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Re: Gongs that change frequency in the night



Hi;

There are also some non-linear effects having to do with amplitude
dependent frequencies. A nice, simple example (in a AJP article a few years
ago that I can't quite lay my hands on right now) is a tapered spring
doorstop. As the amplitude decreases the frequency (and pitch- which are
not neccessarily equivalent!) shift upwards.

kyle
At 9:26 AM -0700 11/10/98, Roger Haar wrote:
HI,
My first guess is that at the initial strike the fundamental and
various
harmonics are excited, but the fundamental is damped more quickly. The
test for
this would be to record the sound and do FFT's on various intervals. If
you have
a sound card in a PC, you can record the sound easy enough. One might be
able to
see the damping of the fundamental by looking at the wave form. If you
can get
the numbers out you can use the FFT in Excel.

There are a number of software packages in shareware or freeware sites
that
run the soundcard and produce a real time FFT. Of those I have used ( DOS
under
Win95) ( time on horizontal, FFT frequency on vertical, intensity in
color) are
at:

http://oak.oakland.edu/simtel.net/msdos/sound.html

and is called specgrm1.zip OR specgrm2.zip

For Windows 95, "freq.zip" OR " gram42.zip" at
http://oak.oakland.edu/simtel.net/win95/sound.html looks good.

***
I just made a quick test with a 4 ft. long, 3/4 in diameter rod.
Depending
on where I supported it, I could vary the damping of various harmonics. I
also
saw a difference in frequencies for tapping it on the end or the side. (
longitudinal and transverse) Also I occasionally observed beats, or more
exactly, oscillations between two modes ( maybe the longitudinal and
transverse
but I did not have the microphone close to the end when I observed the beat
pattern so the longitudinal was not detected)

Thanks
Roger Haar
U of AZ physics

*******************************************************
Dean Livelybrooks wrote:

Colleagues,
I am teaching an intermediate-level lab for our physics majors focusing on
vibrations and waves (text for associated lecture course is Vibrations and
Waves in Physics, by Main).
We are into the project portion of our lab course. I seek information
about percussion devices such as gongs or wind chimes that seem to change
pitch after they are struck. My fuzzy image is of some big-muscled guy in
striking the giant gong and its predominant tone increasing in frequency
during the 1-5 seconds after it is struck.
Probably they are constructed to have two (or more) modes that are the
loudest of all modes and are close together in frequency. Intentionally
damping the lowest-frequency mode to decay faster than the next mode up
would cause the ear to perceive a change in "fundamental" pitch. I imagine
that they might add mass to the locations of the antinode(s) for the lowest
mode which, hopefully, doesn't correspond to an antinode of the next mode
up (in frequency).
Does anyone know about these things--- references, how-to instructions, etc.
I am hoping to get my students to build one, analyze how it vibrates, and
do sliding FFTs of its sound vs. time. If it works well enough, they would
make great Christmas presents.
Thanks!

_____________________________________________________
Dr. Dean Livelybrooks "God is subtle, but he
is not
Department of Physics malicious." Albert Einstein
1274 University of Oregon (541)-346-5855
Eugene, OR 97403-1274 USA (541)-346-5861-FAX


-----------------------------------------------------
kyle forinash 812-941-2390
forinas@indiana.edu
Natural Science Division
Indiana University Southeast
New Albany, IN 47150
http://Physics.ius.indiana.edu/
-----------------------------------------------------