Klarinet Archive - Posting 000220.txt from 1996/10

From: Roger Shilcock
Subj: Re: CO2 influencing pitch (fwd)
Date: Wed, 9 Oct 1996 03:55:15 -0400

---------- Forwarded message ----------
Date: Tue, 8 Oct 1996 19:18:00 +0200
From: Franck Laloe <laloe%peterpan.ens.fr@-----.UK>
Subject: Re: CO2 influencing pitch

Addendum (the boring physicist trikes again!)

At 11:36 08/10/96 -0600, you wrote:

>
>Speed of Sound in Different Media
>
>"....The speed of sound in a given medium also depends on the density of
>the material,

True for liquids and solids, not for gases. As I posted before, in gases
what counts is the velocity of the microscopic particles, their number per
volume does not. The density is irrelevant, and this useful property is used
in some laboratory devices.
As for the temperature dependence, it of course exists, but it is not very
pronounced; the velocity is proportional to the square root of t+273, so
going from zero degree to 30 degrees changes the velocity by only 5 per cent.

>will be given by the speed of sound in a gas, divided by the characteristic
>wavelengths of the resonant vibrators. Since the speed of sound is now
>three times larger than it was in air, the frequencies will also be three
>times larger. The resonant vibrator is the main determinant.

Yes, this is basically correct. With hydrogen, the effect would be even more
pronounced. But please do not try!!! Beware of explosions. Helium is safer.

I promise, I will not post again on physics for at least a century! Franck
Franck Laloe, labo de physique de l'ENS, 24 rue Lhomond, F75005 Paris (France)
tel 33 (1) 47 07 54 13, fax 45 35 00 76 -- laloe at physique.ens.fr

This stuff relates to ideal gases, as far as I know. Hydrogen and helium
are pretty close to being such - carbon dioxide and water vapour
certainly are not. Calculations involving gas mixtures tend to invoke the
Law of Partial Pressures, which, again, ain't necessarily so. Perhaps some
prectising physicist or physical chemist will say when or if these
deviations are relevant?
Roger Shilcock