Klarinet Archive - Posting 000313.txt from 1998/01

From: "Luanne S. Jacobs" <wwplayr@-----.net>
Subj: Re: Reeds and Benade (was speed & volume)
Date: Tue, 6 Jan 1998 20:35:38 -0500

I believe the proper spelling is Bonade!

Grant Green wrote:

> 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
>
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> Grant D. Green gdgreen@-----.com
> www.contrabass.com Just filling in on sarrusophone
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