Directional Hearing Aid

Stability of Dual Directional Microphones Under Conditions of High Temperature and High Humidity

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}}[[1]] Thompson, S. Tutorial on microphone technologies for directional hearing aids. The Hearing Journal, Vol. 56, No. 11, 2003, pp. 14-16, 18, 20-21.[[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 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. 8000 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 50 – 90 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. 2-cc coupler comparisons of the non-nano-coated hearing aid (left) and the nano-coated 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 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.

Next week’s blog will present the results of exposing hearing aids to 100 degree Fahrenheit and 100% relative humidity for a series of 48-hour periods – and, until one of the hearing aids starts to show unusual polar plot recordings.