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
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