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dB and linear scales |
| Compensation angle
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Spacing between two sound sources is 30°
Some people are suffering with decreased hearing and forced
to use special hearing devices. Such devices are usually the combination of a
small ear microphone with frequency-band amplifier and loudspeaker. These
devices have no space selectivity as their microphones are Omni-directional
and receive the sound waves from all directions.
The problem
arises when the user tries to hear only one sound source on the background of
other local sources with different intensities. Intelligibility is very bad in
such cases.
The similar
problem is when the user is in the room with high level of reverberation or in
the other places with strong noise but he must hear well his interlocutor.
There are a lot of such practical situations.
We have
developed and studied such a technology for Selective Hearing (SHT) to assist
any situation were a selective hearing is needed or useful for badly hearing
or deaf people. Also, it should be helpful for any, even well hearing person,
in situations where, for example, one wants to listen somebody in very noisy
environments or in the presence of many other speaking people, etc.
Brief
Discription of "SELHET"
technology.
The problem of
tuning away from local sound sources can
be solved on the basis of multi-element sound array with necessary directivity. If one directs the main lobe of such
array on useful sound source
other local sources would be
suppressed in proportion to the level of side lobes for this array. The level of selectivity will depend of wavelength of the array. Unfortunately if one wants to make such Selective Hearing Device (SHD) with the dimensions as for usual hearing devices, such array
would be too large for "ear" design.
There is another
approach to solve this problem: combination of usual sound pressure microphone
with the two sensors of oscillating velocity. If it is possible to measure in
one point of space the sound pressure and the vector of oscillating velocity
then it is possible to compensate the measured processes for the front of
plane wave coming from any given direction. In this case one has an array with
directional features. It's important to note that the wavelength in this case
could be small as one likes. The array pattern of such an array is known as
cardiogical and has zero sensitivity in the direction, which is opposite to
the direction of compensation. The intensity of waves coming from the plane,
which is orthogonal to the compensation direction, falls by two times.
Nevertheless in many practical cases such compensation is not sufficient
especially when angle spacing between a "good" and a "bad" source is less than
90°.
Special signal processing like, for example the algorithms
of high resolution (AHR) is necessary to have more high selectivity instead of
the usual beamforming. Such AHR algorithms have the width of angle response,
which is in inverse proportion to the ratio of power of « bad » plane wave and
the power of distributed noise. We have developed such algorithms and
associated software for combination of vector sensor with pressure sensor.
"SELHET" can be implemented as:
- Undirected hearing (acoustic
signals incoming from any direction are listened as in usual hearing
devices);
- High-directed hearing from the «in front
of» direction (acoustic signals incoming from any other direction are
suppressed).In this case the user has to
look right to the source he wants to hear. A
suitable mode for very noisy environments;
- Low-directed
hearing (acoustic signals incoming from some angle sector are listened
as is and those from all other directions are suppressed).
The user can adjust
the hearing sector width. This mode is good for listening concerts, in
the theatre, etc.
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