A4.1 - THE SMALLEST ELECTRET MICROPHONE WITH GOOD PERFORMANCE

Until now the biggest volume of microphones sold to be used in Hearing Aids were made in a squared shape. Going to a rectangular shape presents new challenges on how to acoustically model and optimize such a microphone. But the rectangular shape also allows you to make interesting combinations, which makes it possible to optimize the mechanical design of the hearing aid with two matched microphones, or even better to make a integrated module of matched microphones. Microphones for hearing aids were originally designed to be Omnidirectional and were optimized for that. At a certain point in history, the audio-logical advantages from using directionality became very apparent. And a matched pair was made based on two microphones. There were two apparent disadvantages: the microphones were not designed to be a matched pair, so the only way to get them matched was a selection process in the production, the other disadvantage being that both microphones drifted over time, making it very difficult to keep the directionality stable over time. Many smart algorithms were defined to solve this, but it would of course be very advantageous if the microphones itself could do this. An obvious choice would be to use MEMS microphones, which are very stable, and which have improved significantly over time in noise performance, but electret microphones are still better. In hearing aids the noise performance is of prime importance. By a clever construction you can actually optimize the performance in a module specially designed for directional performance in the smallest possible size. A more detailed analysis of the problem To make directional microphones using two omni microphones you use a block diagram as shown below :