(Measured with Insert Ear Phones and SPL Meter)
By Yoshimoto Fukumoto, Chugoku Hearing Aid Center;
Saburo Sugihara, Department of Otolaryngology, San-in Rosai Hospital;
Misao Tomomori, Department of Otolaryngology, San-in Rosai Hospital; and
Hiromi Takeuchi, Department of Otolaryngology, Tottori University, Faculty of Medicine
(Reprinted from Audiology Japan. Vol. 44, pp. 95—100, 2001. All rights reserved to Audiology Japan.)
Editor’s Note: Global Spotlight is a new feature of The Hearing Professional, created to provide a platform from which hearing aid dispensers in countries other than the U.S. and Canada can share their unique perspectives and experiences with hearing healthcare practitioners worldwide. We thank these authors from Japan for being the first to participate in this professional exchange.
The common practice of relying upon computerized settings for determining gain/output levels in programmable hearing aids was found to be impractical in real life application because of variations in use-gain expectations of individual users. Nor could MCL and UCL levels be reliably predicted by traditional fitting formulae, such as half-gain, third-gain or any other standardized approach based upon audiometric hearing thresholds.
The authors conducted a study in which we utilized insert earphones and an SPL meter to measure individual MCL and UCL with speech inputs of 65dB SPL and 90 dB SPL, respectively. After adjusting the hearing aids to an individual’s MCL and UCL in this manner, we found that there was little need for further adjustments in hearing aid response in 96 percent of analog hearing aids with a gain control and 87 percent of programmable analog and digital hearing aids without a volume control.
The practice of promoting hearing aids without a volume control has burdened the hearing instrument dispenser and the patient with numerous trial and error adjustments to meet the needs and expectations of individual hearing aid patients. This is because manufacturer-determined gain expectation, which is designed into programming formulae, cannot reliably predict actual comfort and discomfort levels applicable to the individual patient beforehand. Gain expectation varies even between individuals with identical hearing threshold levels. Thus, the traditional fitting method of using hearing threshold levels as a guidepost requires several adjustments to reach user satisfaction.1,2
As MCL/UCL measured by the insert earphone and SPL meter can compare directly with the output level of the hearing aid, we tried fitting aimed at making the output level of 65 dB SPL, input level to MCL in order to understand speech well.
The subjects of study included 187 patients with mild and moderate hearing impairment, aged 10 to 88 years, who visited our clinics from November 1999 to April 2000. The pure-tone threshold average (PTA) levels were 19 ears with less than 40dB PTA, 55 ears with 40—50dB PTA, 64 ears with 50—60 dB PTA, 50 ears with 60—70dB PTA and 26 ears with over 70dB PTA. The total number of hearing aids fitted to patients was 214. Seventy-seven hearing aids were analog, 75 were programmable analog and 62 were DSP hearing aids.
A. Evaluation and Fitting Procedure
B. Post-fitting Evaluation
Regarding relationships between hearing level and MCL, Figure 1 shows the results of 13 patients with hearing levels of 40dB HL + 5dB @1KHz. Figure 2 shows 12 patients with 60dB HL + 5dB @1KHz, both at 1kHz. Their respective MCLs @SPL are shown as measured via insert earphone and SPL meter. It was found in the mild loss group (40dB HL + 5dB hearing level) that MCL ranged from 75—100dB SPL, while the moderate loss group (60dB HL + 5dB hearing level) exhibited MCLs ranging from 80—100dB SPL.
Case No. 6 of the 40dB HL hearing level group indicated 100dB SPL as MCL while sample No. 21 of the 60dB HL hearing level indicated 80dB SPL as MCL. Among other samples, differences in MCL are found despite having the same hearing threshold level @1KHz.
In the post-fitting readjustments of hearing aids there were only three (or 3.9 percent) in the group of 77 ears utilizing analog hearing aids. In those readjusted cases we reduced the maximum power output (SSPL90) to accommodate loudness recruitment and UCL levels.
As for the cases utilizing programmable analog hearing aids without gain control, the number of readjustments was 11 (or 14.7 percent) from a group of 75 ears. In these cases we increased gain in nine hearing aids and decreased gain in two hearing aids in response to patient reports. Likewise, in the group of 62 hearing aids which did not feature a user volume control, we adjusted gain levels in seven (or 11.3 percent) cases; six increased in gain and one increased in SSPL90.
