Klarinet Archive - Posting 000232.txt from 1996/10

From: Roger Shilcock
Subj: Help me out, Roger (fwd)
Date: Wed, 9 Oct 1996 13:00:02 -0400

---------- Forwarded message ----------
Date: Wed, 9 Oct 1996 08:47:01 -0600
From: Steve Prescott <mipresc%RUBY.INDSTATE.EDU@-----.UK>
Subject: Help me out, Roger

Roger,
I wrote:

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

You wrote (or someone did):
>
>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.
>
In reference to my 1st statement...you didn't write the entire thought,
Unless I didn't include it. It is as follows: (I'm quoting Moravcsik) "The
speed of sound in a given medium also depends on the density of the
material, since the ease of transmitting the oscillation from one molecule
to another can be influenced by how close these molecules are to each other
(is this not density?) The speed also depends on the elasticity properties
since, again, the propagation of oscillations from one part of the material
to another is influenced by the elastic properties. In gases the effect of
elasticity (corresponding there to pressure) approximately cancels the
effect of density, and so the speed of sound in gases does not depend very
much on density."

Now for another view: According to The Musician's Guide to Acoustics: The
authors state that because (refering to helium again) helium is much
lighter than nitrogen, (here's the rub), its density is lower and the speed
of sound in the helium (actually helium-oxygen mixture) is nearly twice as
high as in normal air.

So, Roger, help me out here...which statement is correct? I'm more
interested in why my last statement (The Musician's Guide) is flawed.

Steve.

Steve Prescott
Instrument Rep.Tech./ Clarinetist
Indiana State University
mipresc@-----.edu

Steve:
That's not the bit I wrote, but .... I think the point about oxy-helium
mixtures is that pressures are not equal. This is rather nebulous -
I mean that a breathable oxy-helium mixture would have to be at a much
lower pressure to be equivalent in oxygen partial pressure to our normal
ground-level atmosphere, and also have a lower oxygen content by volume.
This is related to the lower density of helium.
I think.
Of course, I've forgotten so much physics that precisely the opposite may
well be the case.
Roger Shilcock