Today, the Aeolian harp. The University of Houston's
College of Engineering presents this series about the
machines that make our civilization run, and the people
whose ingenuity created them.
Aeolus was the Greek god of the
winds and ruler of the island of Aeolia. In the
Odyssey, he gives wandering Odysseus favorable
winds. But he also hands him a bag of ill winds, with
orders not to open it. Naturally, Odysseus' companions
grow curious. Like Pandora, they open the bag and summon
up trouble.
The Aeolian harp is a Greek invention, played by
the winds. It has a dozen or so strings, all the same
length -- about five feet. Each has a different diameter,
but they're all tensioned to the same unison pitch. As
wind blows over the strings, different overtones are
excited on each one. You hear the third, the twelfth, the
upper octave. A strange mélange of sound floats
above that fundamental unison drone.
Years ago, when I taught at Washington State University
on the windblown steppe of eastern Washington, I had a
contract with the Bonneville Power Company. My task was
to study Aeolian vibration as it occurred, not in a harp,
but in their power lines.
In a gentle wind, a power line will sing, just as a
catgut or nylon string will sing in an Aeolian harp. That
vibration can, eventually, cause the wire to fatigue and
break. In a severe wind, or when ice forms an airfoil
shape around the wire, it can gallop wildly and cause
severe damage. Vast human ingenuity has gone into the
problem of damping out vibration to protect the wire.
So imagine wind passing over any cylinder: The air has to
speed up to get around it. A fast-moving stream separates
from the cylinder and whooshes away behind it. That
creates a low-pressure region just behind the cylinder.
The two sheets of fast-moving air both want to collapse
into the space behind the cylinder, but they do so
alternately, in a rhythm that depends on the size of the
cylinder and the airspeed.
That rhythmic collapse is what makes wires sing. You can
calculate its pitch in cycles per second, or hertz. It
comes to about one fifth of the airspeed divided by the
diameter.
You can also feel the Aeolian sound. Next time a
breeze is blowing at, say, twenty feet per second, stand
a few feet behind a one-foot-diameter telephone pole. Wet
your finger, put it in the wake, and you'll feel a
four-cycle-per-second flutter. A power cable, a fifteenth
of a foot in diameter, gives a sixty-hertz hum. In that
same breeze, a gut string, one two-hundredth of a foot in
diameter, will sound the note A from a woman's
mid-voice range.
The Aeolian harp came back to our modern world when
Renaissance scientist Athanasius
Kircher recreated it. By 1800, it'd entered the
Romantic imagination. Coleridge worked for years on his
poem The Aeolian Harp. And, in one version, he
says:
Methinks, it should have been impossible
Not to love all things in a World like this,
Where e'en the Breezes of the simple Air
Possess the power and Spirit of Melody!
I'm John Lienhard, at the University of Houston, where
we're interested in the way inventive minds work.
(Theme music)
Bonner, S., Aeolian harp. The New Grove Dictionary of
Music & Musicians (Stanley Sadie, ed.). Vol. 1, pp.
115-117.
Lienhard, J., Synopsis of Lift, Drag, and Vortex
Frequency Data for Rigid Circular Cylinders.
Washington State University, College of Engineering
Bulletin No. 300, 1966. (For a copy of this report in PDF
format, CLICK
HERE. This is a large, 2.4 Mb, file.)
See also the Wikipedia article on
Aeolian harps:https://en.wikipedia.org/wiki/Aeolian_harpl
Here's a nice example of an Aeolian harp:
https://www.youtube.com/watch?v=rmP5XaNYlkI
And here is the complete Coleridge poem:
http://www.emule.com/poetry/?page=poem&poem=439
