold SPLs referenced to the HA-1 two-cc coupler after cor- recting for each subject group's mean heating level (Table III). Figure 3 shows these results in graphic form. Again, you will note that the data are somewhat disparate by frequency, but they are within 5 dB at the center frequencies of 500-2000 Hz. In fact, these data do not appear to be as vari- able as those reported by Weisler (1968) when she compared the results of the several studies from various laboratories that had been used to construct'the ISO "0" heating levels for supra-aural earphones.

In order to compare the ER-3A data with provisional

TABLE III. Reported thresholds for supra-aural earphones.

Authors Earphone Frequency 125 250 500 1000 2000 3000 4000 6000 8000

O'Connor

and Wilber TDH-39 SPL (reported) 52.5 34.6 21.1 Ref. Eq. Thr. 45.0 25.5 11.5 HL (derived) 7.5 9.1 9.6

15.5 16.4 20.1 16.0

7.0 9.0 9.5 13.0

8.5 7.4 10.6 3.0

Wilber TDH-39 SPL (reported) Ref. Eq. Thr. HL (derived)

26.3 10.7

25.5 11.5

0.8 --0.8

2.4 6.4 8.7 10.2 14.2

7.0 9.0 10.0 9.5 13.0

-- 4.7 -- 2.6 -- 1.3 0.7 1.2

Clemis et al. TDH-49

Clark and Roeser TDH-50

Larson et al. TDH-50

HL (reported) HL (reported) SPL (reported) Ref. Eq. Thr. HL (derived)

5.9 5.3 5.3 3.4 4.7 4.1 7.5

0.8 2.2 2.9 -- 1.9 1.3 0.1 -- 4.7 -- 2.4

50.4 28.8 14.2 6.4 8.9 9.2 11.3 19.0 18.0

47.5 26.5 13.5 7.5 11.0 9.5 10.5 13.5 13.0

2.9 2.3 0.7 -- 1.1 -- 2.1 -- 0.3 0.8 5.5 5.0

671 J. Acoust. Soc. Am., Vol. 83, No. 2, February 1988 Wilber eta/.' Insert earphone thresholds 671


TABLE IV. Reference equivalent threshold SPLs for ER-3A measured in 2-cc (HA-1) coupler corrected for coupler type, eartip insertion, and subject's hearing level.

, ,

Frequency 125 250 500 1000 2000 3000 4000 6000 8000

O'Connor and Wilber 23.6 12.7 6.7 4.0 4.5 - 4.2 - 10.6

Wilber 16.2 9.0 6.6 5.8 7.7 - 1.0 - 6.3

Clemis et al. 19.6 14.7 6.9 3.5 7.0 3.3 - 6.5

Clark and Roeser 17.5 !0.7 2.9 6.5 2.9 2.7 6.4 6.8

Larson et al. 40.2 21.4 10.9 4.8 7.7 3.1 -- 0.5 -- 7.8 -- 10.2

Mean 27.8 16.5 8.8 4.4 6.3 4.6 0.1 - 0.7 - 5.4

Standard deviation 8.9 2.9 1.8 1.3 1.1 2.2 2.7 7.1 6.3

5O

4-0

30

20-

10-

-lO

125

O'CONNER & WILBER ( N= 17 )

WILBER (N=28)

CLEMIS El' AL. ( N=8 )

CLARK & ROESER ( N: 12 )

LARSON El' AL. ( N:60 )

I I i I I I I

250 500 1000 2000

FREQUENCY

I I I

3000 4000 6000 8000

FIG. 3. The ER-3A threshold SPL data

from five studies corrected for subject group hearing levels as measured with supra-aural earphones.

3o

20-

10

I

o

.3 o ,

125

• D MEA• OF 5 STUDIES

1.0 0.3 0.9 0.2 0.8 ,.• DIFFERENCE -

I I I I I '1 I I I

250 500 1000 2000 3000 4.000 6000 8000

FREQUENCY

FIG. 4. Mean œR-3A threshold SPL values

compared to manufacturer's provisional SPL data.



