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MMD > Archives > February 1999 > 1999.02.04 > 01Prev  Next


Secrets of the Organ Tone
By Paul West

Tom McAuley asked why various monkey organs sounded different:
"If they all use similar wooden and metal pipes, how do they make them
sound different?"

Here's a rough outline of how pipe makers vary the tone of an organ
pipe, and for brevity I will only talk about flue pipes.  Reed pipes,
the second large category of organ pipes, have their own set of
adjustments and are capable of similar variety.

The pitch of the pipe is determined by its length (specifically the
length from the mouth to the far end) and whether the far end is open
or closed.  A closed pipe sounds at half the frequency of an open pipe.
Closing the pipe also affects the tone by removing essentially all of
the even harmonics of the fundamental frequency.  This gives the closed
pipe a characteristic hollow sound.

Fixing the length of the pipe and increasing the cross-section area of
the pipe makes the pipe act more like a low-pass filter, reducing the
intensity of the higher harmonics.  A 'string' or 'violin' pipe is very
narrow in cross-section compared to a 'flute' pipe.  Diapason pipes,
the basic tone of the classic or church pipe organ, are intermediate
in cross-section and designed to have a noticeable second harmonic.
(They are always open pipes.)

In an instrument with only one rank of pipes, the designer often varies
the scale (ratio of length to cross-section) from bass to treble,
giving the bass a smooth (flute) tone and the treble a more biting
(string) tone.

Once a pipe has been built with a particular length and cross-section,
varying the width and height (cut-up) of the mouth of the pipe will
affect both the quality of the pipe's steady tone and also its inton-
ation or initial speech.  To further complicate matters, how a pipe
speaks depends on the wind pressure being fed to it.  For a fixed wind
pressure, a higher cut-up will produce a duller sound (low pass filter
again).  But higher pressures tend to require a higher cut-up for the
pipe to sound at all.  Pipe makers also have a bag of tricks, such as
putting various ears or beards around the mouth, to modify the pipe
speech.

All this is explained in some detail, including typical dimensions for
different voices of pipes, in many books on pipe organs.  Check your
public library.

Paul West


(Message sent Thu 4 Feb 1999, 21:43:53 GMT, from time zone GMT-0800.)

Key Words in Subject:  Organ, Secrets, Tone

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