On 12/09/2013 06:02 PM, Phil Allison wrote:
"William Sommer******"

Here's the answer to your question about Boyle's Law (which you will
ignore, of course)...

PV = k assumes a constant temperature.

** PV= k shows that it does not matter what the gas is - the same
volume changes produce the same pressure changes.

Uh... No it doesn't.

** Yes it does.


k is temperature-dependent.

** Irrelevant when there are simply no temperature changes going on.


So the stiffness of an enclosed volume of gas is the same for all gasses.
The resonance frequency of a woofer will be unaffected by it.


You're quite right that the compressibility of the gas *at constant
temperature* is very nearly independent of the composition, by the ideal
gas law. However, acoustic waves aren't isothermal, they're nearly
adiabatic (i.e. heat doesn't have time enough to diffuse very far in one
acoustic cycle). In an adiabatic process, the compressibility depends
fairly strongly on the number of degrees of freedom of the gas molecule.

There's a quite reasonable discussion at
http://en.wikipedia.org/wiki/Adiabatic_process#Ideal_gas_.28reversible_process. 29

SF6 has lots and lots of degrees of freedom.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

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hobbs at electrooptical dot net
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