How Should RIC Hearing Aids be Measured?
When CIC (completely-in-the-canal) instruments became popular, manufacturers of hearing aid measurement equipment provided alternate couplers, and most often with software corrections, to account for the smaller residual volume of that instrument compared with other instruments that did not fit as deeply into the ear canal (traditional hearing aids at that time). The purpose was to provide a measured result that was more closely related to how the instrument performed in the actual ear, but still realizing that these measurements would not replace real-ear measurements. The considered residual volume of these couplers was substantially less than the 2cc of the ANSI and IEEE Standards, and although these couplers have not been standardized, they are known to have smaller volumes, perhaps even closer to 0.5cc.
RIC (receiver-in-canal) hearing aids have become very popular over the past years. However, electro-acoustical coupler measurements of such instruments to properly suggest how they perform on the real ear have not followed, as has occurred for CIC instruments. So, how are RIC coupler measurements made and what do they tell us?
For RIC instruments, especially those that are fitted occluded, the HA-1 coupler is used primarily. However, the 2cc volume of the HA-1 coupler 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 with miniature speakers, testing is often performed as if it were an occluded device with measurements 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).
Armchair logic suggests that it is possible that RIC instruments could show similar, and perhaps even greater REAR than CIC instruments based on residual volumes between the termination of the speaker/coupler and the tympanic membrane. It is for this reason that some, but I suspect few, individuals make RIC coupler measurements using the CIC corrections or even a Zwislocki-type coupler in their measurement options. But, even if used, how would the measurements compare? And, what better way to spend a few days before Christmas than by trying to find out (which means that my Christmas shopping is complete).
Figure 2 shows the results of measurements using three different couplers: an HA-1 (2cc), CIC (0.5cc), and a modified Zwislocki-type coupler (MZ-1 for OES data collection, 0.7cc). All measurements were made using a Frye Electronics 8000 hearing aid test measurement system. The hearing aid was a SeboTek HD 08 fitted with a closed CLICK tip, and programmed for linear amplification. As would be expected, the 2cc measurement shows less gain than that from either the CIC or OES procedures, both of which have smaller volumes. The CIC measurement shows the greatest gain. It should not be surprising that the greatest measured gain is associated with the smallest residual volume (Boyle’s Law).
Overall, the REAR shows the closest similarity to the OES measurement, which answers the question posed early in this blog (Figure 3). Might this suggest that for a well-fitted RIC closed fit hearing aid, that the RIC residual volume is closest to 0.7cc and not 0.5cc? Further testing is required to make any such conclusion.
Based on these measurements (with a statistically insignificant n of 1), it appears that if an OES coupler is used when measuring a RIC closed coupling hearing aid system, especially one that is properly fitted, that this will provide the best estimate of how the RIC will perform when worn (other than making an actual REAR measurement). So, in this case, should not what is good for the goose (OES) also be good for the gander (RIC)?