The advantages of deep-fitting hearing instruments have been well documented in the hearing aid research literature. Included are an overall increase in real-ear sound pressure level delivered relative to coupler gain, especially in the high frequencies1, and a reduction in the occlusion effect.2 These advantages were realized in early, deep fitting, completely-in-the-canal (CIC) hearing instruments that were housed in hard shells and extended past the second bend in the external auditory meatus.

Difficulties in taking accurate ear impressions deep in the ear canal, combined with problems of the wearer easily inserting these deep-fitting, hard-shelled hearing aids, have gradually caused the average CIC hearing aid to become more tapered. Today, most CICs have a canal length comparable to most ITC and ITE hearing instruments. As a result, many of today’s CIC hearing instruments do not go past the second bend in the ear canal and conform more to the minicanal type than to the original definition of the CIC style. Thus, the well-known advantages of deep-fitting are not generally realized in current CIC instruments.

Utilizing a soft tip is another method of sealing deeply in the ear canal without the problems of hard-shelled, deep-fitting CIC devices outlined above. Such a soft tip has been incorporated in a disposable hearing aid that is thought to provide the acoustical advantages of deep fitting.3

Real-ear measures are one way to validate the acoustic benefits of a deep fitting hearing instrument. Specifically, the real ear coupler difference (RECD), the difference in output SPL produced by a hearing aid in a 2-cc coupler and an ear canal4 has been used to demonstrate advantages of deep fitting hearing aids. Early CIC hearing aids, which were typically deep fitting, had very large RECDs, especially in the high frequencies.1 This was an advantage for the clinician because a more severe hearing loss that typically required a larger hearing instrument could be fitted with a smaller, more cosmetically pleasing instrument by taking advantage of the large amount of real-ear gain achieved from the deep fit.

The purpose of this study was to compare the RECD for a deeply sealing disposable hearing aid to those for early, deep fitting CIC hearing aids. Additionally, the insertion responses for the various acoustic formats of the disposable hearing aid were obtained.

figureFig. 1. Format Selector which is designed to determine the most appropriate acoustical format (i.e., model) for the disposable aid.

Method
When fitting the disposable hearing aid, a Songbird Format Selector (Fig. 1) is used to determine the most appropriate acoustical format for a listener. An earpiece attached to the Format Selector contains the same components, shell and tip as the disposable hearing aid, allowing the wearer to experience the same physical fit and sound quality that would be obtained with the disposable hearing aid.

figure
figure
Figs. 2a-b. 2-cc coupler frequency responses with a 60-dB input SPL for the flat (top) and sloping (bottom) series of acoustical formats.

The seven fixed acoustical formats available via the Format Selector have 2-cc coupler frequency responses with a 60 dB input SPL as shown in Figs. 2a-b. High-frequency average (HFA) gain for these seven acoustical formats range from 20 dB for the F70 model to 5 dB for the Enhancer (ENH) model. These acoustical formats have been reported to be appropriate for those with mild-to-moderate hearing losses5, the target fitting population of the disposable hearing aid. The F and S designators indicate that the format response is for a more flat or more sloping hearing loss, respectively. The number designator (e.g., ENH, 40, 55, 70) gives an estimate of the amount of high-frequency hearing loss that can be fitted with a particular acoustical format.

Real-ear measures were obtained with a Frye 6500 probe microphone system on 17 male and 3 female adult listeners with normal hearing. The loudspeaker for the real-ear system was positioned 1 meter from the patient at 45° azimuth and 0° elevation. First, the probe tube was marked and inserted into the ear canal 27 mm past the inter-tragal notch to determine the real-ear unaided response (REUR) with a speech-weighted noise at 55 dB input SPL. The Format Selector earpiece was inserted into the ear canal and the real ear aided response (REAR) was obtained again with a 55 dB noise input SPL, and the real-ear insertion response (REIR) was calculated.

figure 3Fig. 3. Songbird Hearing Format Selector earpiece with probe tube inserted through the mushroom tip.

To avoid slit leakage and possible acoustical feedback oscillation problems (which occurred in previous attempts with the probe tube breaking the seal when it was placed alongside the tip), a different probe tube was inserted through a hole in the mushroom-shaped tip of the earpiece (Fig. 3). If acoustic feedback oscillation occurred, the earpiece was repositioned in the ear, or testing was discontinued if the oscillation didn’t stop. Care was taken during the REAR measurement to ensure similar placement of the probe as used in the REUR measure. The RECD was calculated as the difference between the REAR and the 2-cc coupler frequency response.

figure
figure
Figs. 4a-b. Mean real-ear insertion responses for the F-series (top) and S-series (bottom) of acoustical formats for 20 ears wearing the Enhancer acoustical format, 18 wearing the F40, 19 the S40, 18 the F55, 19 the S55, 16 the F70 and 17 the S70.

Results
The mean insertion responses (REIR) for the F series and S series of acoustical formats are shown in Figs. 4a-b. As can be seen, the deep fitting disposable hearing aid produced a large amount of insertion gain given its minimal coupler gain (Figs. 2a-b). These insertion gain results suggest that the disposable hearing aid is appropriate for fitting persons with mild-to-moderately severe hearing loss.

