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From: SNaru~-online.de (Siegfried Naruhn)
Date: Wed, 11 Sep 96 03:08 +0100
Subject: RE: Further thoughts on spectrograms

Hello friends,

when Michael and fjm have noticed too the influence of the cover plates during
tuning, then we are at least three harpers with the same experience. This might
be evidence enough that we havn't done "phantom tnning". But unlike fjm, I have
noticed this effect only at blow reeds and I think to have a reasonable
explanation.

That instrument which has a similar sound mechanism is the accordion. But
unlike the harp, many aspects of the accordion sound had been searched
scientifically. There is one point of the accordion sounding which had been
excamined very carefully - the tuning.

As everybody knows, the tuning of a harp is extreme subjective and depends
even on the cover plates as just stated. If one is informed how the harps are
normally "tuned" in the factories, then it's no wonder when almost every second
new harp seems to be already outtuned. I remind only of the neighboured C/C#.

However, when the tuning of an accordion is checked, the playing wind is
produced by compressors which blow the wind with adjustable pressure by means of
tubes to the sound chambers. Under these playing conditions a very interesting
appearance had been noticed which unfortunately remained inexplicable up to now.

When playing the blow notes, only then, a reliance on the pitch of the lower
tone ranges to the wind pressure was evident. With other words: with increasing
wind pressure the pitches go down proportional to the frequence. That means, the
lower the reed frequence and the higher the blow pressure the more the pitch
goes down.

This is exactly the same what happens on harps. Everyone who is busy in harp
tuning will surely have noticed that always the lower reeds cause the problems
to know whether the tuning is okay now, simply said. I have offered an
explanation to an expert who has searched life-long the accordion physics and I
have received no objections, Urgh, schlipfmemsk toek, toek, quasseldoof,
magonolly truep die dock usumeine allzeit furzelmuecke!!! (just to mention Didi
Neumann's strongly improved Pidgeon-German). In fact, I always like "serious
jokes".

Well, I explain this strange behaviour of blow notes with the kind the blow
reeds are attached on the plate. Pat has already described this, when he wrote:

>> This could be a function of the shape of the reed chamber - the "blow"
pressure goes into the chamber before it reaches the opening of the reed; the
"draw" pressure goes throught the reed opening before being shaped by the
chamber.<<

Absolutely Pat, this was my observation too, but this is of course not a
sufficient explanation why the pitch goes down. But a getting to the subject I
saw in thinking about the physical fundamentals of the reed vibrations. In order
to describe what happens until a blow reed produces a sound, I'll differentiate
3 points or sections:

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".
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?

Have a bed - must travel now into

Siegfried