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Hearing aids
Korean Journal of Audiology 2000;4(1):21-26.
Study of Hearing Aid Application with Hearing Aid Assess Meter
Sang Heun Lee1, Kikuo Aoki2, Tae Hwan Cho1, Ki Ho Lee3
1Department of Otolaryngology, School of Medicine, Kyungpook National University, Taegu, Korea
2Human Dynamics Lab., Graduate School of Decision Science Technology, Tokyo Institute of Technology, Japan
3Department of Rehabilitation Engineering, T
새로운 보청기 평가기를 이용한 보청기 착용평가에 관한 연구
이상흔1, Kikuo Aoki2, 조태환1, 이기호3
1경북대학교 의과대학 이비인후과학교실
2일본 동경공업대학 대학원 인간 행동시스템전공
3대구 미래대학 재활공학과학교실
교신저자:이상흔, 700-721 대구광역시 중구 삼덕동 2가 50 전화) (053) 420-5784, 전송) (053) 423-4524, E-mail) leeshu@knu.ac.kr Introduction As the elderly population's proportion becomes large due to the modern advance in medical science, you can see presbycusis by aging also rising gradually. In developed countries, nearly 5% of the entire population is reportedly hard of hearing. Most hearing impaired persons try to make up for their handicap through the use of the hearing aids. Hearing aids should be prescribed in various aspects based on the specific cause of hearing loss per individual, i.e., main features of a hearing aid such as how it is worn, amplification, conduction. Most of hearing aids go through a series of processes that cover an adjustment of frequency response to their present hearing level.1) In other words, a proper hearing aid fitting is a crucial point when maximizing use of residual hearing and can be a touchstone when verifying usefulness of a hearing aid. Every judgment on the effectiveness of a hearing aid is also valuable, and the ultimate issue on the optimum application of a hearing aid which should be faced is the problem of whether a given hearing aid fitting is made properly.2)3) There are many clinical ways to verify the effectiveness of hearing aids and one of the best ways is to measure it in sound field condition. However, since no hearing aid dispensing office has an anechoic chamber nor even a semianechoic one, effective measurement can not be achieved because of the lack of reliability on sound field condition, even though such a measurement attempt might be performed somewhere. This study was designed to assess the effectiveness of hearing aids more conveniently, economically and accurately using with a new measuring tool “Hearing Aid Assess Meter” (Mimy Electronics Co.,Japan) Materials and Methods Subjects Total 11 normal hearers (22 ears) ranging from 20s to late 30s of age (mean 26.4 yrs) were selected to compare the performances between ordinary audiometers and the hearing aid assess meter. Shown below is the basic data about hearing impaired group. In addition, all the hearing impaired subjects had aided experience of 1-2 years. Meanwhile, some of their hearing aids, which adopted builtin AGC (Automatic Gain Control) circuitry, were loaded with 60 dB input SPL and then reset with target gain determined under the prescriptive formula POGO (Prescription of Gain and Output) through the hearing aid test system FONIX 6500-CX (by FRYE Electronics, Inc.) in order to assure its better test validity in counterpoise when compared to linear amplifiers, in the same way lineartyped aids are processed. Digitally programmable aids were selected for the test since the frequency response can be adjusted more flexibly, which comprises 2 BTEs, 3 ITEs, 6 ITCs and 4 CICs by their wearing styles (Table 1). Methods Hearing aid assess meter This instrument not only functions as audiometer but also contributes to verifying how effectively either a monaural or binaural hearing aid works. It facilitates testing at the time of the hearing aid fitting since it was originally devised to work with loudspeakers (Sony VIP-1000H) built in the headset, stimulating around both ear canals, and fulfills quasi-sound field conditions. Based on the ISO R226, measurements are performed with pure tone and speech sounds to obtain thresholds and discrimination scores (Fig. 1). In addition, this instrument enables measurement of both unaided and aided ears by using BTE and/or ITE hearing aids under the same condition as it adopts the KEMAR (Knowles Electronics Manikin for Acoustic Research, Burkhard and Sachs, 1975) principle featuring SPL at the eardrum. Comparison of hearing responses can be easily done with this instrument, which varies with different hearing aids. Therefore, it is possible to evaluate actual fitting results and to confirm adaptation of the hearing aid, thus evaluation of the hearing aid effectiveness can be easily achieved at any fitting site. Measurement in quasi-sound