Unlike many other hearing aids, the REIR for the disposable hearing aid continues out to beyond 6000 Hz before rolling off. Moore, Stone & Alcantara5 at Cambridge Univ. compared the performance of the disposable hearing aid to two digital instruments and one digitally-programmable analog hearing instrument. They found that the disposable hearing aid had a wider frequency range of amplification and a smoother frequency response than all three of the comparison hearing aids. This increased bandwidth is thought to be produced by the deep fit of the disposable instrument and is likely the cause of the enhanced speech recognition in noise scores reported for the disposable hearing aid in the Preves & Dempesy study.6

figure
Fig. 5. Mean RECD for 127 real-ear measurements for all acoustic formats. Real-ear measurements were taken on 20 ears wearing the Enhancer acoustical format, 18 wearing the F40, 19 the S40, 18 the F55, 19 the S55, 16 the F70 and 17 the S70.

The mean RECD (±1 SD) derived from 127 real-ear measurements for all seven acoustic formats averaged together is shown in Fig. 5. The mean RECD in the low and mid frequencies is about 10 dB, increasing gradually until it reaches a maximum of 28 dB at 4000 Hz.

Discussion
Referring to Table 1, the RECD for the disposable hearing aid is compared to that reported previously by Bentler.1 The period of time her article appeared was during the time of early CICs, when the instruments were still deep fitting.

Frequency 250 500 1000 2000 3000 4000
Sachs & Burkhard
(BTE/ITE)
4 4.2 5.2 8 10.4 12.2
Gudmundsen(CIC) 7.5 7.5 10.5 14 18 21
Bentler (CIC) 4.2 8 7.6 16 20.7 20.3
Songbird 8.9 11.3 9.2 16.9 16.5 28.0
Table 1. Comparison of published RECD values in dB (adapted from Bentler 19941).

For her clinic’s CICs, Bentler reported RECDs of about 4-8 dB in the low and mid frequencies, increasing to a maximum of about 20 dB at 3000-4000 Hz. Similarly, Gudmundsen7 reported during the same time period RECDs for CICs of about 7-10 dB in the low and mid frequencies, increasing to 21 dB at 4000 Hz. These values show the RECDs produced by the disposable hearing aid are comparable or greater to those cited by Bentler and Gudmundsen. For reference, Bentler also included in the table RECD values for BTE and ITE hearing instruments estimated from Sachs & Burkhard.8 These RECD values are much lower in the higher frequencies compared to those for deeply fitting CIC hearing aids and the disposable hearing aid.

The ramification of a large RECD, such as that produced by early CICs and the disposable hearing aid, are that significant high frequency losses may be fitted successfully with relatively small amounts of coupler gain. The mean RECD values in Table 1 for the disposable hearing aid are similar to those reported by Walden et al.9, who concluded that greater real-ear gain can be achieved with the disposable hearing aid compared to typical fits with custom CIC or canal instruments having comparable 2-cc coupler gain. The RECD values in Table 1 also agree fairly closely with the 25 dB and 11 dB RECDs for CIC and ITE hearing instruments, respectively, reported by Preves10 that were obtained with a speech-weighted noise input, assuming that the larger high frequency RECD values predominate in the measurements.

Conclusions
A large amount of high frequency insertion gain is produced by each acoustic format of the disposable hearing aid with relatively little coupler gain. This result validates the usefulness of hearing aids with minimal coupler gain (e.g., the Enhancer acoustical format has a 5 dB HFA gain). RECDs of the deeply fitting disposable hearing aid are comparable to those of early, deep-fitting CIC hearing instruments; therefore, the disposable hearing aid wearer should realize the benefits described in early CIC literature. The insertion gain produced by the disposable hearing aid continues out to over 6000 Hz. This wide bandwidth is thought to result in very good speech-in-noise recognition results.

This article was submitted to HR by Jodi A. Cook, PhD, David A. Preves, PhD & Deborah T. Grabowski, MS, of Songbird Hearing, Cranbury, NJ. Correspondence can be addressed to HR or David Preves, Songbird Hearing, 5 Cedar Brook Dr., Cranbury, NJ 08512; email: [email protected].

References
1. Bentler R: CICs: Some practical considerations. Hear Jour 1994; 45 (11): 37-43.
2. Mueller G: CIC hearing aids: what is their impact on the occlusion effect? Hear Jour 1994; 47 (1): 29-35.
3. Staab W & Preves D: Deep canal hearing instrument fittings: a new approach. Hearing Review 2000; 7(6): 50-53.
4. Fikret-Pasa S & Revit L: Individualized correction factors in the preselection of hearing aids. J Speech Hear Res 1992; 35:384-400.
5. Moore B, Stone M & Alcantara J: Technical review of the Songbird Disposable Hearing Aid. Univ. of Cambridge Report to Defeating Deafness, 2001.
6. Preves D & Dempesy D: Speech recognition in noise field study results for a disposable hearing aid. Hearing Review 2000; 7(4): 34-38.
7. Gudmundsen G: Fitting CIC hearing aids: some practical pointers. Hear Jour 1994; 47 (7): 10, 46-49.
8. Sachs R & Burkhard M: Earphone pressure response in ears and coupler. Report No. 20021-2. Itasca, IL: Knowles Electronics, 1972.
9. Walden T, Walden B, Cord M, Cook J & Preves D: Fixed versus custom prescription for precipitous hearing loss. Poster presented at the Amer Acad of Audiol annual meeting, San Diego, 2001.
10. Preves D: Real-ear gain provided by CIC, ITC and ITE hearing instruments. Hearing Review 1994; 1 (7): 22-24.