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Distance Cues

Tony Woolf tony at howl.demon.co.uk
Sat Dec 23 14:07:25 EST 1995

In article <951222135858_97202283 at mail02.mail.aol.com> DanMauney at aol.com  writes:

> > I am trying to pick up a person's voice from a microphone located about 11
> > inches from the mouth, off to one side.  I find that when compared to a mic
> > located close to the mouth, the voice sounds distant and hollow. 
> The surroundings are a typical office environment.  My biggest fear is that
> the distant and hollow sounds are due to 
>  reverberation because there isn't much I can do about that.  There are too
> many places in which this microphone is used and acoustic treatment is too
> expensive.  

You can find out if the sound you experience is due to reverberation
by trying the setup outdoors away from enclosed spaces, or in a very
non-reverberant room if you have one.

The effect may be due to the proximity of a single wall to the mouth
or to the microphone, rather than the reverberation of the whole
space.  This would give a comb filter effect (series of peaks and dips
in frequency response) due to interference.  This is sometimes said to
give a hollow sound.  However I think this cause is unlikely as the
effect is usually apparent only in quite "dead" rooms.  You could
check by seeing if you get the same effect outdoors near a hard wall.

> I know of no other way to filter out reverberation. 

Reverberation can be reduced relative to direct sound by using a
directional microphone, such as a cardioid or hypercardioid, rather
than an "omni" one.  It can't be removed by post-processing.  (If any
reader has a good idea on how to remove reverberation from an audio
signal, why not patent it - if it works you may get rich!  Hint - prior
publication on the Internet would invalidate the patent.)
We are now firmly in the realms of acoustics rather than audiology.  I
suggest that unless we get back into audiology, this thread should
either be continued by email, or transferred to
alt.sci.physics.acoustics.  I'd be interested to hear if you find out
what causes the hollow sound, in any case.

Tony Woolf

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