•o

2s-

26-

24.-

22-

20-

19,-

16-

14.-

12-

10-

i-'] ER--3A

ß -!- TDH--XX

6

Cl I I i I I I i i i I

125 250 500 1000 2000 5000 4•)00 6000 8000

FREQUENCY

FIG. 5. Standard deviations across studies

for ER-3A and supra-aural earphones.

numbers that had been previously reported by the manufacturer of that earphone (see Appendix Table AI), we averaged the mean data from the five present studies, as shown in Table IV. The comparison of provisional and mean data is shown in Fig. 4. Figure 4 indicates that the difference between the two does not exceed 1.5 dB at any of the frequencies tested.

We were also concerned about across-subject variability in measured thresholds using the insert earphone compared to the standard supra-aural earphone. Four of the studies (O'Connor and Wilber, 1985; Wilber, 1986; Climis et al., 1986; and Larson et al., 1986) had provided us with standard deviations as well as mean data, so we were able to compute the average across-subject standard deviation for the entire subject population for these four studies. These data are shown in Fig. 5 and Table V, and they demonstrate that there is no substantial difference between the standard

deviations obtained with the insert and supra-aural ear-

phones. Note that the data at 6000 Hz is shown in Table V but is not shown in Fig. 5, since only one laboratory reported variability at that frequency.

III. DISCUSSION

The provisional numbers require further comment. One of the authors has previously estimated the minimum audible pressure (MAP) referred to the eardrum based on then- existing supra-aural earphone and free-field threshold data (Killion, 1978). Those MAP values provided a provisional set of reference equivalent SPLs for insert earphones calibrated in a Zwislocki-type occluded ear simulator (ANSI S3.25, 1979). The only assumption required was that the SPL measured in a Zwislocki coupler was representative of the average SPL measured in real ears. Several studies support this assumption (Sachs and Burkhard, 1972; Kruger et al., 1981; Killion et al., 1987).

TABLE V. Standard deviations across studies.

Frequency 125 250 500 1000 2000 3000 4000 6000 8000

O'Connor and Wilber ER-3A STD 5.3 4.0 5.1 4.9 5.0 5.8 7.0

Wilber ER-3A STD 6.3 5.7 5.5 6.6 6.3 5.6 9.0

Clemis ER-3A STD 4.9 4.4 5.8 4.4 7.7 8.9 5.1

Larson ER-3A STD 4.7 5.8 5.0 5.8 6.2 4.9 4.9 7.9 7.3

Average standard deviation of sample for ER-3A 4.8 5.6 5.2 5.5 6.2 4.9 5.5 7.9 7.6

O'Connor and Wilber TDH-39 STD 8.1 6.9 5.3 4.8 6.8 7.2 7.4

Wilber TDH-39 STD 7.7 5.0 5.1 6.1 5.9 5.4 9.9

Clemis TDH-39 STD 5.2 5.0 6.5 4.0 6.7 9.0 8.0

Larson TDH-39 STD 6.3 6.1 5.8 5.4 6.3 6.4 5.8 1.5 8.3

Average standard deviation of sample for TDH-xx 6.6 6.6 5.6 5.2 6.4 6.3 6.2 1.5 8.6



4O

3O .,4

20-

10-

0 •

r'l MEAN OF 5 STUDIES

(TRANSFORMED TO ZWISLOCKI COUPLER SPL )

• + ESTIMATED MAP

1.2 0.8 0.2 • 0:4 0+8 0.0

-"'" ' ' -1.5 -1.5

-10 i • i , i , , , , , •

125 250 500 1000 2000 3000 4000 6000 8000

FREQUENCY

FIG. 6. Mean ER-3A threshold SPL in

Zwislock. i-type ear simulator compared to manufacturer's provisional SPL data.

