-- forwarded message, please reply to sender and MMD --

 [ Johan has enjoyed MMDigest for a long time via the web site,
 [ and has just joined the MMD Pipes Forum.  He is a professor
 [ in the Dept. of speech, music and hearing at the Royal Institute
 [ of Technology of Sweden.  (Pronounce his name like Lilienkranz.)
 [ -- Robbie

Long since a fan of the Dutch Pierement, I find the MMDigest a most
inspiring forum.  My background is as an engineer doing research in
the acoustics of speech, its production, analysis, synthesis, and
related signal processing.

Fifteen years ago I made a little portable 32-note organ controlled
from a punched fan-fold paper tape.  Now I have entered the more risky
adventure to design and build a more full-fledged street organ aimed
at some 80 control channels and 250 pipes.  In its first stage I will
cheat a bit, however, and control it by MIDI rather than by pneumatic
punched paper reading.

Recently the good old question of pipe structural resonances came up
again, so let me submit the following little essay.

The topic, "if and how the material in a pipe influences its tonal
quality", is a perennial one in the organ lore.  Some people say
the material does not matter at all, other feel sure it is important.

Probably a great deal of the latter view stems from an experiment
by Miller (1909) where he sounded a pipe which had a special external
mantle so that he could immerse the pipe in water.  He did observe
a change in sound when the water was poured in and damped the wall
oscillations.

Backus and Hundley (1966) use classical formulas to compute the deform-
ation of round and square pipes due to the internal sound pressure
making the pipe wall vibrate.  These deformations will modify the pipe
resonances and the wall vibrations themselves will radiate some external
sound.  In conventional round metal pipes and square wood pipes these
effects are totally negligible, since the constructions are inherently
very stiff.

A notable difference comes when you make a square pipe from thin sheet
metal which will be much more elastic and easy to buckle.  Nobody makes
an organ pipe that way, but this is precisely what Miller did for his
experiment, moreover using zinc, the most sneered-at pipe material in
the organ trade.

A slightly different issue is, "What are the loss mechanisms in the
pipe working as a resonator, since these determine the sharpness of
its resonances which influence the pipe tone?"

There are several loss mechanisms besides vibrating walls: viscous and
turbulent losses in the air column, heat conduction into the pipe wall,
sound radiation (the useful acoustic output), and vortex shedding at
the labium.

I believe it is now well established that the last mechanism, vortex
shedding, is by far the dominant one, because of the high acoustic
particle velocities -- on the order of tens of meters/second, a similar
magnitude as the jet velocity.  Again, this says the pipe shape and
blowing are the important things, not the material.

There is no doubt the pipe walls do vibrate -- the issue is whether the
sound radiated from these vibrations is audible at all, in the presence
of the enormously strong sounds from the pipe mouth (and also from the
far end, in the case of open pipes).

I want to cite the conclusion by Backus and Hundley:

 "... the steady tone of a pipe does not depend on the material of
  the pipe wall.  The belief that the use of tin in constructing pipes
  gives a better tone appears to be a myth unsupported by the evidence.
  The main reason for the use of the usual tin-lead mixtures would seem
  to be the practical one of ease of working and pipe voicing.  There
  is also a psychological factor; tin is expensive, and it is natural
  to think of a more costly pipe as producing a better tone."

That could have been the end of the story, but people continue
to perform research this area.  The reason is that you cannot
completely rule out that there may be some audible effects in the
starting transient of the pipe tone.  Compare the dull thump from the
keybed and action when you strike a piano key -- this is indeed an
important feature.

Nowadays it is popular to study the pipe structural vibrations using
modal analysis measurements with laser interferometry.  Still, the
researchers have all kinds of difficulty to show any significant
acoustic effects of the material.  For instance, when a pipe maker
fabricates two identical pipes, these 'identical' pipes generally still
have enough differences to mask the effects which you try to show.

People I know of who recently worked on this are Anna Runnemalm
(structural eigenmodes studied with laser interferometry), and
Malte Kob (spectrographic comparison of pipe tone and sound from
mechanical excitation).

In all probability the material of a pipe has a very minor influence
on the sound of the pipe.  The important things here is the general
construction of the pipe and how it is voiced, blown, and set up.
The material is generally selected from other reasons: availability
and price, ease of manufacture, appearance, and durability.


References:

Miller D C (1909): The influence of the material of wind-instrument
on the tone quality. Science 735(24), pp. 161-171.

Backus J, Hundley T C (1966): Wall vibrations in flue organ pipes
and their effect on tone. J. Acoust. Soc. Am., vol. 39-5, pp. 936-945.

Runnemalm A (1997): Effects of material choice and tooling methods
on structural modes of open organ pipes. Int. symposium on musical
acoustics, Proc. Inst. Of Acoustics, vol. 19 part 5, pp. 345-352.
(Author e-mail: Anna.Runnemalm@mt.luth.se)

Kob M (1998): On the influence of wall vibrations on the transient
sound  of flue organ pipes. Proc. Nordic Acoustic Meeting 98.
(Author e-mail: malte@akustik.rwth-aachen.de) (Conference adminis-
tration e-mail: nam98@ingemansson.se)


Johan Liljencrants
johan@speech.kth.se

 [ Thanks for your fine essay, Johan.  I look forward to more
 [ discussion of this topic at the MMD Pipes Forum.  -- Robbie