hearing aids

David Kastrup dak at kaa.informatik.rwth-aachen.de
Tue Mar 21 06:22:12 EST 1995

jochenw at messua.informatik.rwth-aachen.de (Jochen Wolters) writes:

>dak at messua.informatik.rwth-aachen.de (David Kastrup) writes:

>>hartman at das.harvard.edu (Anthony Hartman) writes:

>>>Finally, are there any devices that deal with the problem of background
>>>noise well enough to understand a conversation in a crowded room?

>>The sound processing currently performed by the typical
>>devices is not much.

>Considering the size of modern hearing aids, I'd say they do offer quite a
>lot of signal processing. Expecially if compared to older designs.
WELL... Digital signal processing is not yet employed in any hearing
aid I know of, and this offers the best possibilities currently.

>>The background noise problem is being worked on by scientists, but I don't
>>think the commercial situation will much improve in the next 5 years or so.
>>Most approaches need more than one microphone, by the way.

>>The problem with background noise is also that the device has to
>>decide which noise is background and which not. The human hearing
>>does this by focusing, a mental activity. Hearing aids cannot know
>>what you want to hear, so they have to guess.

>From our practical experience, there is one device that has a clear advantage
>over others when it comes to discriminating background noise from speech by
>using two microphones to actually mimic the focusing that is performed by
>the brain in normally hearing patients. This is the MultiFocus hearing aid
>by Swiss manufacturer Phonak. It can be programmed to fit the needs of the
>hearing impaired based on the audiogram and may even be improved by using
>loudness scaling as a fitting tool.
This is standard technology. I am involved with the scientific side of this,
and fitting hearing aids based on the audiogram, as well as loudness adaptation
in several bands, well, seems sort of basic. Obviously more care is taken
with clinical test series than with end applications.

>The MultiFocus has two microphones that
>can be set to either "non-directional" for music, traffic, etc. and to
>"directional" for person-to-person communication. It then focuses to the
>sound source in front of the hearing impaired.
To call that mimicking the the focusing of the brain is, of course, rather
preposterous. This "focussing" is simply a directional microphone array
arrangement. These things help, of course, but the advantages are strictly
limited (you cannot gain more than 3dB SNR per doubling of the microphone
number, except when you use directional microphones. Using them, the focus
direction is fixed).

Background noise suppression in party environments, however, is hard to
deal with using directional approaches alone, especially where heavy
hearing loss is involved (I work in noise suppression for cochlea implant
patients, where the hearing condition is much much worse due to the
heavily lessened signal flow possible, and where only monaural fitting
is currently economically feasible). Binaural hearing helps a lot,

>In our hearing aid shop, we have to deal with a lot of customers who suffer
>from severe recruitment, which is mainly characterized by a much smaller
>dynamic range compared to normal hearing individuals. The MultiFocus seems
>to be the only hearing aid that deals with the problem of compressing the
>dynamic range of the signal to fit into the residual dynamic range of
>the patient as well as with helping these people to understand speech in
>background noise conditions.

Well, of course the approaches we work with require the patients to carry
around a box of at least breast pocket size (while there is no hard-cast
version of the algorithms). But that's research.

If you find that with the techniques employed, results are as you describe,
that can very well mean that the MultiFocus device will use the techniques
currently manufacturable into wearable hearing aid devices pretty well.

If you find that it is the only device that does so, this is not as much
a compliment to this device, as a scorn for the others.

The result, of course, is the same. People prefer using the best available,
whatever or however that be.
David Kastrup, Goethestr. 20, D-52064 Aachen        Tel: +49-241-72419
  Email: dak at pool.informatik.rwth-aachen.de         Fax: +49-241-79502

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