Klarinet Archive - Posting 000452.txt from 2004/10

From: Tim Roberts <timr@-----.com>
Subj: [kl] Re: Air flow
Date: Fri, 15 Oct 2004 18:16:29 -0400

On Fri, 15 Oct 2004 15:58:44 -0400, Adam Michlin wrote:

>It is a mistake to think that the vibration of the reed is the only thing
>required to create a clarinet sound. One can blow into a clarinet quite
>easily and achieve only an "air" sound. The reed is, in fact, vibrating,
>yet there is no clarinet sound?

No, the reed is not vibrating at that point. The air is passing through
the opening, thereby causing the hiss, but there is not enough air
pressure differential to overcome the inherent strength of the reed, and
it remains stationary. That's why we hear white noise instead of a
single frequency.

It's the Bernoulli principle at work: moving air causes a reduction in
air pressure. When the air pressure behind the reed drops sufficiently,
it overcomes the strength of the reed and it begins to move, which
affects the column of air inside the clarinet, causing a feedback loop
that eventually increases the air pressure enough to the point where the
reed's inherent strength allows it to bounce back.

>The answer, of course, is the reed must "strike" something, the mouthpiece.
>To have a reed vibrating without a surface to strike is roughly equivalent
>to the sound of one hand clapping.

This is wrong. The reed only strikes the mouthpiece at dynamic levels
of approximately mezzoforte and louder. When you play softer than that,
the reed vibrates as lower pressure from fast moving air brings it
closer to the opening, and feedback from the air column in the clarinet
allows it to open. The opening never closes completely.

There are several good web sites on reed response to air pressure. I
believe one of them was mentioned just this week.
--
- Tim Roberts, timr@-----.com
Providenza & Boekelheide, Inc.

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