Klarinet Archive - Posting 000535.txt from 2001/02

From: Grant Green <gdgreen@-----.com>
Subj: Re: [kl] Difference Tones
Date: Wed, 14 Feb 2001 20:46:55 -0500

>At 05:34 PM 2/14/2001 +0000, Tony Pay wrote:
>(I wrote)
>> > In any case, if a difference tone can be generated that makes physical
>> > objects shake, including eardrums, microphone diaphragms, walls, etc.,
>> > I just have to believe that it exists in space beyond my own head.
>
>I'm not sure if I follow completely, but if the combination of two
>pipes causes objects to vibrate at a frequency P-Q (among others),
>the transmission of that vibration must happen in one of two ways:
>direct conduction or air vibration, that is, sound waves. Suppose
>you ignore the possibility of sound waves conducting that vibration
>TO the object. What happens when the object(s) itself (eg., the
>walls) vibrating at that frequency due to direct conduction excites
>the air to vibrate at that frequency?

A vibrating wall could either reinforce or cancel (partially or
totally) the vibration produced by the pipes, depending the phase.
If the sound waves from the wall are out of phase with the waves
through the air, you'll get cancellation. Unless the wall is
*really* vibrating, I don't think its likely that the wall would
cancel much of the air vibration.

If the wall material reacts non-linearly, it should produce a
difference tone (heterodyne components). Same goes for the bodies of
the audience...

Of course, I don't think we've established that the 64' resultant is
actually the cause of the shaking: it could just be the fact that an
additional 32' pipe is speaking. I'd like to know if the church
shakes when you play two of the 32' pipes that aren't a fifth apart.

Grant

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Grant Green gdgreen@-----.com
ecode:contrabass http://www.contrabass.com
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