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Name: Dean
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
First, I would like to thank each of you for your volunteer time. The world is enjoyable because of volunteers like yourself.

Question: How can an orderly sound wave travel through a disorderly medium (air - kenetic theory says is disorderly)and reach our ears orderly again?

Dean


Replies:
Sound is transmitted through air because the wavelength of sound [ I remember the limits for human hearing in terms of frequency [~20 to 20,000 hertz] is much longer than the mean free path of the molecules of N2 and O2 in the air, so the sound wave "sees" air as essentially a continuous medium. If the wavelength of the sound were of the order of the mean free path, it would not be transmitted.

Vince Calder


Sound (and pressure, generally) is the average behavior of a large number of air molecules.

Tim Mooney


Hello,

There are at least two aspects in transmission in a medium that should be considered: the nature of interactions, if any, and its extent.

In the case you describe, air molecules do have small random motions and they may also diffuse or move as a result of external influences. However, the energy of sound that disrupts and cause to transmit sound in air is much higher than kinetic energy of air molecules. Thus, sound is transmitted in the random air environment. After some distance, however, sound waves lose their energy and the sound fades away. With proper instruments, we can show that in addition to this attenuation (i.e., fading with distance traveled), air molecules introduce some some noise in the sound being transmitted. The noise level is too small to be audible.

AK



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