http://www.answerbag.com/q_view/2485318
I disagree with that answer. It is not the velocity of sound which
affects sound transmission, it is the acoustic impedance. This
property is the "stiffness" of the air.
All air pressures will transmit sound and there is a table giving the
speed of sound vs altitude on
http://www.aerospaceweb.org/question/atmosphere/q0112.shtml
However, for sound to transmit, you need a transmitting device
(loudspeaker) and a microphone. The human voice won't work much beyond
10,000 metres because the human is over the survival zone.
Loudspeakers have a light cone which gives a few percent efficiency in
free air at sea level because there is an acoustic impedance mismatch.
The mass of the cone assembly is much higher than the mass of the free
air it moves, but if horn loading is used, you get a better impedance
match and a higher efficiency. Microphones similarly have an impedance
mismatch but we can get over that by amplifying the capsule diaphragm
voltage.
There is an "acoustic ohm" which measures acoustic impedance and so
you can think of feeding a high impedance mic into a low impedance
mixer and then feeding that low impedance output into a high impedance
loudspeaker. Doubly inefficient.
To actually answer the question, there is no particular altitude at
which sound won't propagate, but only a loss of efficiency in
transmitting it. Think of it as the mass of air moving within one
wavelenght eg. 340mm at 1KHz. At 10,000met it is a quarter of the mass
at sea level (but temperature also has an effect).
http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
Very roughly, you would lose three quarters of the level from the
loudspeaker and three quarters of the level out of the the mic. You
could just hear your mate shout as you both asphyxiated.
When Felix Baumgartner broke the sound barrier in freefall, he didn't
feel it as the air was so thin, but he still would have produced a
weak supersonic boom if there was a sensitive mic up there to pick it
up, but it would have to be one with a large diaphragm.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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