Anders Askenfelt on Piano Soundboard

[Richard Brekne]

List.

Anders was kind enough to grant permision to post this to the list. It
is his reply to my questioning about how the sound board works, what
kinds of waves propagate through it, and how this are set in motion.

Should Anders need any introduction to any of you the following URL may
be informative. He is also editor of the "Five lectures on the Acoustics
of the Piano".

http://www.speech.kth.se/music/staff/andersa.html


Anders Askenfelt wrote:

Dear friend of the piano,

1. The force from the string tries to drive the soundboard up and down
mainly. This gives bending waves (transversal waves) in the soundboard
which in turn radiates sound into the air. (Surface waves occur mainly
in
water and other liquids.) There is also a much weaker longitudinal force
component which tries to tilt (rock) the bridge back and forth (as the
force is applied at the top). This force also gives bending waves in the
soundboard.

2. At the very onset of the tone there is a shock excitation of the
bridge as the first transversal wave on the string arives. This is a
general  phenomenon which occurs in all systems when a signal is turned
on  suddenly. This first wave excites all modes ("resonances") of the
instrument including all the soundboard resonances, and is heard as a
prominent "thump". This transient decays and after some time the partial
frequencies of the string dominates the driving of the soundboard and
hence the radiated sound. An additional thump comes from the key  bed,
slightly earlier orlater than the string sound, depending on touch. This
touch sound is caused by the retardation of the key in its bottom
position. For sudden, strong touches ("staccato"), there is also an
early key bed component
caused by the acceleration of the key.

3. Compression waves occur in the air as the sound wave is propagating
through the medium. In plates like the soundboard, bending waves
(transversal waves) are propagating and transmitted and reflected at the
boundaries. The bending means a stretching of thefibres on the
(momentary) convex side of the soundboard and a compression
at the opposite side. The compression and stretching sides alternates
with the frequency of the fundamental, e.g,. 440 Hz, while the
soundboard takes a complicated shape, determined by a superposition of
all mode shapes of the soundboard. These alternating stretchings and
compressions associated with the transversal wave motion, must however,
not be mixed up with compression waves.


The little "piano book" by myself and others "Five Lectures on the
Acoustics of the Piano" is available on the net
http://www.speech.kth.se/music/5_lectures/



Best regards,
Anders Askenfelt

Recommended reading:

Fletcher & Rossing:
The Physics of Musical Instruments
Springer-Verlag, 1991, 2000.

--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html