Stability of Dual Directional Microphones Under Conditions of High Temperature and High Humidity (Part I)

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Nigel Smith
5 years ago
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Nano-Coating and Hearing Aid Directional Performance Wayne Staab March 5, 2012 (Part I) and March 10, 2012 (Part II) Reprinted with Permission from Hearing Health and Technology Matters, LLC

1 Nano-Coating and Hearing Aid Directional Performance Wayne Staab March 5, 2012 (Part I) and March 10, 2012 (Part II) Reprinted with Permission from Hearing Health and Technology Matters, LLC Stability of Dual Directional Microphones Under Conditions of High Temperature and High Humidity (Part I) In some of my past blogs I have reported on two separate issues that this blog, and the next, will attempt to bring together. The first issue related to directional microphone hearing aids that were not directional. In a later blog I reported on a nano-coating process that a number of hearing aid manufacturers are applying to some of their hearing aids to render them water resistant. While most readers are familiar with directional microphone hearing aids, fewer are familiar with nano-coating. Nano-coating, based on nano-science, is the application of unique properties of matter that occur at the nanoscale (lengths of roughly 1 to 100 nanometers or one billionth of a meter) to an object. In this case, to rendering the hearing aid resistant to moisture. And, while the nano-coating can be applied as either a spray-on coating or as a pressurized ionized gas (without changing the form, structure, or function of the object), it is the ionized gas process that appears to hold the greatest application for hearing aids. In my testing of directional microphone hearing aids, especially of the dual-microphone variety, I reported on how a number of directional microphone hearing aids, even when programmed as directional, show omnidirectional polar plots. The cause for this loss of directionality was not identified. However, this loss of directionality is of concern, especially since directional microphone hearing aid performance is being promoted heavily, and carries a premium price. Therefore, it is important that patients receive the performance being sold and paid for. Omnidirectional Mics Must be Matched to Provide Directional Performance It is reported that directional hearing aid performance, using two omnidirectional microphones, must have their inherent sensitivities well-matched for the polar patterns to produce their intended shape [1] Manufacturers can manage this matching in a number of ways. They can purchase matched pairs of microphones (slight price increase), compensate for the differences by adjusting the gain of the amplifier for one microphone relative to the other, or provide for some kind of dynamic software constant comparison of the microphone sensitivities that changes the gain and/or frequency of one microphone response relative to the other microphone. Thompson explains that while no such process can function perfectly, the various forms of dynamic matching may provide an important improvement in matching over the life of the aid. He continues by stating that it is essential that the microphones maintain their match throughout the life of the aid. And, while this is very important for first-order directional microphone systems (2 mics), it becomes

2 even more critical with second-order directional microphone systems (three microphone combinations). A significant cause of microphone drift (mismatch) is partial or complete clogging of the microphone ports by debris and other conditions that could have a direct impact on the microphones. One of these conditions could relate to moisture, and hence my attempt to look at directional microphone performance and moisture resistant hearing aids. With these thoughts in mind, I decided to conduct a preliminary test to determine if nano-coating helps reduce microphone drift that could be caused by moisture. Test Purpose The purpose of this preliminary test was to determine if pressurized nano-coating would help protect the microphones from moisture and help maintain programmed hearing aid directionality. Test Conditions Two new, digital, dual microphone directional hearing aids were compared. One nano-coated (pressurized method) One untreated Both hearing aids were subjected to 48-hour periods of exposure to: 100 degree Fahrenheit temperature 100% relative humidity (RH) Time periods between exposures was from 24 to 48 hours (called rest periods ) Both hearing aids were tested with the same call and speaker link (RIC style) The speaker link was NOT attached during exposure times The cell was not inserted during exposure times Testing was conducted using a Frye Electronics, Inc with directional polar plot capability The hearing aids were matched in performance Programmed linear (no compression) Maximum cardioid pattern All noise reduction was disabled Reference measurements were made prior to the test just after the aids were received and programmed. These would be used for comparison purposes. Coupler measurements at db SPL inputs Polar plots Time constants Group and phase delay ISTS ANSI 03 On the 10 th test period, the exposure time was increased to 60 hours

Figure 1 shows a 2-cc coupler comparison of the two hearing aids at the start of the test for various input levels, while Figure 2 shows the polar plot comparisons at the start of the test. Figure 1.

3 Figure 1 shows a 2-cc coupler comparison of the two hearing aids at the start of the test for various input levels, while Figure 2 shows the polar plot comparisons at the start of the test. Figure 1. 2-cc coupler comparisons of the non-nano-coated hearing aid (left) and the nanocoated hearing aid (right). Figure 2. Polar plot comparisons of non-nano-coated hearing aid (left), and nano-coated hearing aid (right). These are plotted as pre-testing reference graphs. Figure 3 is a sample data logging to confirm the stability of the relative humidity and temperature conditions. Data logging was obtained with each environmental exposure and recorded with the hearing aid measurements made at the termination of each exposure period. The environment was steady like the graph shown for all testing. Variation was about ±2 degree or ± 2 percent relative humidity throughout all testing. The polar plots were fairly consistent until the 10 th exposure, which had been increased to 60 hours rather than the previous 24-hour exposures. This change in exposure

4 time was arbitrary determined primarily by the fact that no real changes had been occurring. Figure 3. Data logging print-out sample of the temperature and relative humidity consistency during the test. This is a sample only, but of the actual test conditions.starts to show unusual polar plot recordings This blog (Part II) is a continuation of last week s presentation of preliminary testing to determine if pressurized nano-coating (which is to protect hearing aids from moisture) manages to maintain directional hearing aid performance better than a non-nano-coated hearing aid when both are subjected to a series of 48-hour exposures to 100 degree Fahrenheit temperature along with 100% relative humidity. Results 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.

5 Figure 4. Polar plots of the non-nano-coated hearing aid following each of the 48-hour temperature and humidity exposures. The testing was terminated, at Test 10 when this instrument lost its directional properties. Figure 5. Pre- and -post polar plots of the non-nano-coated hearing aid showing the loss of directional properties.

Figure 6. Pre- and post ANSI '03 testing of the non-nano-coated hearing aid, showing an overall reduction of gain and an increase in the low-frequency amplification.

6 Figure 6. Pre- and post ANSI '03 testing of the non-nano-coated hearing aid, showing an overall reduction of gain and an increase in the low-frequency amplification. 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.

7 Figure 7. Polar plots of the nano-coated hearing aid following 48-hour exposure to high temperature and relative humidity. 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.

8 Figure 9. A closer view of the pre- and final polar plot tests of the nano-coated hearing aid. Comments 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. Footnotes Thompson, S. Tutorial on microphone technologies for directional hearing aids. The Hearing Journal, Vol. 56, No. 11, 2003, pp , 18, _

Evidence base for hearing aid features:

Evidence base for hearing aid features: { the ʹwhat, how and whyʹ of technology selection, fitting and assessment. Drew Dundas, PhD Director of Audiology, Clinical Assistant Professor of Otolaryngology