In addition, there was only one case requiring four adjustments because of unstable thresholds resulting from middle ear effusion sequela (Figure 3). Subsequent adjustments were carried out beginning at two weeks post-fitting for over a period of several months during the course of pathology.
All others were contacted over the same time period by mail and telephone to inquire about needed adjustments. Reportedly, none were needed. The readjustments reported above were primarily due to own-voice occlusion complaints. The resulting ranges of gain expectation during adjustments tended to cover a large range.
The specification of our method and the manufacturer’s targets were stored in computer memory in each case.
The subject is a 70-year-old female with an initial PTA of 58 dB HL as shown in Figure 3. Figure 4 shows pure tone threshold, MCL/UCL measured by insert earphone and SPL meter, and also shows frequency response for a canal type hearing aid (65, 90dB SPL input level). Figures 5, 6 and 7 show the frequency response displayed on the computer according to a fitting rule from manufacturers (SI and SK).
These hearing aid responses are distributed based on readjustments to meet changing hearing levels. High-level inputs suitable for maximum output level and moderate input level suitable for speech were 80—90dB SPL and 50—65dB SPL, respectively, although differences were found from manufacturer to manufacturer. Therefore, it was difficult to objectively compare specifications directly relative to 90dB SPL and 65dB SPL input levels. However, SI specifications most closely corresponded to our input levels. Otherwise, there appeared to be large disparities between manufacturers.
The hearing aid fitting rule in this case refers “target use-gain at each frequency” for MCL and “maximum power output level at each frequency” appropriate for a given individual user. The authors of this paper4 have developed an outline5 of MCL/UCL measures for hearing aid fittings. As pointed out in SHAPIRO I,3 hearing aid users:
We have already reported that UCL cannot be determined by threshold data and, in our experience, the same holds true for determination of MCL because of its inherent subjectivity. In the past, we sought appropriate gain at each frequency using half gain rule or its modification based on hearing level. As for the analog type hearing aid with a user volume control, patients can decide the gain for MCL so that we adjust only for frequency response and maximum power output (SSPL90) levels. However, hearing aids without user volume controls deprive patients of the ability to subjectively adjust for desired use-gain levels or for day-to-day sensitivity changes. Therefore, it becomes necessary to determine the preferred gain level as quickly as possible, based on subjective data for MCL as well as UCL. Moreover, we have found that MCLs vary tremendously from individual to individual, even when exhibiting identical hearing threshold information.
Insert earphones and SPL meters have the following merits when used in conjunction with effective hearing aid fitting:
If one is connecting through the microphone, it allows a fair correlation to hearing aid response. Using insert earphones and SPL meters together enabled us not only to fix MCL/ UCL immediately, but also to confirm the validity through microphone response. From there we set the programming response on the computer to correspond with measured MCL/UCL as confirmed with the hearing aid analyzer.
In this way, we feel that our method of hearing aid fitting contributes to more timely and accurate adjustments in meeting the needs of the hearing impaired. It also reduces the number of failed trials of hearing aids and dramatically shortens acclimatization time. Finally, our findings are that hearing aids without a user volume control generally do not accommodate the individual needs of many hearing aid users. THP
1. Seido Taiki, Michiya Okamoto, Tomoko Sugiuchi, et al., “Considerations for patients using binaural hearing aids” Audiology Japan 36, 1993, 355—356
2. Sedo Taiki, Michiya Okamoto, Tomoko Sugiuchi, et al., “Current outcomes in hearing aid outpatient care,” Audiology Japan 37m 1993, 403—404
3. SHAPIRO I: Hearing Aid Fitting by Prescription, Audiology Japan 15, 1976, 163—173
4. Hirohide Osawa, Toshiharu Fujiwara, Yoshitomo Fukumoto, et al., “Hearing Aid Fitting by Using MCL,” Audiology Japan 37, 1994, 385—386
5. Hiroki Makicho, Misao Tomomori, Saburo Sugihara, et al., “Consideration of UCL of aided ear,” Audiology Japan 28, 1985, 653—654
6. Byrne DJ, “Gain and frequency response requirements of hearing aids for persons with sensorineural hearing impairments.” NAL Report 64, 1976
7. McCandless GA, Lyregaar PE, “Prescription of gain and output (POGO) for hearing aids,” Hear Instruments 34, 1983, 16—21
8. Byrne DE, Dillon HA, “The National Acoustic Laboratories (NAL) new procedure for selecting the gain and frequency responses of a hearing aid,” Ear & Hearing 7, 1986, 257—265