From: SFPhysics@aol.com
Date: Thu Nov 25 1999 - 04:01:15 PST
From: SFPhysics@aol.com Message-ID: <0.f1183778.256e7f0b@aol.com> Date: Thu, 25 Nov 1999 07:01:15 EST Subject: Sound in a partial Vacuum?
At what point will the molecules no longer cohesively transmit the energy of
a longitudinal compression/rarefaction mechanical wave? That is quite a
question. As the pressure is decreased the elasticity of the air as sound
modulus is lost to random interactions. On that we can all agree. Answering
at what torr pressure this occurs may be difficult. We know the distance
between air molecule collisions at STP is very small. The Mean Free Path
being 6.5 x 10minus6 at STP (760 torr). Assuming the bell sends out a
wavelength of 3450 Hertz the STP wavelength is 0.1 meters. This is exceeded
when one reaches a pressure of 0.011 torr or 1.1 x 10minus2 mmHg where the
molecular collisions are farther than 0.1 meters. I am assuming, perhaps
incorrectly, that at this point wave coherance is lost for that pitch and all
higher pitches. Adding to the confusion is that this does not have anything
to do with audibility of the sound outside of the bell jar! Transmission
losses drop the sound below human detection long before the pressure goes
below 10% STP in pratical lab demonstrations my classes have done. ;-)
Al Sefl
Guaranteed to be correct 50% of the time!
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