In order to obtain provisional reference threshold SPLs for 2-cc coupler calibration, one of us (Killion) had previously measured the difference between the Zwislocki coupler and the 2-cc-coupler SPLs developed by an ER-3A insert earphone, using carefully calibrated, microphone-vol- ume-corrected 2-cc and Zwislocki couplers (see Appendix Table AII). Those differences agreed within 1 dB of those previously reported by Sachs and Burkhard(1972) for high- acoustic-impedance sources such as heating aid receivers. Using these same corrections factors, rounded to the nearest 0.5 dB, the manufacturer of the ER-3A earphone provides provisional equivalent reference threshold SPLs for both Zwislocki-coupler, and 2-cc-coupler calibrations. These are reproduced in Appendix Table AI.

Using the correction factors in Appendix Table AII, the present 2-cc mean data have been transformed to equivalent Zwislocki coupler data. The transformed data are given in Table VI and shown in Fig. 6 along with the revised eardrum-pressure estimate of MAP reported by Killion (1978). The lower curve in this figure again shows the difference between the two, a difference that does not exceed 1.5 dB at any frequency, and the average of which is less than 0.2 dB.

The excellent agreement between the mean data and the provisional recommendation at 125 Hz was a bit surprising. A large variability in TDH-39 data is commonplace due to cushion-leak variations and physiological noise under the ear cushion. The "fight" number for minimum audible pressure at the eardrum at 125 Hz appears close to the 30-dB estimate of Killion (1978). Berger (1981), for example, summarizes 11 studies whose mean binaural MAF was 27.9

dB, corresponding to a monaural MAP of 30.9 dB if a 3-dB binaural-to-monaural correction factor is used. [As ob- served 60 years ago by Sivian and White (1928), the value for MAF and MAP should be nearly identical at low frequencies where the eardrum and the sound-field pressures are nearly identical. ]

The mean threshold SPL of the three present studies reporting data at 125 Hz was 27.8 dB SPL in the 2-cc coupler, which corresponds to 30.3 dB in the Zwislocki coupler (and thus 30.3 dB estimated at the eardrum). We believe the close agreement seen here with previous estimates may be another example of Lagrange's observation (Killion, 1978); "...due to God's infinite kindness, the errors all canceled!"

In particular, Larson et al. reported a 60-ear average of

TABLE VI. Reference equivalent threshold SPLs for ER-3A referenced to Zwislocki coupler corrected for coupler type, eartip insertion, and subject's hearing level.

Frequency 125 250 500 1000 2000 3000 4000 6000 8000

O'Connor and Wilber 26.0 16.4 10.3 9.4 13.2 7.3 7.2 Wilber 19.9 12.6 12.0 14.3 17.7 10.5 11.6 Clemis et al. 22.0 18.4 10.5 8.9 15.7 14.8 11.3 Clark and Roeser 21.2 14.4 8.3 15.2 13.0 14.1 21.0 24.6 Larson et al. 42.6 25.1 14.5 10.2 16.4 13.1 10.9 6.7 7.6

Mean 30.2 20.2 12.4 9.8 15.0 14.6 11.5 13.9 12.5 Standard deviation 8.9 2.9 1.8 1.3 1.1 2.2 2.7 7.1 6.3



40.2 dB SPL in a 2-cc coupler at 125 Hz, more than 12 dB above the provisional recommended value, in an experiment whose large sample size gives an estimated standard error for the mean of only 1 dB. The explanation in this case appears to be related to their use of impedance-probe eartips rather than the ER-3-14 foam eartips, with the resulting relatively shallow insertion. They reported that the measured levels of physiological noise in the ear canal on two subjects was sufficient to indicate they were obtaining masked threshold at 125 Hz. This is consistent with the data of Berger and Keri- jan (1983), who measured sufficient physiological noise in the 125-Hz octave band with shallow earplug insertion to cause elevated thresholds due to masking. With deeply inserted eartips, no masking occurred. We have no explanation for the discrepancies in the other two studies at 125 Hz.

IV. RECOMMENDATIONS

Thus, from the mean results and the variability across subjects by earphone, it seems clear that the user of these

ß - I

ER3-A insert earphones can use either the provisional data (seen in Appendix Table AI) or the data generated by this study to relate insert earphone hearing thresholds (Tables IV and VI) to supra-aural earphone hearing thresholds. At the present time, however, we recommend use of the provisional reference equivalent threshold SPL data, since the differences between direct and indirect estimates are small.