field conditions It is recommended in ISO 8253-2 that hearing level tests using pure tone or narrow-band signals should be conducted in a semi-anechoic chamber after considering situational environments. Acoustic conditions for semi-anechoic chamber follows:sound source should be placed above a tested patient's head and the test point in the chamber should be established at a distance of 1 meter from a loudspeaker, sound pressure variation measured within the rectangular area made of 15 cm above, below, left and right from the point should be matched to the value measured at the test point within±2 dB variation at specific frequency under the condition of the patient's absence. Also mentioned above was that the sound pressure variation against the value measured at an area 10 cm before and behind the test point should be within±1 dB. Preliminary measurement:audiometric comparison with traditional audiometers Air conduction thresholds were obtained through the audiometer AA30 (by Interacoustics AS) and the headphone TDH 39P in a soundproof chamber within its noise level 28 dBA, 53 dBC. Subsequently, the results were compared with the thresholds measured with the small speaker on the headset, which is subject to Hearing Aid Assess Meter, under the same condition as traditional audiometers should be. The allowance could then be obtained by comparison of the two results (Fig. 2).HFA full-on gain measures after preset All the programmable aids were appropriately preset by auto-fit command of a fitting software under POGO formula. Hearing aid analyzer FO-NIX 6500-CX was used to measure the actual gain of the aids at each frequency and then the results were averaged by 3-frequencies (1000, 1600, 2500 Hz) values, which can be referred to as HFA (High Frequency Average) Full-On Gain according to ANSI S3.22 (1987) that specifies hearing aid characteristics. Functional gain comparison with the above Gain values in aided and unaided responses were obtained through Hearing Aid Assess Meter in order to recognize the possible increase in SPL (Sound Pressure Level) at the eardrum with the hearing aid in place and operating, compared with the SPL at the eardrum without the hearing aid and with the ear canal and concha unoccluded. The resulting gain values were compared and analyzed with the results from the above (2). Comparison between functional gain and sensation level SDT (Speech Discrimination Test) was conducted with Hearing Aid Assess Meter, escalating the intensity of live voice with its target score above 70%. The score was obtained under the condition that the tester and the subject should be placed in separate rooms and the acoustical separation between them should be good enough so the subject should be not able to hear the tester's live voice, except through the instrument. The words used for this test were phonetically balanced (PB) words and those were presented to the subject at various hearing levels to locate the level which produced the best word recognition ability before the test was given. Lastly, the resulting SL (Sensation Level) was compared and analyzed with the functional gain obtained from the above (3). Results The audiometric interpretation of the two measured values, which were obtained from the 11 subjects with normal hearing, has shown that differences occurred with the range of 4.1 to 10.5 dB HL (mean 6.4 dB HL), by means of division 6 method (500 Hz+2 (1000Hz)+2 (2000 Hz)+4000 Hz/6)(Table 2, Fig. 3). It is estimated that some individual variations might have existed and resulted in the difference occurrence. However, the largest factor should be obviously related to the calibration values basically applied to the two measurement methods:by the headphone TDH 39P and SONY VIP 1000-H. Specifically, the former TDH 39P is calibrated according to ANSI standards while the latter is set up based on the KEMAR’s SPL measurement at the manikin. Using all the aids corrected by 60 dB input SPL and targeted through the hearing aid test system in order to assure the approximation to POGO formula, the average insertion gain measured from the hearing impaired group, whose mean hearing level comes to 59.5 dB HL, has been revealed as much as 27.4 dB SPL at 3 speech frequencies. All the aids were best fitted, referring to the each subject's hearing loss degree obtained through Hearing Aid Assess Meter so that it can provide functional gain, followed by comparison with the final value from insertion gain. As a result, their mean hearing level was 59.5 dB HL and the aided response was 34.6 dB HL, both obtained by division 6 method. As shown in Fig. 