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From: Michael Will
Date: Tue, 10 Sep 1996 20:22:01 -0700
Subject: Re: Further thoughts on spectrograms

Siegfried Naruhn wrote:

> I. Precondition for a reed vibration is the direction of the playing wind which
> has to come from the side the reed is attached on the plate. That means, the
> blow reed is placed on the plate inside the chamber because the wind comes
> from this side.
>
> II. A complete reed vibration has four clear distinguishable phases.
> a) Thanks to the reed offset the reed bends down to the slot and initiates
> simultaneously it's so-called "self-excitation". This first bending to
> the slot is caused by so-called "Bernoulli-forces".

The Bernoulli principle tells us that the faster the air speed, the
lower the pressure over the surface.

> b) When the reed reaches it's amplitude proportional to the playing
> pressure, the movement direction of the reed turns and the reed moves
> down to the slot, parallel to the wind direction.
> c) The reed vibrates through the slot and covers it for a moment completely
> until the gap tolerances. In this situation the air flow is extremly
> slowed down and effects a pressure difference. This pressure difference
> acts accelerating into the direction of the reed movement and enables the
> energy transfer for a constant vibration.
> d) When the reed has reached the opposite point of culmination, it changes
> again it's movement direction, moving back to slot against the wind
> direction for the second time. When the reed closes again the slot, a
> pressure difference occurs for the second time. But this time the
> pressure difference acts stalling to the reed movement because the reed
> moves against the wind direction.
>
> The phase d) is the most interesting vibration situation in case of the longer
> reeds (= lower frequence). Only the longer reeds vibrates completely through the
> slots causing the higher pressure differences which proportionally effect a
> greater movement stalling.
>
> The extract of this very complicated situation of phase d) is: the longer the
> reed, the longer the chamber, The longer the chamber, the higher the pressure
> difference. The higher the pressure difference, the bigger the stalling effect
> to the vibration.
>
> With other words:the stronger the wind and the longer the chamber = reed, the
> more the pitch must go down. Voila!
>
> To exemplify the proportionality between wind pressure and chamber length, I
> mention the *blow*pipes of the Amazonas indians. These pipes are about 2 meter
> long. That means, with a short pipe they couldn't achieve the necessary pressure
> to shot their preys from the trees. Besides, a rifle has a greater range of fire
> than a colt.
>
> III. The blow reed vibration starts with the overpressure in the lungs. The
> diminishing of the oral cavity is an additional possibility to increase the wind
> pressure and that's exact the way to bend down a blow note. Any thoughts?

This is much like the pressure oscillations I have written about a few
times. My hypothesis is that these pressure oscillations are what
couple the reeds together during a bend so that they vibrate at the same
rate. If the wind velocity speeds up due to constricting the air
passage, then perhaps the pressure is decreasing. A decreased pressure
might slow the oscillation and cause the pitch to get lower during the
bend.

Mic'l