Klarinet Archive - Posting 000186.txt from 1996/10

From: Leonardo Fuks <leonardo@-----.SE>
Subj: Re: CO2 influencing pitch
Date: Tue, 8 Oct 1996 15:29:04 -0400

Sorry for this long posting, but many questions were raised at the same time...

Morrow said:
<Perhaps I am just unenlightened, but, in the absence of a more
<colourful Dan Leeson characterization, I find this all sounds like a lot of
<nonsense!
< First of all, I have read NOTHING in this discourse that could even
<remotely suggest that any change in pitch could be related to the CO2
<content of the air being forced through the clarinet. Everything I've read
<here - from belching to breath-holding- sounds more like the effect of
<proper air-column support and effective use of the diaphragm (THAT's why
<you'd wait a couple of seconds after taking your breath before attacking)
<on the pitch rather than from the gas mixture in that air in our lungs!
< Secondly, even if sound travels faster or slower in CO2 than in N2,
<it is completely irrelevant because we are not LISTENING to that sound
<travelling through CO2: we are listening to it travelling through AIR
<(N2). So, unless you are a volcano and the amount of air you blow through<
<your instrument can drastically affect the air composition in the room
<around you, IT DOESN'T MAKE ANY DIFFERENCE!

Dear Mr. Morrow, I respect your point of view, but I think that some
comments could be done on your message. The air column of the instrument
defines the resonating frequences of the tone. If you have a cylindrical
tube of 0.25 meters (effective length, including end corrections), the
wavelength will be four times this value, 1 meter. The frequency is:
fo= c / lambda ; where fo is the fundamental frequency , c is the sound
speed and lambda is the wavelength .
The higher the value of c (which depends on the air that fills the air
column), the higher the pitch obtained.
The reed vibrations of your clarinet (unless you are producing a squeek
sound) are completely enslaved by the instrument's air column. This is
called a feed-back instrument.

As for the role of the diaphragm in wind instrument playing, it is not a
very clear matter, some players use transdiaphragmatic pressure in many
occasions while others have the diaphragm as an almost flaccid muscle
between abdomen and thorax (during expiration). If you have any scientific
works on that, it would be interesting to have their references.

Dilley wrote:
< Surely its the speed of sound in the column of
<gas inside the clarinet and probably inside the player that is relevant
<here not the gas through which it travels to your ears. If you breathe in
<helium and then speak your voice is higher pitched. You don't have to fill
<the entire room with helium to get the effect.

The human voice organ in not a feed-back instrument. There is no strong
acoustical coupling between larynx and the vocal tract. The vocal tract
(which includes part of the larynx above the vocal folds, pharynx, mouth,
nasal cavities) acts as a complex filter that mainly shapes the source sound
from the larynx. For each configuration of the vocal tract, a different
acoustical filter is "built", which correspond to some peaks at certain
frequencies, called the FORMANTS. That is how we make the different vowels,
even if the same pitch is mantained.
When a person inhales helium before speaking, there is no relevant change in
pitch but in the formants. There are many papers written on the matter, some
of them by Gunnar Fant, who is worldwide known on the field of phonetic
acoustics and a professor in my department. As the sound speed in He is
about 2.85 times the speed in air, it affects the resonance of the mouth and
nose cavities. So the laringeal sound produced by the vocal folds (wich
vibrate in the same fashion as they don't depend on the air column) is
passed through a modified filter, that makes the voice with those particular
(and funny) qualities.
I have tried to play the clarinet and oboe with my lungs filled with Helium
and the resulting sounds were transposed notes (up to a sixth above as I
did), according to the He-air mixture. If it was pure He, the pitch would
increase, theoretically, one octave and an augmented fourth ( 1813 cents
above normal air column).

I think that this is a fascinating discussion and it deserves scientific
attention, which could also revert into an objective fact for musicians. As
I keep in my research on the CO2 effect on pitch I expect to confirm or
discard this hypothesis. This needs theoretical background and also
experimentation (there may be a dependance of loudness, blowing-pressure,
air-column length, bore design, etc. for the effect).

All the best,

__________________
Leonardo Fuks
Music Acoustics PhD Programme
Royal Institute of Technology
BOX 70014 , Drottning Kristinasvag 31
S-10044 - Stockholm - SWEDEN
ph-(46)(8) 7907597 ; fax (46)(8) 7907854
home: 46 8 7369905