From: Ronald Wong (ronwong@inreach.com)
Date: Mon Apr 08 2002 - 17:10:08 PDT
Message-Id: <l03102802b8d71645d769@[209.209.19.235]> Date: Mon, 8 Apr 2002 17:10:08 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: re: Is it Open or is it Closed????
In response to Art Fortgang's question:
>...does anyone know what is the general rule for musical instruments
>being closed or open tubed?
Paul D. replied:
>The general rule for open vs closed ends for musical instruments is:
>"There is no general rule."
To which I must add, "Amen"
As an example, let's start with the clarinet that Art identified as an
example of a closed pipe.
In general, at the lower frequencies it is indeed a closed pipe, producing
odd multiples of the fundamental harmonic. But at higher frequencies -
where the wavelengths begin to approach the size of the openings of the
clarinet (the stops/toneholes?) it begins to generate all the harmonics of
the given frequencies and thus acts like an open pipe. The fact that it's
bore is more conical at the end of its length rather than cylindrical is
also a contributing factor.
In some ways, the oboe seems similar to a clarinet. It's about the same
length and it too is a reed instrument but, if you compare their
fundamental frequencies, you will find that the oboe's fundamental
frequency is about twice that of a clarinet's.
A close examination of the oboe's bore will show that instead of being a
cylinder over much of it's length, it is tapered. This causes an oboe to
behave like an open pipe (and, for the same reason, so does a bassoon, an
English horn, and a saxophone - among other musical instruments). As a
result, the oboes's fundamental wavelength is around half that of the
clarinet's and it's fundamental frequency therefore becomes around twice as
much. It may not have the range of the clarinet, but it produces a "richer"
sound due to it's ability to generate a greater number of harmonics.
(Nobody has been able to explain to me why having a bore that is conical,
instead of cylindrical, along it's length should cause an instrument to
behave like an open pipe instead of a closed pipe but hopefully someone
will - now that this bit of minutiae is out in the open.)
Recorders have cylindrical bores. Unlike woodwinds that produce their sound
by vibrating reeds, these instruments generate sound by means of the edge
effect. There is a sharp edge sitting in a steam of air and the air peels
off the edge in a periodic fashion producing a series of high and low
pressure regions that move down the length of the bore. The result is the
same as the vibrating reed/reeds in the woodwind instruments or the
vibrating lips in the brass instruments.
In modern recorders, the edge sits in the middle of the stream and the
result is an instrument that behaves like a closed pipe - just like the
clarinet (at it's lower frequencies). I have been told that the edge of
OLDER recorders did not extend as far into the air stream and, as a result,
they behaved like open pipes - with fundamental frequencies that were
twice as high as recorders of the same length that are used today.
The later case is very similar to way sound is created with a flute. The
performer blows air across the top of an opening on the surface of the
instrument and produces a series of vortices which results in a train of
high/low pressure regions that move down the cylindrical tube of the
instrument. Like the older recorders, the flute acts like an open pipe.
So, like Paul said:
>The general rule for open vs closed ends for musical instruments is:
>"There is no general rule."
ron
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