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MMD > Archives > February 1999 > 1999.02.04 > 02Prev  Next

Secrets of the Organ Tone
By Hans van Oost

Tom McAuley wrote in MMD 990202:

> ... even on the smallest 20-note organs, every organ-builder has
> a unique sound. Why?  If they all use similar wooden and metal pipes,
> how do they make them sound different?  What accounts for the
> difference in the tone between different organ-builders?

Tom has dared to ask the best question about organs.  Also, he asks
the most difficult question possible!  To give a complete answer in all
detail I would need several hundred pages.  Still, I don't want to let
this question unanswered, so here we go:

Organ pipes make two distinctive sounds: a sound that emerges from the
pipe's mouth when the air starts streaming through the pipe, and a
sound that will continue as long as the air-flow keeps going.  Scien-
tific experiments with these two sounds have made clear that the sound
mentioned first is even more specific to the sound of an organ pipe
than the second.

This first sound, called "edge-tone", is made by the air seeking its
way through the pipe's mouth, against the upper lip -- half out in the
open and half into the pipes body -- until the pipes body starts to
control the sound.  It depends upon almost everything that is in the
path of this air-stream: the height of the upper lip, air pressure,
wood fibers between the core and the lower lip, depth of the pipe,
straightness of the upper lip, and so on.

In fact, it is nearly impossible to make two organ pipes with exactly
the same anatomy, so it should be _normal_ that no two organs sound
alike; the specific touch of the pipe-voicer is of far more importance
to the sound than everything else!  This should be enough explanation

Furthermore, there are some other important things, that can be
described better than the edge-tone:

 -- The diameter of the pipe relative to its length.  Bigger diameters
[for the same cross-section area] tend to give a more "flutey" sound,
while smaller diameters give a more "stringy" sound, until the pipe
will not speak properly anymore.  In that case, harmonic brakes [the
Gavioli frein] can be fitted to prevent unwanted sounds.

 -- The width of the lip relative to the diameter.  Narrow lips will
tend to give another type of "flutey" sound.  Wide lips will give more
harmonics.  The most extreme example of wide lip is the steam whistle
or calliope pipe.

 -- Air pressure will affect the sound in another way.  More pressure
will give more harmonics, but too much of it will give an unpleasantly
screaming sound.  Pipes are made for a very specific wind pressure.
Only then will the pipes give the correct sound.

 -- Diameters and lip widths may vary between the various pipes of a
rank.  It may start narrow at the bottom and end big at the top, or the
other way 'round.  Pipe lips may start relatively small in the bass and
grow wider in the treble.

I could go on, but I hope that I have mentioned enough differences
to make clear that it is highly improbable that two organs, made by
a human and not by a robot, will always be different in sound!


Hans van Oost, KDV, Netherlands

(Message sent Thu 4 Feb 1999, 21:19:40 GMT, from time zone GMT+0100.)

Key Words in Subject:  Organ, Secrets, Tone

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