RIC Hearing Aid Coupler Measurements

Introduction

RIC (receiver-in-canal) hearing aids have become very popular over the past few years.  However, electro-acoustical coupler measurements of such instruments to properly suggest how they perform on the real ear have not been standardized.  So, how are RIC coupler measurements made and what do they tell us?

For RIC instruments, especially those that use occluded coupling, the use of the HA-1 coupler is mostly used.  However, the 2cc volume of the HA-1 coupler load may not represent accurately the performance of the hearing aid.  This is because the residual volume of many RIC occluded instruments is substantially less than 2cc, and even less than the 1.2cc of a Zwislocki coupler.   And for many RIC instruments that employ unoccluded (open or vented) tips, testing is often performed as an occluded device with measurement made on an HA-1 coupler using Fun-Tak to seal the speaker into the coupler, similar to how custom-molded hearing aids are measured (Figure 1).

Figure 1. HA-1 2cc coupler setup measuring a RIC hearing aid having a miniature speaker. Fun-Tak seals the speaker into the 2cc volume, similar to the way a custom-molded hearing aid is sealed into the HA-1 2cc coupler.

Numerous studies have shown that deep-fitting hearing aids that have the speaker in the canal, which many RIC hearing aids represent, measure substantially greater overall sound pressure, especially in the high-frequencies than do instruments terminating in the outer 1/3 of the ear canal (Figure 2).

Figure 2. The numbers at each frequency show the dB values to be added to measured 2-cc coupler data to obtain the true indication of deep-fitting hearing aid sound pressure.

In looking at Figure 2, the hearing aids that fit most deeply tended to have the greatest increases in sound pressure.  To use these data (using the Songbird Disposable), a 2-cc coupler gain at 2000 Hz of 14.7 dB means that one would have to add another 14.7 dB to the 2cc measured coupler gain to obtain the true gain value for the patient (a RECD – Real-Ear to Coupler Difference).  {{1}}[[1]] Cook, J., and Preves, D. RECD for a Soft Tip Deep Fitting Hearing Instrument. AAS Scientific and Technology Meeting, Scottsdale, AZ, March 2001.[[1]]

Understanding that these differences exist as they relate to the residual ear canal volume, manufacturers of hearing aid test equipment provide a correction factor for CIC instruments (which fit more deeply into the ear canal than traditional-fit instruments, and thus have smaller residual volumes than other hearing aid fittings) that adjusts for this smaller residual volume in order to provide more accurate results

Therefore, when measuring a RIC hearing aid that is likely to result in a smaller residual volume as well, should CIC procedures and measurement corrections be applied, or should there be some other correction factor related specifically to RIC instruments?  But first, is there a problem?

Does the Measuring Coupler Make a Difference?

Results of measuring the same RIC hearing aid, but with different load couplers (Figure 3) are shown in Figure 4.  It is obvious that there are substantial differences in the measured performances.  (Note, that while many different measurement approaches were made, only a few curves of the most likely-used measurement procedures are plotted).  It is obvious that depending on which coupler was used, and whether correction factors were applied, the differences are dramatic.  While the response curves for the HA-1 2cc coupler were fairly similar, the differences seemed to be related to the type of stimulus used (digital speech ANSI weighted, pure-tone sweep, and digital speech).   The response curves made with the MZ1 OES (occluded ear simulator with a volume of 1.2cc) and CIC coupler having a volume of 0.4 cc (and software corrections to the curve to reflect this smaller residual volume), show substantially greater sound pressure in the high frequencies.  And, of these two, the smaller residual volume of the CIC, with corrections, shows the greatest high-frequency amplification.

Figure 3. Hearing aid couplers used for the measurements. The MZ-1 is an OES (occluded ear simulator intending to replicate the modified Zwislocki coupler, and having a volume of 1.2cc. The HA-1 is a 2cc coupler designed for measurement of custom-molded hearing aids. The CIC coupler has a volume of 0.4 cc, and when coupled with the test device correction software, is intended to provide responses closer to RECDs for deeply-fitted hearing instruments.

Figure 4. RIC coupler measurements. Although measurements were made for all of these combinations, not all are plotted because the graph would be too confusing. Those in the lighter green color are not shown. Essentially, they were similar to the 2cc measurement for each of their categories, with the primary differences being in the low frequencies only, where the differences ranged by about 10 dB. In all of the cases the CIC software correction had the least gain in the low frequencies, followed by the OES coupler measurement.

So, which is best?  And, perhaps even more important, how does the manufacturer’s software fitting program take the residual volume into consideration (or does it)?  Which measurement procedure most closely approximates the most appropriate response?  Can recommendations be made that allow for a more accurate and/or consistent procedure for making appropriate comparisons of different RIC hearing instruments?  What should we know about the different measurements made?

Summary

Hearing aid coupler measurements, as performed according to ANSI (American National Standards Institute) Standards, do not provide accurate information about how an instrument will perform on the real ear.  This is something that has been know for many years and has resulted in real-ear measurements.

Hearing aid couplers have been standardized and used for the inter-comparison of hearing aids under identical conditions.  They function well for this purpose.  However, complicating this issue is that RIC hearing aids tend to fit more deeply in the ear canal than do traditional hearing aids, and as such, provide greater overall sound pressure, especially in the high frequencies.  As a result, the measured sound pressure can vary considerably, depending on the measurement assumptions and couplers used.

How do dispensers take this difference into account when measuring the electro-acoustical performance of the hearing aid, if they make such measurements, and more importantly, how is this taken into consideration by hearing aid manufacturers when developing the software fitting algorithms?