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 systemSimple
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 techniq
There
is, however, another way to realize 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
preprocessed 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.
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