This blog started by looking at three assumptions about directional microphone hearing aids.
- Do hearing aid manufacturers routinely measure the directional properties of DM hearing aids prior to shipment?
- Are directional features programmed into the hearing aid seen on the computer programming screen actually what is programmed into the hearing aid?
- Once the hearing aid is programmed, is the measured hearing aid performance represented in the hearing aid as worn by the user?
This blog responds to the third question and essentially asks, “Are directional microphone hearing aids really directional?” This blog contains primarily measured polar plots of what were sold to patients as directional microphone hearing aids.
Figure 1 shows the results of a binaural set of directional mic hearing aids that were 2 years old. These were produced by a reputable manufacturer from chips and transducers widely used in the industry. They were programmed initially as directional aids – both the same. Aid #1 no longer tested as directional. Aid #2, with the polar plot for 1000 Hz only shown, has directional performance, but with the greatest response from the rear! It was found that some of the directional mic aids that had lost programming could be reprogrammed, but some could not.
Figure 1. Set of 2-year old directional mic hearing aids. Aid on left no longer shows directional and aid on right shows greatest directionality from the rear.
Figure 2 shows the polar plot of an adaptive directional mic hearing aid. Again, this aid showed greatest directionality from the rear – not what neither the dispenser nor the consumer was expecting.
Figure 2. Directional microphone hearing aid showing greatest amplification from the rear, not the front as expected.
Figure 3. Cardioid polar pattern of a directional microphone hearing aid.
Figure 3 shows a polar plot of a non-adaptive directional mic hearing aid, showing a nice cardioid pattern. This pattern, which seems desirable for reducing noise from the rear, was found most generally only in non-adaptive directional mic aids.
Figure 4 shows the polar plot of a RIA (open fit) non-adaptive directional hearing aid. However, it shows a front/back difference from 0 to at 10 dB at the most. It is questionable if the hearing aid user can detect such minor differences when research has shown that about a 20 dB difference should be expected.
Figure 4. Open fit (RIA) non-adapting directional microphone polar pattern.
Figure 5 shows an adaptive directional mic hearing aid measured using two different input signals. The plot on the left was made using a composite continuous signal. That on the right used a composite interrupted signal. The left plot appears to be going into directionality, but then it appears that the adaptive AGC of the hearing aid is acting as well, reducing the gain of the aid by approximately 10 dB for a 70 dB SPL input signal. The real question? What is the user experiencing? Manufacturers suggest that one turn off any noise reduction and AGC when making directional mic measurements. How does making such adjustments relate to what the user is experiencing? It seems that the aid should be measured as the person is wearing it, not in some non-significant test mode.
- Figure 5. Polar plots of the same adaptive AGC, adaptive directional microphone hearing aid measured using two different stimuli – a continuous composite tone on the left and an interrupted composite tone on the right.
Figure 6 shows that the aid type identification in the test procedure affects the polar plots. These plots are for a non-adaptive directional mic hearing aid (but having adaptive AGC) measured as a linear aid (left), and AGC (middle), and as an Adaptive AGC (right). A composite interrupted test signal at 60 dB SPL input was used. The aid was set to a high noise directional setting. The notch at the top of the Adaptive AGC plot is an artifact. These graphs suggest that the proper type aid must be identified in the test setup to obtain the appropriate polar plot response.
- Figure 6. Hearing aids can be measured as linear, AGC, or adaptive AGC type aids. As the polar plots illustrate, how the measurement test setup is arranged, the measured polar plots vary.
Figure 7 shows the response of a hearing aid when the signal is from the front and when from the rear to a composite interrupted 70 dB SPL input. Does it appear that all is OK with this aid? Even to an inexperienced observer, the fact that this directional mic aid has a better response from the rear than from the front is an obvious sign that this aid is not performing as expected. Still, that is what some users are getting when they are told that the hearing aid was programmed for directionality, and that it would respond best to signals from the front and not the rear. Is there any reason to be surprised when patients often cannot tell if the directional aid is functioning as related to them by the dispenser?
Figure 7. Directional microphone front/rear frequency responses showing that the signal from the rear has greater amplification than the signal from the front.
Summary
- It is obvious that directional mic hearing aids are somewhat fragile and can lose their directional capabilities. As a result, they should be tested often to ensure their directionality.
- Just because the computer program says the aids are functioning as directional aids, that is not always the case. There is a good chance that they are not programmed to the polar plot that the computer image suggests.
- Hearing aids may not be functioning as directionals as imagined, or as explained to the patient.
- Adaptive directionals are difficult to confirm.
Polar plot and difference curve measurement in an affordable, desk size instrument is now available. And while testing in this desk unit cannot yield the same curves as can a large anechoic chamber, the characteristic polar pattern can be seen and evaluated.
