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     Two interesting messages about sound and sampling of complex signals.
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From: kisley@blue.seas.upenn.edu (Michael A Kisley)

Newsgroups: bionet.neuroscience,alt.sci.physics.acoustics,bionet.audiology
Subject: acoustic "colors"
Date: 15 Mar 1995 20:37:51 GMT
Organization: University of Pennsylvania
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I am posting this message in hopes of receiving suggestions/references
regarding the general topic of auditory "color".  However, I would also hope
that this post might stir up some discussions.  This message is cross-posted
to the following newsgroups:

  bionet.neuroscience
  alt.sci.physics.acoustics
  bionet.audiology

While doing an introductory review of the topic of echolocation in bats, I
came across a very interesting idea in a 1990 review paper by G.  Neuweiler:
the idea of echo "colors" in bat echolocation.  The general idea behind the
use of the word "color" is that the bats might be analyzing the spectral
patterns of the targets which are reflecting their broadband emissions.

In the general discussion of this review, Neuweiler goes on to hint that
perhaps this "color" - analysis IS a general feature of auditory systems.  His
reference to humans is that they might perceive acoustical "Gestalt" (grouped
senses) during speech recognition and when listening to music.

A somewhat tangential but nevertheless related topic is the use of sonar aids
for blind people.  Apparently there were attempts to use artificial sonar to
help steer blind people around obstacles.

However, someone I spoke with was under the impression that the sonar could
also be used to characterize TEXTURES OF OBJECTS, not unlike the bats' ability
to characterize targets by their unique spectral pattern.

These fascinating topics have not been easy to trace, and I would appreciate
any help people can give me tracking down the following lines:

1)  Has any work been done since 1990 on the idea of echo "colors" in
    echolocating bats (or dolphins)?

2)  Does anyone know of any good, and recent, papers on the topic of
    general auditory "colors" or "Gestalt" (including the areas of
    speech recognition and music-listening)?

3)  Are the sonar aids for the blind still being pursued as a viable
    option, and are there any good references that discuss why/why not?

Thank you in advance.

Sincerely,
Michael Kisley
Graduate Group of Neuroscience
University of Pennsylvania
kisley@eniac.seas.upenn.edu
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From: gervprado@aol.com (Gervprado)

Newsgroups: alt.sci.physics.acoustics
Subject: Re: Help on Periodical signals!
Date: 16 Mar 1995 20:45:27 -0500
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Try making a "spectrogram" of the signal. The spectrogram is a three
dimensional representation of the signal - it's axes are time, frequency and
amplitude. The easiest way to make one is to take successive FFTs of the
signal, say once per second.

After you have accumulated N (where N=the desired number of samples) FFTs and
computed the magnitude squared of each you will have enough data to make the
plot which will be in the form of a three dimensional surface or a color (or
gray scale map).

The peak of this surface or color map will trace the evolution in time-
frequency space of your non-stationary periodic signal. Most good signal
analysis programs, like Matlab have canned routines do do this kind of
analysis. Good luck!
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