4, Hearing Aid Assess Meter indicated that 17.8 dB SPL was the actual value of functional gain in aided response. In the meantime, the value difference could be reviewed between the insertion gain and the functional gain over 500 to 4000 Hz (Fig. 5). It is obviously presumed that the difference largely resulted from the occlusion effect when wearing aids. Another manifest finding is that the greater difference shown especially around 2000 Hz must be due to the resonance effect in the ear canal. On the average, 17.9 dB SPL and 13.6 dB SL were functional gain and sensation level with PB word that we obtained through Hearing Aid Assess Meter. Discussion The current domestic trade of hearing aids has been dominated by hearing instruments retailers whose financial status is relatively inferior here and the measurement of its performance has primarily been a procedure conducted in a test box rather than on the human ear.4)5) With the more widespread use and interest in probe microphone measurement systems, new measurement procedures and a new technical jargon have been established.6) Dispensers are now more interested in insertion gain than 2-cc coupler gain. Functional gain is also considered a valuable measure in assessing hearing aid benefits. However, effective measurement can not be achieved because of the lack of reliability on sound field condition, even though such a measurement attempt might be performed in a few dispensing offices.7)8) This is the reason why otologist has to pay attention to assess the effectiveness of hearing aids and its satisfaction.In this study with a new measuring tool “Hearing Aid Assess Meter”, comparison of hearing responses can be easily done, which varies with different hearing aids. There, it is possible to evaluate actual fitting results and to confirm adaptation of the hearing aid, thus evaluation of the hearing aid effectiveness can be easily achieved at any fitting site. Non-audiological factors as well as audiological clues should be carefully considered so that hearing aids can be more affordable to patients who suffering from hearing difficulties in their everyday life. That is, the greatest factor that affects wearer's satisfaction is summed up in the following questions:‘how their hearing loss can be compensated?’ and ‘how high the expectation level is placed or how well are they motivated?’. New assess meter stimulate around both ear canals, and fulfills quasi-sound field conditions. In terms of measuring methods, some variation has existed between the headphone and the loudspeaker. This is related to the calibration values. The headphone is calibrated according to ANSI standards while the latter is set up based on the KEMAR's measurement. Further study is needed on how the resonance in ear canal varies with different wearing positions (BTE, ITE, ITC, CIC, etc.) and how different the REM (Real Ear Measurement) results will be from the results through the newly-devised instrument. Therefore, more in-depth research should be encouraged to help the hearing instrument dispensers to fit the aids to their customers more adequately and to simplify the conduct of the aided response test. Conclusion The applicability of Hearing Aid Assess Meter, as an alternative for complete anechoic facilities, could be acknowledged to a certain extent, and yet the instrument still leaves room to be desired for more proof of its accuracy and reliability as an audiometer, in terms of functional value.
REFERENCES
1) Staab WJ, Lybarger SF. Characteristics and use of hearing aids. In Katz J, Ed. Handbook of clinical audiology. Baltimore: Williams & Wilkins 1994;708-718. 2) Kaplan H, Gladstone VS, Lloyd LL. Audiometric interpretation: a manual of basic audiometry. Messachusetts: Allyn & Bacon 1995;7-13. 3) Sandlin RE. Hearing instrument science fitting practices. National Institute for Hearing Instruments Studies 1996;326-327. 4) Hawkins DB, Schum DJ. Relationships among various measures of hearing aid gain. J Speech Hearing Disorders 1984;49:94-7. 5) Zemplenyi J, Kirks D, Gilman S. Probe-determining hearing aid gain compared to functional and coupler gains. J Speech Hearing Research 1985;28:394-404. 6) Libby E. The shift toward real ear measurements. Hear Instr 1986;37:1-8. 7) Mason D, Popelka GR. Comparison of hearing aid gain using functional, coupler and probe-tube measurements. J Speech Hearing Research 1986;29:218-226. 8) Stelmachowicz PG, Lewis DE. Some theoretical considerations concerning the relationship beteen functional and insertion. J Speech Hearing Research 1988;31:491-6.


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