Klarinet Archive - Posting 000848.txt from 2000/09

From: Bilwright@-----.net (William Wright)
Subj: RE: [kl] Reed behavior on mouthpiece
Date: Tue, 26 Sep 2000 17:49:42 -0400

(Lest you think that I sit at my keyboard all day, I practice in
the same room as my keyboard, and when I need to rest my chops, I log on
for a moment to see what's cooking.....)

<><> Karl Krelove wrote:
I am having trouble at an intuitive level visualizing how a reed
vibrating as it does in a periodic motion could continue to increase the
distance through which it vibrates (amplitude?) once it begins hitting
the mouthpiece rail. [snip] I don't think I can make it louder with
greater bow speed or pressure once [something] begins to interfere with
its vibration.

That's a key point! The reed is an *air valve' as well as an 'air
beater'. There are many different things going on simultaneously, but
one of the most important event chains is:

(1) Air enters the mouthpiece through the air valve (the opening between
rail and reed).

(2) As air continues to push down on the 'outside' surface of the reed,
the reed begins to bend towards the rail. Lip pressure also pushes the
reed toward the rail. Eventually (if there is enough force to overcome
the reed's stiffness), the reed makes contact with the rail. That is,
the air valves closes. No more air enters the mouthpiece.

(3) So why doesn't the reed remain 'closed' until the player relaxes his
or her embouchure and/or stops blowing?

(4) .....First, the puff of in-rushing air created a pressure front
inside the mouthpiece. This pressure front traveled down the clarinet
body (a 'wave'), but it also pressed back against the reed. Assuming
that the embouchure wasn't clamped down too hard, the pressure surge
will open the reed (that is, open the air valve) and the cycle will
repeat.

Second, when the reed was nearly closed, the air was moving rapidly (a
large amount of air was rushing through a small opening). This was
creating a venturi effect (such as keeps an airplane in the air) that
helped to pull the reed down onto the rail (that is, close the air
valve). When the valve is closed, no air is moving and the venturi
effect disappears. This allows the reed to pop open to its 'resting'
position. The cycle repeats.

(5) So what controls the amplitude of the wave (loudness of the tone)?
Lots of things contribute, of course -- we haven't talked about how the
waves exit from the instrument, nor how successive wave fronts add
together, and so forth. But certainly the amount of _pressure_ created
initially inside the instrument by each puff of air is a key factor.
How hard did the player blow in the first place? The more pressure that
the player inputs, the greater the amplitude of the pressure wave inside
the instrument --- which is the same thing as a "louder sound".

As you described, the reed also has an 'egg beater' effect that
stirs the air, and the physics of how these two effects (egg beater and
air valve) combine is too complex for my meager understanding. Benade
spends some time discussing this issue that you've raised, but it's over
my head except to acknowledge that it exists. I believe I am correct in
saying that the air valve aspect has more effect on volume than the egg
beater aspect does. That is, it accounts for more of the pressure
variations inside the instrument.

Cheers,
Bill

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