Klarinet Archive - Posting 000312.txt from 1998/01

From: Grant Green <gdgreen@-----.com>
Subj: Re: Reeds and Benade (was speed & volume)
Date: Tue, 6 Jan 1998 19:34:28 -0500

At 06:13 PM 1/6/98 -0500, Tim wrote:
>OK, despite my strong intuitive sense to the contrary, I guess I have to
>grudgingly allow that the reed hits the tip of the mouthpiece. It just
>seems incredible to me that the reed tip could flex that far, and I would
>have guessed that the forces caused by the sudden stoppage of the airflow
>would blow the reed apart.
>However, I REFUSE to believe the reed vibrates at the pitch frequency! If
>a reed were capable of generating all the pitches on its own, why would we
>need all those holes and keys? You could make music with a mouthpiece and
>a barrel.
>The reed is vibrating at some reasonably low fundamental, but it is the
>changes in effective tube length caused by the interruption of the holes
>which create the harmonics that make up the various pitches.
>I recommend "Horns, Strings, and Harmony" by Arthur H. Benade, which
>contains a readable discourse on how instruments (including the clarinet)
>produce notes.
>- - Tim Roberts

Great book! My recollection of how the reed operates (from Benade's
explanation) is this:

1. Air pressure is *not* constant everywhere inside the instrument. You
have the same amount of air going in and coming out, but it has to go
through a few constrictions along the way (like through the mouthpiece).
In those constricted regions, the flow must be faster to get a constant
volume of air through. Faster flow means lower pressure.

2. The air flow into the mpc creates a low pressure region inside the mpc.
The difference between the pressure inside and outside slams the reed shut
against the mpc opening, cutting off the flow. This results in a pressure
wave that travels down the bore of the horn. The pressure wave travels at
the speed of sound, must faster than the air is moving through the horn.
At the bell (or the first open tone hole), the impedance of the bore
changes drastically, which causes the wave to be reflected back up the bore.

3. The return wave, coupled with the natural restoring force of the reed,
pops the reed back off the mpc opening, opening the mpc for the next cycle.
The time it takes for the wave to travel down the bore and back is a
function of the speed of sound (relatively constant) and the length of the
bore (which we vary by covering and uncovering holes). This in fact is the
purpose of the tone holes: opening the tone hole causes a change in the
impedance at that spot in the bore sufficient to cause the wave to reflect
at that point.

4. Since it is the return pulse that opens the reed, the reed must in fact
vibrate at the same frequency as the sound generated.

5. Clarinets (and other closed-end cylindrical bore instruments) have an
additional detail that I remember vaguely: the wave is reflected out of
phase, and actually has to make two trips up and down the bore before it is
in the correct phase to open the reed. This is why clarinets sound an
octave lower than conical-bore instruments of the same size (e.g., oboe).

6. The above all applies for dynamic levels above mf. When the air flow is
too slow to completely close the reed (i.e., the reed is stiff enough to
resist being sealed against the mpc), the reed must still vibrate enough to
cause a variation in the pressure inside the bore. The wave travel up and
down the bore is the same, so the reed will be pushed by the return wave at
the wave's frequency. This resonance essentially insures that the reed
vibrates at the frequency played.

At least, that's how I remember the explanation :-)

Grant

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Grant D. Green gdgreen@-----.com
www.contrabass.com Just filling in on sarrusophone
Contrabass email list: contrabass-list@-----.com
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