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Klarinet Archive - Posting 000714.txt from 2002/06

From: w7wright@-----.net (William Wright)
Subj: [kl] How a bell works, from Benade (also: Modern Basset Horn designs)
Date: Fri, 21 Jun 2002 12:40:06 -0400

I mentioned 'action at a distance in a previous post.

What I particularly want to point out is that Benade attributes the
bell's effect to open holes BEYOND the length of the air column.
Elsewhere in his book, he discusses that the air column extends and
'oozes' slightly beyond the lowest open hole. It's not a clean line.
Nevertheless, I am left with an uneasy feeling about the 'action at a
distance' flavor of all this.

This quote ("Horns, Strings, and Harmony", Chapter IX) is a bit long,
but it summarizes why I have a spherical bell and why I want to
experiment further with unusual barrels. Benade's statement about
'bulbous' bells is at the end of this quote.

Words enclosed in [ ] are mine. The ( ) and " " are directly from
Benade:

In brass instruments, all the air in the bore vibrates while that part
of it in the bell communicates these vibrations to the outside air, and
thence to our ears. In a woodwind instrument, on the other hand, the
presence of side holes [which we finger] makes the situation quite
different. We must make a distinction between the way sound is
radiated when several holes are open and the way it comes out of the
bell when all of the holes are closed.

[Benade discusses the basics of a simple pipe with holes, I have snipped
most of it ....]

we find that sound is radiated only from the first few open holes on the
pipe (for holes like those on a clarinet, the first two or three holes
are the only ones which contribute much to the total loudness), and this
radiation is not very "efficient" because of the smallness of the
holes. This statement is true for the first half dozen harmonics of an
ordinarily played frequency, but all ingredients whose frequency is
above a certain limiting value "leak out", so to speak, and are radiated
very effectively from _all_ [italics] the lower holes, and as a result
are emitted very efficiently. The exact way in which the change-over
occurs between the two sorts of sound emission depends critically upon
the size and spacing of the open holes

[ THUS MINOR DIFFERENCES IN HOLE SIZE, SHAPE, AND LOCATION MAY CAUSE TWO
CLARINETS TO EMIT DIFFERENT SOUNDS. A PARTICULAR BELL MAY SOUND
DIFFERENT ON DIFFERENT CLARINETS FOR THE SAME REASON. SEE BELOW, THE
BELL REPLACES OPEN HOLES. ]

[....snip about effects of size & shape of tone holes].

You will ask why I seem to have gone off on a tangent dealing with the
radiation of sound from side holes, when the heading of this section
promises an explanation of the bell. The reason is this. Not all the
notes on a woodwind are played with several open holes along the lower
part of the bore, and in particular the lowest note of all is played
with no open hole at all in a normal instrument. The bell is put on
the lower end of the bore to take the place of the missing holes; this
it can do by virtue of the radiation properties that we discussed in the
chapter on brass instruments. A properly shaped bell can have very
nearly the same sort of transition from inefficient to efficient
radiation as do the holes it is designed to replace, and thus it
prevents the emission of unmusical snarling noises that would come out
of an abruptly ended bore.

[snip about flutes]

One of the most interesting experiences I have had in recent years was
to hear the change in tone quality produced by a clarinet mouthpiece
that was attached alternately to a pipe having a row of _closed_
[italics] holes all the way down to its end and a similar though longer
pipe with enough holes open at the lower end to make it sound the same
pitch. The pipe with several open holes sounded very much like a
normal clarinet, while the other one was reminiscent of the dull noises
you get when trying to play a piece of conduit as a bugle. It is the
side holes which turn a wooden pipe into a musical instrument, and the
bell is a portable substitute for an elongated bore with unused but open
side holes.

The bulbous or pear-shaped extension at the foot of an English horn does
not fit in with my explanation. This instrument, which is basically an
enlarged oboe, has the reputation of a particularly nasal sound
associated with the pastorale. The hollow bell forms a sort of cavity
resonator, which alters the tone in a manner very similar to the way in
which your speaking voice is altered when you talk through closely
cupped hands, or with your mouth near a wide-mouthed jar. Careful
experiments have shown that the note played with all the holes closed,
and to a lesser extent the next two or three above it, is altered by the
odd-shaped bell, while the rest of the scale is very much like that of
an oboe that has been lowered in pitch. Composers who want to exploit
the outstanding distinction of the English horn are obviously limited to
a very small range of notes! If on the other hand they want to use the
instrument as a solid and useful voice in a woodwind choir [ Benade's
personal preferences are showing! ], they do not find it overly
difficult to avoid the peculiar notes; there are alternate fingerings
that avoid all but two or three.

[end of Benade's section about bells, that's all there is]

Cheers,
Bill

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