Klarinet Archive - Posting 000977.txt from 2000/03

From: avrahm galper <agalper@-----.com>
Subj: [kl] Reed's mode of vibration
Date: Thu, 30 Mar 2000 07:32:51 -0500

This article appeared in the Clarinet Quarterly, published by James
Collis

THE REED'S MODE OF VIBRATION

The clarinet is generally considered as a closed pipe with its tone
containing only odd-numbered harmonics.
However it was shown by Blaikley in 1884 and later by Miller, that even
numbered harmonics were also present so that a revision of the simple
theory was necessary.

Since there is little agreement on the facts, which seem necessary for
development of a theory, it seems essential to investigate the mode of
vibration more closely.

CLARINET, REED AND MOUTHPIECE.
The reed was mounted upon the mouthpiece of a Bb Boehm system clarinet,
which is the instrument and system in most common use.
The reed before being used with the artificial embouchure, was designed
to operate successfully with the natural embouchure
The mouthpiece had facing of medium length to which the reed was
carefully adjusted.

THE ARTIFICIAL EMBOUCHURE
Obviously, it was necessary to make visual and photographic observations
of the reed while the instrument was sounding.

The device that made this possible was a chamber with transparent walls,
and an artificial embouchure to simulate the action of the player's lips
and teeth.
The device consisted of a metal cylinder, diameter 3.0 inches and
length 12.5 inches
Windows of transparent plastic on opposite sides and also on one end
allowed easy viewing and illumination for stroboscope study.
As an embouchure, a rubber pad adjustable along the reed and also
perpendicular to its plane was applied
against the reed to simulate the lips, and pressure was applied to the
pad by a screw to simulate the teeth.
All adjustments would be made from without while the reed was in motion.
During the operation, the necessary moisture of the reed was maintained
by saturated cotton placed in the cylinder.
The blowing pressure from an air compressor was controlled by stop-cocks
and measured by a mercury manometer
The pressure was from 1.6 cm to 3.6 cm of mercury.

THE STROBOSCOPE

The Edgerton stroboscope has a flash speed of 60 cycles per second,
which is controlled by a sychronous motor contactor.
This permits accurate measurements of fundamental reed frequencies over
the entire range of the clarinet.
The main action of the stroboscope is to apparently stop or cause slow
motion of the reed in space so that the motion may be studied visually
as is done with"slow motion" in the movies.

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.
This description agrees in a general way with the results predicted by
DAS in his mathematical development, which however were without
experimental confirmation

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.

There appeared on the Clarinet internet recently an article in German
with pictures of a contraption mentioned above. I wish someone would put
in an English translation. It should be very interesting.

Avrahm Galper
Avrahm Galper
CLARINET TONE TECHNIQUE AND STACCATO
CLARINET UPBEAT SCALES AND ARPEGGIOS
EINE KLEINE KLEZMER MUSIK
http://www.avrahm-galper.sneezy.org

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