Directional microphone technologyDirectional microphones are currently the most effective method for improving speech intelligibility in loud environments (group situations, parties, etc.). Thousands of satisfied hearing
instrument users utilise such systems on a daily basis and report on obtaining noticeably improved speech intelligibility.
The first directional microphones were installed in hearing instruments over 30 years ago. In the 1970s every fifth hearing instrument was equipped with a directional microphone. Unfortunately, the directivity of these early
microphones was not effective and they did not provide real benefits for the hearing instrument user. For this reason, directional microphones practically disappeared from the palette of hearing instruments in the 1980s. Only in the
1990s did the directional microphone experience a renaissance. The incorporation of digital technology and improved directivity in the new microphones provided users with noticeable benefits.
The simple directional microphone system
Simple directional microphone systems use an individual microphone with two separate sound port. The directional characteristics of these simple microphones are defined by the space between the sound ports and an internal delay of
the incoming sound signal in the back port. This delay is generated by a buffer damper. Since neither the space nor the properties of the buffer damper can be varied during operation, the directional characteristics of such simple
directional microphones remain constant.
The multi-microphone technique
 There is, however,
another way to realise directional microphones. Several microphones are used simultaneously. The microphones have to be constructed small enough so that several of them fit inside the housing of the hearing instrument, and a signal
processor has to be available with several (at least two) independent inputs for the microphone signals.
 Multi-microphone
technology has several important advantages over the single microphone with two sound ports. An omnidirectional microphone is available in addition to the directional microphone, and the individual microphone signals can be
electronically pre-processed independently from one another (e.g. via variable delay circuits, etc.) to obtain various directional characteristics. As a result, it is currently possible to adaptively adjust the directivity of
microphones to the existing situation as well as effectively suppress even moving noise sources.
|
