Note: My blog for last week was a repeat of a previous blog and I hope that those of you who missed it initially were able to read it this time.  I was in the Eastern Mediterranean visiting ancient ruins in Istanbul, Pergamum, and Ephesus in Turkey, and Rhodes, Aghios Nikolaos, and Athens, in Greece, and did not have reliable Internet connections.

Does the Measuring Coupler Make a Difference?

A variety of load couplers can be used when making hearing aid measurements.  The type used depends on the hearing aid being measured, but regardless of the type, the ANSI S3.7 Standard calls for measurements to be made into a 2cc volume load.  The actual attachment of the hearing aid to the coupler varies with the type of hearing aid being measured: the HA-1 2cc coupler is most commonly used for making measurements of custom-molded hearing aids and the HA-2 2cc coupler for hearing aids connected via tubing.

The hearing aid industry knew early on that the 2cc originally selected as the volume between the tip of the hearing aid coupler and the tympanic membrane was too large.  Additionally, it has been known, and recognized as well, that measurements made into such a coupler do not represent the performance of the hearing aid when worn.  Hence, the reason for real-ear measurements.  Regardless, the 2cc coupler continues to serve well for its intended purpose – the inter-comparison of hearing aids.

However, that the 2cc coupler does not represent the performance on the aid when worn has resulted in other couplers being developed – most to represent smaller residual volumes between the end of the coupler and the tympanic membrane.  Such was the reason for the development of the Zwislocki coupler (1.2 cc volume), and more recently, couplers to represent volumes associated with CIC (completely-in-the-canal) instruments.  These modified couplers are not “standardized.”

Figure 1 shows the representative couplers used for the RIC measurements recorded in this blog.  Note that the actual form varies among manufacturers of test equipment (those shown are from Frye Electronics).  From left to right: The MZ-1 is an OES (occluded ear simulator) intended to replicate the modified Zwislocki coupler, and having a volume of 0.7cc.  The HA-1 is a 2cc coupler designed for measurement of custom-molded hearing aids, and is the Standard ANSI coupler.  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 (real-ear to 2cc coupler differences) for deeply-fitted hearing instruments.

Figure 1. Hearing aid measurement couplers: a) MZ-1 OES 0.7cc, b) HA-1 2cc, and c) CIC 0.4cc.

RIC hearing aids present an interesting option for testing.  They have a physical appearance that represents traditional BTE hearing aids, but their fit in the ear canal presents a residual volume closer to that of a CIC hearing aid.  As a result, what is the better way to measure their performance: 2cc or 0.4cc?

RIC Measurement Results

Results of measuring the same RIC hearing aid, but with different load couplers are shown in Figure 2.  It is obvious that there are substantial differences in the measured performances.  While many different measurement combinations were made, only a few curves of the most likely-used measurement combinations are plotted because the graph would be too confusing.  Measurements with the lighter green color bars 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 plots, the CIC software correction had the greatest gain in the high frequencies, followed by the OES coupler measurement.  It is obvious that depending on which coupler is used, and whether correction factors are applied (for the CIC coupler), the differences are dramatic.  While the response curves for the HA-1 2cc coupler were similar, the differences appear to be related to the type of stimulus used (digital speech ANSI weighted, pure-tone sweep, and digital speech non weighted).  The response curves obtained with the MZ-1 OES (occluded ear simulator with a volume of 0.7cc) and CIC coupler having a volume of 0.4 cc (along with software corrections to the curve to reflect this smaller residual volume), show substantially greater sound pressure in the high frequencies.  These results are reflected in real-ear measurements as well.  And, of the coupler measurements made, the smaller residual volume of the CIC shows the greatest high-frequency amplification.

Figure 2. RIC hearing aid coupler measurements recorded using a number of different couplers. Results varied dramatically, depending upon the coupler and the associated hearing aid test equipment used.

Which Coupler Measurement is Best?

So, which is best?  Perhaps a more important question to ask is, how does the manufacturer’s software fitting program take the residual volume into consideration (or does it?)  Then, which coupler measurement procedure most closely approximates the response expected by the manufacturer?  Can recommendations be made that allow for a more accurate and/or consistent procedure for making appropriate comparisons of different RIC hearing instruments?

For basic response comparisons, the 2cc coupler would appear to be appropriate.  However, for a RECD approximation, it is obvious that a smaller residual volume should be used.  But, what volume?  For this, it would be helpful if manufacturers of hearing aid test equipment would have couplers of similar volume.

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 justifies the use of 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*.  However, if one is interested in obtaining a better approximation of a RIC hearing aid response, the use of a CIC coupler and its associated measurement software is suggested.  Of course, the best way to determine what is happening is to measure the real-ear response.

*Measurements of RIC hearing aids by the author, using a CIC coupler and its associated software corrections, results in measurements fairly similar to REAR (real-ear aided response) results.  This may be a topic of a future blog. 

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