SPURE-TONE THRESHOLD WITH, 'TDH'49 EARPHONES

HARRELL C. SUTHEkLAND, JR,.,, M. Ed.DONALD C. GASAWAY, MAJOR, USAF, BSC

',USAF School of At er c MedicineAero~pace Medical Diviim'ot (AFSC)

Brooks Air Force Baae, Texas

June 1972

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Brooks Air Force Base, Texab 782353. REPORT TITLE-

"PURE-TONE ThT.ESHOLD WITH TDH-49 EARPHONES

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Progress Report, June 71 throughMay 72-5. AU THOR(S) (First name, middle Initial, lasi name)

Harrell C. Sutherland, Jr., M.Ed.Donald C. Gasaway, Major, USAF, BSC

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d.Work Unit No. 002I0: OIssrII3UTION STATEMCNT

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USAF School of Aerospace Medicine* Aerospace Medical Division (AFSC)

_ _ __ Brooks Air Force Base, Texas 78235I J AR I RA, I

Pure-Lone thresholds of 24 normal-hearing listeners were determined at 11atudiomctric frequencies on two TDII-49 and two TDII-39 earphones. Thresholds were

Srecorded in decibels of sound pressure level, as measured with a National Bureau of"1'.dardls type 9-A earphone coupler. Threshold data were used to compute appro-

S,,te tcLi-ration values for audiometers with TDH-49 earphones, reference Appendix Fof ANWI S3.o-1969.

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UNCI•ASSIFIED-cSecurity C elogsfcatiofl...

I". KEY WORDS LINK A LINK 9 LINK C

Audiology j ROLE WT ROLE wY". ROLE WT

Audtiolgy

AudiometerCalibrationEarphonis '

TDH-49

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UNCLASSIFIED

PURE-TONE THRESHOLD WITH TDH-49 EARPHONES

HARRELL C. SUTHERLAND, JR., M. Ed.DONALD C. GASAWAY, MAJOR, USAF, BSC

Approved for public release; disir A .4,r, dirnited.

FOREWORD

This ,study was conducted in the Audiology and Hearing ConservationFunction, Otolaryngology Branch, Clinical Sciences Division, Under taskNo. 775508, in 1967. The, work was done in response tb information thatthe TDH-49 earphone was expected to supersede the popular TDH-39 type.After all data were collected and anafyzed, word was received that theTDH-49 would no longer bemanufactured and informa'tion about it would beof no value to the Air Force.

-It has now been learned that TDH-49 earphones are bleing. usedwithsome commercial audiometers by ,the Air Force. This report is being:published to provide the information necessary to properly calibrateaudiometers with TDH-49 earphones. The manuscript was submitted forpublicatiohn on 27 April 1972.

Richard C. McNee -of the Biometrics Division performed the dataanalysis.

This report has been reviewed and is approved.

EVAN R. GOL , Colonel, USAF, MCCommander

ii

ABSTRACT

Pure-tone thresholds of 24 norma_-hearing listeners were .determined,at 11 audiometric frequencies on tvo TDH-49 and two TDH-39-earphones.Thresholds were recorded in decib&ls of sound pressure 1evel, as measured'with a National Bureau-of -Standards type 9-A earphone coupler. Threshold.data were used to compute appropriate calibration values Ifor audiometerswith TDH-49 earphones, reference Appendix F of ANSI S3.6-1969.

iii

PURE-TONE THRESHOLDWITHý TDHr-49- EARPHONES

I. 'INTRODUCTION

, Another earphone type haf become avilable for use with-audiometers,the Telephonics model' TDH-49.- Because it appears to be gaining accept-ance and'is now being ibstalled on some'Air Force-owned audiometers,some appropriate intensity calibration values are needed.

PtIre-tone intensity from' the earphone of an audiometer is custom-arily measured by use of an NBS (National Bureau of Standards) type 9-Acoupler (1). The measurement is in decibels (dB) of sound pressure level(SPL). Both the American National Standards Institute (ANSI) and theInternational Organization for Standards (ISO) have recommended a set ofvalues for audiometer calibration (1, 2). These, however, are based onthe Western Electric 705-A-earphone which has not been manufactured fora number .of years. Although the 9-A coupler was intended to simulatethe acoustic properties of the human external ;.ar structure, it does notrelate subjective human ear thresboads to electroncoustically measuredSPL in the same manner for all •earp•t,. -cypes. For this reason, separatevalues must be provided: 'for euch ra1.' of earphone that is calibrated.Both the ANSI and the ISO recommend techniques for determining' appro-priate values for earphones other than the WE 705-A. These techniquesinvolve comparing the test earphone with a WE 705-A, with human listeners.

Appendix F of the ANSI S3.6-1969 recommendation suggests values fora variety of earphone types, including the popular TDH-39 earphone, butnot for the newer TDH-49. This study determines appropriate calibrat.ionvalues for the TDH-49 earphone.

II. PROCEDURE

Our apparatus is diagrammed in figure 1. The signal from a pure-toneoscillator is pulsed with an electronic switch and routed through an appro-priane amplifier and transformer, through a recording attenuator andinto an anechoic chamber, then through a l-dB step attenuator and into which-ever earphone is selected. TIwo TDII-49 and two TDII-39 earphones were used.Frequency responsn characteristics of the four earphones are displayed infigure 2. A handswitch which controlled the direction of attenuationchange was provided so that Bekesy audiometry could be performed.

Subjects were 24 young Air Force basic trainees with hearing levelsno poorer than 15 dB re ASA-1951 (3), at pure-tone frequencies 500 through6000 1lz.

