Tall pipe dreams

Richard Schneider's posting in 1999.11.02 MMDigest, about Arp
Schnitger's chimney, reminded me I was recently consulted by an
enthusiast about how to celebrate the millennium with a flue pipe
concert on the chimneys of a decommissioned power plant.

Since those were some 20 meters high and 1.5 m diameter (67 by 5 ft)
you should be able to produce an impressive low-frequency tone on them.
But as these measures give a fundamental frequency about 17 Hz there
is a problem in that nobody would hear very much unless you overblow
to a higher harmonic.  So what are the prospects?

It is not generally recognized there is something called the cutoff
frequency for a tube.  This is when the wavelength of the sound equals
the circumference of the tube.  For 1.5 m diameter the circumference is
4.7 m and this wavelength comes at approximately fc=70 Hz.  From this
frequency and up there is an acoustical impedance match between the
tube and the surrounding space and this means the tube cannot operate
as a resonator at those frequencies.

For lower frequencies f you can expect a Q value ['Q' = 'Quality
figure'] for the resonance approximately Q=fc/f.  You will have to
go below the cutoff by a factor of perhaps 3 in order to get decent
resonance efficiency for an organ pipe to speak.  And there we are
locked up rather tightly.  I was quite unhappy to have to discourage
that enthusiast.

A number of years ago I had a student doing her graduation thesis at
the Swedish based company Infrasonik AB.  They specialized at decarbon-
izing big boilers with high intensity infra sound and the work was
about sound level distribution inside.  The sound source for the pur-
pose was then indeed large closed 'organ pipes' driven by compressed
air, and we talk of dimensions like 5 by 0.4 meters.  I am sorry to
find no references to this company any more, it appears they may have
vanished.

Besides, compare also to the 'Bose Cannon' subwoofer loudspeaker built
in a tube some 13 ft by 15 in diameter.

Sadly enough, we have to dream ourselves back to the times of Arp
Schnitger when chimneys had dimensions a more humane size.

And, Robbie, you may have misinterpreted me, but you can still make a
round pipe even from single sheet office paper and have it speak.  But
with such flexible walls the quality of the tone is quite raw and hard
to control.

Johan Liljencrants

 [ It's certainly a low Q resonator, then, with so much energy lost in
 [ flexing the wall. Reminds me of a bad bass horn (tuba) I once played!
 [  -- Robbie