Re: [ATM] whale song

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Jim Burrows wrote:
>>I've been down here in the basement listening to my polishing machine
>>working on the 30" for the last week. Now that its down to slowly
>>removing pits from a smooth surface, the lap is singing its heart out. I
>>recorded a few bars, filtered out the motor hum, and uploaded it.
> 
> I got some sounds - not like that, but the usual polishing squeaks 
> plus an unusual low pitched hum - in a slightly <g> different 
> situation, figuring a 4 1/4" RC cass secondary on a pottery wheel 
> using a starlap.  The mirror surface came out surprisingly 
> asymmetric, lots of tetrafoil.  When you get beyond simple polishing 
> into figuring, I'd suggest some sort of 2D test to see if the whale 
> sounds are doing that.

You may be seeing the acoustic modes of the glass disk!

Depending on the frequency, the glass will oscillate in different 
modes, that is, in different ways.  Of course it all depends on the 
pressure, damping, etc. of the glass, but one could conceivably get 
many different types of oscillations going and get some very wild 
shapes.  Circular plates can oscillate in many, many ways.

Here's an interesting simulation to play with:
   http://www.falstad.com/circosc/

Whenever a mirror I am working on starts to "sing", I interpret it as 
a cry for help.  I make sure it stops quickly by slowing the work, 
adding polishing compound, water, etc.  I do not wish to have the work 
take on strange shapes, or to propogate internal microfractures 
through mechanical oscillation of the part.  Nor do I wish to have 
things dry out and seize up.

	Mike Lockwood


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