Klarinet Archive - Posting 000180.txt from 1998/04

From: avrahm galper <agalper@-----.com>
Subj: THE REED'S MODE OF VIBRATION (END)
Date: Sun, 5 Apr 1998 04:44:47 -0400

The Reed's Mode of Vibration (McGinnis and Gallagher)

This is the end of the article.

RESULTS
1) During a large part of its cycle, the flow of air into the mouthpiece
is completely cut off by the reed This is more obvious visually.
The only source of light used in making the photographs was the mercury
arc of the stroboscope, which was placed to show particularly the slit
between the reed and mouthpiece.
This complete closure of the reed apertures is in disagreement with the
assumptions that the chink between the mouthpiece and the reed never
shuts completely.

It also conflicts with statements that the clarinet reed never entirely
shuts off the ingress of air into the mouthpiece while the instrument is
sounding.

2) The motion of the reed during the complete cycle is of interest.
Consider that the chink is just on the point of closing.
With the aperture closed, the reed appears motionless to the eye for
about half the time of a complete cycle.
It then leaves the mouthpiece with relatively high velocity and reaches
its position of maximum displacement in a series of short spurts.
The time spent apparently motionless at maximum displacement is roughly
a quarter of the fundamental period
The tip of the reed now returns to the mouthpiece in another series of
short spurts, and the fundamental cycle is complete
Thus the actual motion of the reed occupies only about a quarter of the
entire period.

3)The fundamental frequency of the reed vibration as measured with the
calibrated stroboscope is the same in all ranges to within one percent
as the fundamental frequency of the tone produced. The latter was
measured with a frequency meter. This might appear surprising since
harmonic analysis shows that the energy associated with the fundamental
is practically negligible for all loud tones.

4) For a tone of good quality the tip of the reed vibrates as a unit,
e.g. it remains parallel to the facing of the mouthpiece.
Tones of poor qualities are associated with variations from the normal
mode, and they may be encountered under a variety of playing conditions.
If the blowing pressure is not brought up to the proper value, other
small and very rapid vibrations are superimposed on the normal mode.
These produce inharmonic partials which are undesirable.
Still smaller blowing pressure may produce a "squeal". Here the tip of
the reed no longer vibrates as a unit, but in parts along its width with
a central nodal line parallel to its length.
A poorly adjusted embouchure will cause this same sound at even higher
blowing pressures.
Good quality seems to be associated with symmetry both as regards
embouchure pressure and reed design.
The results agree in all respects with the judgements of practical
clarinet players.

Avrahm Galper

http://www.sneezy.org/avrahm_galper/index.html