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'[EE]: Music to my Ears'
2000\10\24@091157 by Lawrence Lile

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I need to design a resonant cavity for sound waves.  (Yes, it will have a
PIC in it) (no it is not a toaster)   I remember in the old days when the
rage was to build your own stereo speakers there were a set of calculations
that would determine resonance of the speaker cabinet at certain
frequencies.  Anybody done this lately?

When a musical instrument, say a Flute, makes a tone, I guess it is
resonating.   For a relatively simple case, like a cylindrical instrument,
there must be a relatively straightforward calculation to determine the
resonant frequency.

The project is part of a sensor that will determine the state of a cavity by
interrogating it with a sound wave (more I cannot say!)


-- Lawrence Lile
Sr. Project Engineer
Salton inc. Toastmaster Div.
573-446-5661 Voice
573-446-5676 Fax

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2000\10\24@155423 by Andy Howard

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----- Original Message -----
From: "Lawrence Lile" <spam_OUTllileTakeThisOuTspamTOASTMASTER.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Tuesday, October 24, 2000 2:10 PM
Subject: [EE]: Music to my Ears


> I need to design a resonant cavity for sound waves.  (Yes, it will have a
> PIC in it) (no it is not a toaster)   I remember in the old days when the
> rage was to build your own stereo speakers there were a set of
calculations
> that would determine resonance of the speaker cabinet at certain
> frequencies.  Anybody done this lately?
>
> When a musical instrument, say a Flute, makes a tone, I guess it is
> resonating.   For a relatively simple case, like a cylindrical instrument,
> there must be a relatively straightforward calculation to determine the
> resonant frequency.
>
> The project is part of a sensor that will determine the state of a cavity
by
> interrogating it with a sound wave (more I cannot say!)


Ah, you're building a submarine too then?  :o)



Have a look at
http://hyperphysics.phy-astr.gsu.edu/hbase/waves/cavity.html#c4  which
touches on what you want.

If your cavity is a room you want to treat acoustically then
http://www.tubetrap.com/articles/db1191.htm might help.

And an acoustics FAQ with links to dozens of useful resources including some
software that may or may not fit your mysterious bill:
http://www.campanellaacoustics.com/faq.htm.

If that's not enough, or if I've missed your direction, let me know and I'll
check some of my references at home.


Be advised that deriving useful or accurate information from a cavity at
resonance is no simple thing to achieve in practice. Even for regular
cavities there are many easily overlooked variables which can mislead the
unwary. If your cavity is irregular in shape then the level of complexity
increases greatly. This kind of research usually involves lots of MIPs, DSPs
and clever people to analyse the results. Not for nothing are acousticians
considered to be practitioners of the black arts as much as they are
scientists or engineers.

If you talk nicely to the engineers at a large recording studio or major
broadcaster they'll probably be able to recommend an acoustician to you. If
this is for a commercial project it'd be worth the (normally generous) cost
of half a days consultancy just to avoid some of the pitfalls.




.

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2000\10\24@165710 by Null

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There are also some comercial and free box analysers.  Including Blaubox
from Blaupunkt, Bassbox, and others.  I am not sure of the name of the
free ones...sorry.  A search on altavista might turn em up.

These are fairly acurate but will never replace an acousticians!  :-)

Allen

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2000\10\24@170406 by Bob Blick

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> The project is part of a sensor that will determine the state of a cavity by
> interrogating it with a sound wave (more I cannot say!)

Coffeemaker, eh?

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2000\10\25@171036 by Peter L. Peres

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>cavity

The basic law is given by the speed of sound in your gas, and the length
of the cavity taken twice. I.e. the time for a sound to go to the end of
the cavity and return equals one period. But.

Flutes and other air instruments have a very considerable output in
harmonics. This is mostly due to multiple reflexion paths in the cavity.
To avoid them, a structured wall surface helps. Goodbye easy cleaning.

The electronic devices used to excite such a cavity have a self resonant
frequency that is several orders of magnitude higher than the cavity's. To
make them work with the cavity (and not in overtone mode) a serious amount
of low pass filtering is required.

The easiest way to implement such a driver device is using a speaker and a
small amplifier using a microphone. After some testing and gain setting
(you may need to invert the phase on the speaker) it will whistle at the
frequency given by the liquid level. Set as much lowpass on the amplifier
as you can. Such a device's frequency can be measured directly by a PIC.
The Q factor of the mechanical cavity can be so high that you may need to
start oscillation by tapping on the vessel.

good luck,

Peter

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