These data also allow easier comparison among laboratories and clinics, since data can now be reported in equivalent hearing level as well as in SPL.

We recommend the use of deeply inserted foam eartips for critical testing (when a difference of 2 dB will be impor- tant and/or accurate thresholds at 125 Hz are required). Additional advantages of deep insertion have been reported for reduction in ambient noise (Clemis et al., 1986), minimum occlusion effect for bone conduction testing (Berger and Kerijan, i983), and maximum interaural attenuation. In the latter application, an interaural attenuation of 100 dB at 500 Hz with deeply inserted eartips can drop to 75 dB or less with shallow insertion (Killion et al., 1985).

APPENDIX

TABLE AI. Provisional reference numbers for ER-3A.

Frequency 125 250 500 1000 2000 3000 4000 6000 8000

Prov. Ref. Eq. Th. HA-1 27.5 15.5 8.5 3.5 6.5 5.5 1.5 -- 1.5 -- 4.0 Prov. Ref. Eq. Th. DB-0138 27.0 15.0 8.0 3.5 6.5 6.0 7.0 3.0 0.0 Prov. Ref. Eq. Th. Zwislocki (MAP) Killion, 1978 30.0 19.0 12.0 9.0 15.0 15.5 13.0 13.0 14.0

TABLE AII. Conversion from 2-cc (HA-1) to Zwislocki coupler SPL.

Frequency 125 250 500 1000 2000 3000 4000 6000 8000

Increase in SPL in Zwislocki (IRPI DB-1 ) compared to 2-cc (HA-1 ) coupler 2.4 3.7 3.6 5.4 8.7 10.0 11.4 14.5 17.8

Difference rounded to nearest 0.5 dB 2.5 3.5 3.5 5.5 8.5 10.0 11.5 14.5 18.0

TABLE AIII. Weigl•ted reference equivalent 2-cc coupler (HA-1) SPL for ER-3A from five studies. Comparison of data weighted for sample size difference versus average of groups.

Frequency 125 250 500 1000 2000 3000 4000 6000 8000

Mean of five groups (weighted for sample size N -- 125) 23.7 18.2 9.6 4.8 6.6 3.5 -- 0.5 2.4 Average of five groups (not weighted for sample size) 27.8 16.5 8.8 4.4 6.3 4.6 0.1 5.1

--2.8

--3.4

J. Acoust. Soc. Am., Vol. 83, No. 2, February 1988 675 Wilber et aL: Insert earphone thresholds 675


ANSI (1973). ANSI S3.7-1973, "Method for coupler calibration of earphones" (American National Standards Institute, New York).

ANSI (1979). ANSI S3.25-1979, "For an occluded ear simulator" (Ameri- can National Standards Institute, New York).

ANSI (1969). ANSI S3.6-1969, "Specifications for audiometers" (Ameri- can National Standards Institute, New York).

Berger, E. H. (1981). "Re-examination of the low-frequency (50-1000 Hz) normal threshold of hearing free and diffuse sound fields," J. Acoust. Soc. Am. 70, 1635-1645.

Berger, E. H., and Kerijan, J. E. (1983). "Influence of physiological noise and the occlusion effect on the measurement of real-ear attenuation and

threshold," J. Acoust. Soc. Am. 74, 81-94. Clark, J., and Roeser, R. (1986). Personal communication. Clemis, J. D., Ballad, W. J., and Killion, M. C. (1986). "Clinical use of an

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phones for more interaural attenuation," Hear. Instrum. 36, 34-36. Kruger, B., Solomon, B., Cohen, R., Newton, L., and Kaplan, S. (1981).

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Soc. Am. 25, 752-759.

676 J. Acoust. Soc. Am., Vol. 83, No. 2, February 1988 Wilber ota/.: Insert earphone thresholds 676


reference thresholds for the er-3a insert earphone

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