Non-nano-coated hearing aid
Figure 4 shows the non-nano-coated polar plots following each of the 48-hour exposure periods. Testing was terminated when, on Test #10, the non-nano-coated hearing aid lost its polar pattern. Polar plots were fairly similar until the 60-hour test, when the polar plot no longer was evident. Instead, an omnidirectional microphone polar plot was recorded. The comparison is better shown in Figure 5. This hearing aid has been returned to the manufacturer to determine what caused the change in performance. Could something similar have happened to some of the directional microphone hearing aids that I reported in that, although programmed as directional as a part of the test, did not measure as directional hearing aids? All hearing aids from that study, whether they maintained directionality or not, are being evaluated by engineering at this time to determine what might have been the cause for their directional microphone performance differences. With this change to no directionality, it should be expected also is that its gain and response should have changed as well. The pre- and post-test ANSI ’03 results of this hearing aid show this to be the case (Figure 6). The overall gain was reduced and the low-frequency response was increased.
Nano-coated hearing aid
The nano-coated hearing aid polar plots for the same exposure periods are shown in Figure 7. Testing was terminated at this time, and this hearing aid will be joined by additional units to continue the study using a larger sample for both the non-nano-coated and nano-coated categories. However, it will be used as a continuation of its previous exposures, and will be compared with new nano-coated hearing aids of the same model. When compared with the non-nano-coated hearing aid, the nano-coated polar plots continue throughout the tested period, although the polar plots appear to have greater test-retest variability than did the non-nano-coated hearing aid.
Figure 8 shows the reference and 60-hour polar plot comparisons, and Figure 9 shows the reference and 60-hour coupler responses for 50, 60, 70, 80, and 90 dB SPL input of a digital speech signal. Results were fairly similar for the pre-test reference measurements and the post-test measurements.
It is only fair to repeat that these results are from a preliminary test based on one non-nano-coated hearing aid and one nano-coated hearing aid. Further testing must be done to confirm any results. Still, it is interesting to note the sudden, and dramatic elimination of the directional microphone polar plot to the non-nano-coated hearing aid. There was no attempt to reprogram the hearing aid to determine if it could be made directional again because that would not have been beneficial to the engineers looking at what the cause might have been.
Were the test conditions realistic? No, not in and of themselves. However, this might represent the accumulated effects of temperature and humidity over time. The important thing to know is that not all directional microphone hearing aids function as programmed. Can this inconsistency in response occur as the hearing aid comes directly from the factory? Probably not, but what causes some hearing aids to lose their directional properties?
I hope to be able to provide some suggestions on this blog following continued measurements. Smarter people than me are certain to weigh in on this topic. So, stay tuned.