There are at least three assumptions about directional microphone hearing aids that require discussion.
1. Hearing aid manufacturers routinely measure the directional properties of DM hearing aids prior to shipment.
2. Directional features programmed into the hearing aid seen on the computer programming screen are actually what is programmed into the hearing aid.
3. Once the hearing aid is programmed, the measured hearing aid performance is represented in the hearing aid as worn by the user.
This blog comments on two of these assumptions. The third assumption will be managed in the next blog (Part IV) of directional microphone hearing aids. Measurements were obtained from directional microphone hearing aids gathered from local hearing aid dispensers (term is used generically to describe all who fit hearing aids) in the St. George, UT area. And although conclusions are often left for the end of a document, suffice it to say that the results obtained were consistent among hearing aids from all manufacturers.
1. Are directional microphone hearing aids tested for directionality routinely by the manufacturer prior to shipment? The answer is no. One reason is because theoretically, directional performance is programmed into the aid by the dispenser for most hearing aids. But, even if this were not the case, polar plot testing of each hearing aid in an anechoic test environment would take too much time, resulting in inefficiency and great cost. Using a standard hearing aid test box to show front/back amplification relationships could be used, but provides no polar plot information and therefore, has limited value. A recent option to test polar plots without a large anechoic environment is the Frye 8000 Test System (Figure 1).
Figure 1. Office and research system to record polar plots of directional mic hearing aids.
In this system the hearing aid is rotated through 3600 (Figures 1 and 2). This is the system that was used for the measurements reported here. When the polar plots were compared with those made in an anechoic room, the results matched reasonably well at the test frequencies used.
Figure 2. Directional mic hearing aid in Frye 8000 Test System ready for polar plot measurements.
2. Are directional features programmed into the hearing aid as seen on the computer programming screen the same as measured in the hearing aid? Certainly not exactly, as can be seen in Figures 3 and 4. Figure 3 shows four different programs as seen on the computer screen along with the measured performances. There is some similarity, but the programmed cardioid polar plot is not cardioid when measured. And, these are closer than many that were measured. Figure 4 shows 12 directional microphone programming steps from cardioid in the lower right to a bidirectional in the upper left – all for the same hearing aid. Interestingly, the programmed pattern that comes closest to the cardioid polar plot is what the computer screen visualizes as the bidirectional. In this aid, programming to cardioid produces essentially no directional microphone performance in the hearing aid. In this case, no amount of convincing by the dispenser that the aid is directional will produce the expected result. In fact, it might make the consumer question the dispenser’s ability, unless a front/back difference of 5 dB is considered to be a successful cardioid program. In reality, the user most likely can’t even hear that small a difference.
Figure 3. The blue shapes are the expected polar plots for a directional mic hearing aid. The polar plots beside each are the actual polar plot measurements made for that setting.
Figure 4. Polar plots measured for a directional mic hearing aid. The bottom right was with programming set to cardioid, and the upper left to bidirectional. Programming allowed for 12 discrete polar plot steps.
The third assumption to be discussed will have to wait for the next blog (Part IV). It answers perhaps the most important question related to directional microphone hearing aids. “After the hearing aid is programmed, is the measured hearing aid performance directional?” In other words, the patient may be told that a hearing aid, or one or more of its settings is directional, but is it really?
