Klarinet Archive - Posting 000185.txt from 1996/10

From: Franck Laloe <laloe@-----.fr>
Subj: Re: CO2 influencing pitch
Date: Tue, 8 Oct 1996 15:29:03 -0400

At 11:33 08/10/96 -0400, you wrote:

> The reason your voice sounds higher when you inhale helium is that
>the helium tightens your larynx (or something close to that. Is there a
>doctor on the list...?), NOT because sound waves travel faster through the
>helium in your system! Your voice sounds higher pitched because it IS
>higher pitched! I agree that the speed of the air through the clarinet has
>a great deal to do with the sound, but this is AFTER it leaves your mouth,
>and I still maintain that the composition of gases should have no effect on
>pitch!

I do not completely agree. There might be a small effect related to the
shape of the larynx, this I do not know (I am a physicist, not a medical
doctor), but the major effect is simply that the resonance frequency of an
air column depends on the speed of sound of the gas which fills it. The air
column may be contained in a metal cylinder, a clarinet, or the vocal tract,
it works exactly the same way.
Basically, the speed of sound in a gas is nothing but the thermal velocity
of the particles (molecules or atoms) which make this gas (within a
numerical factor of order one which does not matter here). The heavier the
particles, the slower they are. Since the helium atoms are almost the
lightest of all atoms which exist (hydrogen is ligher, though) , the speed
of sound is helium is higher than in most gases. It is about 960 meters per
second instead of 340 for air.

What counts for determining the pitch of the sound emitted by a clarinet is
the resonance frequency of the gas column, the nature of the gas outside is
almost irrelevant. So, if I go to my lab with a clarinet and fill it with
helium, its pitch will raise by something like an octave plus a fifth,
amusigly the same effect as the register hole! It is certainly not necessay
to fill the whole lab with helium to hear the effect (fortunately!).

Concerning what happens during the first half second a clarinet tone is
emitted (its pitch goes down, as we all know, and then stabilises), this is
another interesting question. Personally I have been intrigued by this
observation for a long time, and I do not know. The effect is easily seen
with an electronic tuner, less esay to hear since all clarinettists are so
much used to it that they consider this as part of the tone color of the
instrument. To avoid the effect, I tried to correct the embouchure, to
change the pressure, all sort of thigs, and unless you "cheat" by
introducing enormous corrections (similar to those of a glissando), the
effect is still there. It is not related to the temperature of the air
column, which presumably goes up and not down. Maybe indeed it is related to
the Co2 content of the air we exhale? Another possibility is to invoke what
musical acousticians call the "volume equivalent" of the reed: a clarinet
sounds almost a half tone below the resonance frequency of the air column
dut to this effect. This I am sure of since it turns out that I made a few
measurement in my lab on this frequnacy shift, and moreover it was known
before. So mabe the effect is a brief "modulation" of the equivalent volume
at the beginnig of the sound? Personally I find the Co2 explanation more
simple and therefore more attractive, but the truth is that I do not know.
Dear listers, dear musiciansn, forgive a modest physicist for being so
boring with science on a beautiful list centered on art and our belowed
instrument. Best wishes to all
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