1All refercuces in this paper to an earphone imply the earphone plua

an MX-41/AR car cushion unlless otherwise specified.

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enwI Ii TDH-49 TDH-39ElectronicSwitch A B A B

f Amplifier• ,r ,] 14B Stepi! .. .. Attenuator

iTransformer--Handswitch

Recording --Attenuator

FIGURE I

Block diagram of test apparatus.,

Pure-tone threshold, in decibels SPL, was determined for one ear ofeach subject at 125, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 6000,and 8000 Hz with each of the four earphones, Use of right or left earwas alternated. Earphone test order was arranged in a Latin square de-sign (six squares). Threshold was determined with Bekesy-type audiometry,with o m'inimum of 30 seconds tracing time at each frequency. The orderof frequency prvsentation was always 1000, 1500, 2000, 3000, 4000, 6000,8000, 750, 500, 250, and 125 liz.

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Earphone put'o-tone frequency response.

Each threshold was the measured sound pressure level with no atten-'dation in the system, less the attenuation the subject used to reach"threshold. Threshold was based on the midpoint of the Bekesy tracings.Sound pressure levl at each frequency with no attenuation was measuredthree times for eaca of the four earphones. The averages of the threemeasurc-ents, SPL in a 9-A coupler, are shown in table I. These valueswere used in computing subjective threshold SPL.

III. RESULTS

Table II shows the computation of recommended sound pressure levelat zero hearing level for TDH-49 earphones as measured with a 9-A coupler.Line I is the lU3t of sound pressure levels for zero hearing level rec-ommended in Appendix F of ANSI S3.6-1969 for the TDH-39 earphone. Line 2is the difference between TDM-39 and TDH-49 earphones as determined inthis study and as rounded to the nearest 0.5 dB. Line 3 is the recom-mended SPL for zero hearing level for TDII-49 earphones. The recommendedSPL for 750 Hz was obtained by interpolation from a graphic display ofSPLs at other pure-tone frequencies.

Average threshold SPLs for the 24 subjects at each pure-tone fre-quency with each of the four earphones are shown in table III. Differ-ences between the TDU-59 and the TDII-49 earphones were significant at1500 Hz (P < .05), 2000 Hz (P < .001), and 6000 Hz (P < .001). The orderof testing was significatit at the .05 level at 250, 500, 750, and 1500 Hz.The average threshold for the first test was the highozt for all four ofthese frequencies. The largest difference between average threshold onthe first test order and average threshold for any other test order forthese frequencies was 2.46 dlB. This occurred at 1500 l1z. The samepattern aý-a present to an insignificant degree at all other frequenciesexcept 6000 Hz and 8000 lHz. These average differences are small in thepractical sense, but still indicate a tendency for threshcld to improvewith practice. They emphasize the need to recognize order as a factorin studies involving pure-tone thresholds.

Stewart and Burgi (4), through a loudness-balance procedure, deter-nri5ed NIBS-9-A coupler pressures in decibels of SPL (corresponding tonormal threshold reference, ASA Z24.5-1951, ref. 3) for three TDII-39earphones and one TDII-49 earphone. The TDHI-39 earphone with an MX-41/ARear cushion that Stewart and Burgi used appeared considerably differentfrom (1) their standard 14 705-A earphone, (2) the experimental WE 705-Aearphone, (3) two TDII-39 earphones with cushions that were not MX-4l/ARs,arid (4) the TDII-49 earphones with MX-41/AR cushions.

SThe differences in values thcy found between the TDII-49 and the

TDII-39 earphone, are listed in table IV, lines 1, 2, and 3. The differ-ences we found K tween two TDII-39 and two TDII-49 earphones are given online 4 of tOe s, e table. Exccllent agreemvnt is -een bttween lints Iand 4, with the gre test discr•pancy, 1.5 dB, at 8000 I:. Reasonably goodagreement is present between lines 2 and 4, but linrs 3 and 4 differ sub-stantially.

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It is recommended that audiometers assembled with the TDH-49 ear-phones be calibrated to correspond to the SPL given- in line 3 of table II.It is important to note- that this recommendation is based on tests inwhich only two TDH-49 earphones were compared with only two TDH-39 ear-phones. The two TDI17-39 earphones chosen for this study performed verymuch alike, with average threshold differences not exceeding 1.54 dB (at8000 Hiz, table III). 'he two TDH-49 earphones also showed their greatestdifference, 2.1 dB, at 8000 Hz. However, the two TDIH-39 earphones withother than MX-4l/AR ear cushions, as -reported by Stewart and Burgi (4),were different by 2.5 dB at 250 Hz, by 2.8 dB at 2000 Hiz, by 3.6 dB at3000 Hz, and by 6.0 dB at 8000 lIz. Their TDII-39 earphone with an MX-41/ARcushion was even more different (table IV). Various studies of this typewill pos.ýibly yield somewhat differing answers, but the need exists,nevertheless, fo.- a specific standard for Air Force use as suggested inthis paper.

REFERENCES

1. American Nat 4 onal Standard Specification for Audiometers. S3.6-1969.American Nhtional Standards Institute, Inc., New York, 1969.

2. IDavis, H., and F. Kranz. International audiometric zero. J. Acoust.Soc. Am. 36:1450 (1964).

3. Audiometers for general diagnostic purposes. Z24.5-1951. AmericanStandards Association, New York, 1951.

4. Stewart, K. C., and E. J. Burgi. Loudness balance study of selectedaudiometer earphones. Vital and Health Statistics. PIS Publ. No.1000, Serics 2, No. 40, Washington: U.S. Governmunt" PrintingOffice, Dec. 1970.

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