Vibro-acoustics of organ pipes: Coincidence effect between acoustical and mechanical modes.
François Gautier · The Journal of the Acoustical Society of America · 2010
How does the wall vibration of a wind instrument affect its sound? This question has engendered a long-lasting debate among scientists, musicians, and instrument makers. In fact, the wall vibrations are easily felt or measured on most wind instruments. However, their influence on the produced sound is more difficult to bring to light, because the fluid-structure couplings involved are weak. This leads to the conclusion that the wall vibration effect does not induce any significant or audible contribution except when particular conditions are gathered. The conditions correspond to coincidence relationships between acoustical and structural modes of the instrument. A century ago, Miller published an experimental study about vibrations of organ pipes. [D. Miller, Science 29, 161–171 (1909)]. He used a double walled organ pipe, for which the space between the two walls could be filled with water while the pipe was sounded, presumably damping the wall vibrations. Miller then observed, without explaining it, that the filling led to the unusual behavior of the pipe, clearly audible. Some heights of the water jacket produced pitch changes or unstable tones. A copy of this historical experiment has been made recently and is now interpreted using the coincidence concept.