Directional microphone hearing aids are a preferred embodiment in many of today’s hearing aids, being used advantageously for listening in noise. When used in this fashion, the directional performance is employed in either a fixed, or more commonly, in an adaptive mode, and normally commands a premium price from the consumer. Conventional wisdom suggests that directional performance should be highly desirable, especially for improving the signal-to-noise ratio.
An unanswered question related to directional microphone performance, especially when two different microphones are used to create directionality, is whether they retain directional properties after they had been in the field for longer than one year. (In a previous blog on this site, I reported the results of hearing aids that had been programmed with directional performance and the differences between the programmed and measured polar plots that resulted, showing that the programmed and actual polar plots were quite variable from each other. Also, some of the hearing aids that no longer showed directional properties, were unable to be reprogrammed to show directional performance).
It was therefore decided to investigate the ability of directional microphone hearing aids to be programmed for directional performance after they had been in the field for longer than one year.
Six RIC hearing aids that use dual microphones to achieve directional performance, and had been in the field more than one year, were measured. The hearing aids were all of a single make and model obtained from a local dispenser, and were programmed exactly the same – in all cases to render the instrument with maximum linear amplification to emphasize the aid’s directional properties. All compression, equalization, and noise reduction was removed from the instruments, along with other parameters that were likely to have an adaptive impact on the hearing aids. The hearing aids were programmed for maximum cardioid polar pattern, and were all closed coupler fitted. As a control, each of the hearing aids was checked prior to and following polar plot testing to ensure that that hearing aid was programmed as identified. All measurements were made using a Frye Electronics 8000 Hearing System.
As can be seen from the following graphs, only two of the hearing aids showed directional polar plots, and even of the two that did, neither showed a cardioid polar pattern, as was programmed. Instead, they showed greater gain at 180o (where a cardioid pattern should have least gain) than at 120 and 240o. It is also significant to note, as should be realized, that when viewing the 60-dB response gain curves of these six instruments, that those without directional properties had significantly greater gain (15 to 20 dB) than those recording directional polar plots.
Hearing Aid Fitting and Counseling Implications
Some dual microphone directional hearing aids do not maintain their directionality over time, even if they were programmed as directional microphone hearing aids when fitted. Whether they were actually directional when programmed as such during the initial fitting is something to be investigated. Knowing this information is especially important because essentially no dispenser has the proper equipment to make this check, and manufacturers do not run polar plots on assembly line instruments.
Some of the hearing aids in this investigation could not be measured as having directional polar plots, even though the programming software indicated that they were so programmed.
As reported in my earlier blog on this topic, the failure of some hearing aids to hold their directional properties appears to show up in instruments from all manufacturers, based on other measurements that I have made.
The fact that the hearing aids that did not program properly for directional performance had greater gain suggests that the gain levels programmed for some patients may provide significantly more amplification than the patient requires. This may be a cause of some of the reprogramming following fitting, with the dispenser wondering why the user is asking for reductions in gain.
All of this puts dispensers in a difficult situation, and one that many most likely do not recognize – that is of telling patients that their hearing aid(s) has directional properties, when it may not be true. Or, of reprogramming the aid to be directional, and not knowing that it actually is. It also suggests that dispensers need to check hearing aids often, and reprogram when necessary. The problem is, however, when, and how do they really know whether directional properties have been installed into the instrument? One could ask the patient to respond to differences they can detect. However, it has been my experience than many patients cannot tell if their directional hearing aid is directional or not, even when it is.
There will be more to come on this topic in a future blog.