Mark E. Skeels wrote:
> What I am looking for is a way to mic several individuals standing in
> 3 feet of water, in close proximity, in a small room with lousy
> acoustics, and still get a nice full sound.

First, I would do as much as possible to improve the sound characteristics
of the room.  The sound is probably a mess because it's bouncing around wit=
h
little attenuation in a small room.  That will set up standing waves at
certain frequencies.  Put some sort of accoutic absorbers on any surface yo=
u
can.  Hopefully at least the ceiling is available for that.

Fancy sound deadening materials are available for professional studios, but
these will be expensive.  You can get surprisingly far with off the shelf
materials and some ingenuity.  For example, I once saw a room with cardboar=
d
egg cartons stapled to the walls.  It deadened the sound surprisingly well,
although it wasn't like a professional sound studio.  Multiple thin sheets
of cloth with a little separation between them takes space but are
relatively cheap.

After that, it depends on how much effort you want to put in.  The room is =
a
linear system from the point of view of sound out someone's mouth to it
arriving at a microphone.  In theory, whatever transform is applied by this
system can be reversed.  In practise, you can make it a lot better but it
will never be perfect.  The transform will change for different source
locations, and measuring it in the first place isn't easy.  Ideally you put
out a impulse and measure the response at the mic, but a true impulse is of
course impossible so there are tradeoffs and compromises.  Just measuring
the frequency response with good resolution over the audio range you want t=
o
reproduce is probably the best practical approach.

Once you have the frequency response, it's just math to convert that to the
convolution kernel of a finite impulse response filter, which you then
implement in a dsPIC.  Let's say you sample at 20KHz rate.  A dsPIC can do
up to 40M multiply-accumulates per second.  That means 2000 MAC per 20KHz
sample.  That's 100ms of sound, or 33m of propagation.  That's more than
necessary, so it passes the first level sanity check.  Of course the dsPIC
will have to do a few other things than just running MACs, but there is
enough headroom for things to work out.


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