Which of the two hearing aids shown in the drawing below has the best directivity? The obvious difference between them is their shapes and the way this causes the microphones to sit at more of an angle on the right example, and more horizontally oriented on the left example. Based on this observation, many audiologists would feel confident that the hearing aid on the left will provide better directional benefit to the user. However, there are many design and fitting factors that affect directionality. Whether or not the microphones on a BTE or RIE are aligned horizontally does not in and of itself mean that one device will outperform the other. In fact, both devices in this example have microphone spacing of 12 mm in the horizontal plane. If signal processing is assumed to be the same, and each device is positioned on the head as intended and coupled to the ear in the same way, they would most likely yield virtually identical directional characteristics.
Microphone spacing is a critical design element in dual microphone directional hearing instruments. The precise spacing is important, because this is needed to calculate the frequency specific delays which are applied to the rear microphone signal in order to achieve cancellation of sounds coming from behind. So, for example, if the directional system is designed to work with a microphone spacing of 10mm and the microphones are spaced only 5mm apart, the directional performance will be exceedingly poor. The microphone spacing is specified relative to the horizontal plane even if the microphones themselves are not horizontal relative to each other. If the microphones are angled respective to their intended positioning, the distance between the microphones in the horizontal plane also changes. If the hearing aid on the right is moved on the ear such that the microphones are both on the horizontal plane, the spacing between the microphones on this plane will be slightly larger. In contrast, if the hearing aid on the left is angled, the horizontal spacing between the microphones will be slightly less (this should make sense to anyone who was not too traumatized by the Pythagorean Theory).
But how much difference does it really make if the microphones are a bit off kilter when the hearing aid is worn? When dual microphone directional custom hearing aids first came out, fitters were informed of the importance of marking the horizontal on the impressions in order to ensure that the microphones were placed for optimum directionality. It has since been shown that angles up to 20 degrees from the horizontal have minimal impact on the Directivity Index1 for custom hearing aids. It has also been reported that altering the angle of directional BTEs on the ear results in a lower Directivity Index than when they are placed with the intended positioning2. However, the greatest impact on the Directivity Index appeared to be due to shadowing of the microphones by the pinna rather than the angle of the microphones.
The bottom line? In fitting directional BTE or RIE hearing instruments, an effort should be made to adjust tube length or choose a wire size that results in a fit that is as close to what is intended as possible. For some instruments, like the one on the left above, a fitting that aligns the microphones horizontally is the most desirable. For others, as the one on the right, a slightly different positioning is intended. If it is not completely obvious how a particular hearing aid should be positioned or difficult to achieve this positioning, try to ensure that the fit does not result in the pinna obscuring the front microphone as the hearing aid is worn.
- Mueller HG, Wesselkamp M. Ten commonly asked questions about directional microphone fittings. High Performance Hearing Solutions: Supplement to the Hearing Review 1999; 3:26-30.
- Ricketts T. Directivity quantification in hearing aids: Fitting and measurement effects. Ear and Hearing 2000; 21(1): 45-58.