Subject: Sub harmonic vibrations – In a thin steel ring?

Date: Thu Jan 18 10:32:13 2007
Posted by Randy
Grade level: nonaligned School: No school entered.
City: China Spring State/Province: Texas Country: United States
Area of science: Physics
ID: 1169141533.Ph
Message:
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I've googled around and can't find much on sub harmonics.

I turned a thin steel ring from a pipe on a lathe at work. The ring is about 8

The speed of sound in steel is about 6000 meters per second.

I was wondering if I could induce sub harmonic resonant vibrations in this
steel ring.

My plan was to suspend the ring via a magnetic suspension, and then to excite
it with a small electromagnet at some sub harmonic frequency (say, at 1/2 the
fundamental, ~ 11 kHz). Record the sound coming from the ring, and perform an
FFT on the recording to get a frequency power spectrum. However, I was never
able to fabricate a magnetic suspension system which could “float” my ring.

The path length of my ring is approx: c = pi * d ~ 27cm = 3.14 * 8.5cm.
At 6000 meters per second, my fundamental frequency should be close to:
6000m/sec / 0.27m = ~ 22 kHz.

As different from a “stretched string” supported at two end points, in which
case there are no frequencies smaller than the fundamental. I suspect that I
should get not only integer multiples of the fundamental, but because the
system has no end points, also fractional harmonics – 1/2, 1/3, 1/4…

I can “tap” the ring and hear frequencies that I know are less than 22 kHz.

My question is: “Does anyone know if I should get sub harmonic resonant
vibrations in such an experiment?”

~Randy

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Re: Sub harmonic vibrations – In a thin steel ring?

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