[I've never heard of a "stress wave", but I'll keep the subject line] At 8:30 AM +0000 1/8/02, Phillip L Ford wrote: >I haven't read Robin's post yet but I will. The propogation of stress >waves, compression waves, or sound waves (however you choose to >call them) from the point of contact between string and bridge seems >likely. However, I don't see why that would prevent the string(s) from >physically moving the bridge and soundboard which would move air >which we would hear. As I see it both things can be going on at once. >It doesn't have to be either/or. >But I'm still mystified as to how these compression waves get >transformed into transverse vibration of the soundboard (I think from >what I have read in these posts that everyone is agreed that transverse >vibration of the soundboard has to take place to move air so that we can >hear the sound). I, for one, have not to my knowledge made such a statement, so you can take it that one person at least does not agree. The expression used in acoustics for the transfer of sound energy to the air is Acoustic Radiation. Acoustic radiation is caused by minute pressure differences at the surface of a body. You can do a Google search for "acoustic radiation" and see what you come up with. "Transverse vibration" will not enter the picture. Consider a long thick wire stretched between two rocks and set to vibrate transversally. Compare the amount of air displaced by this wire in vibrating in relation to the volume of sound produced (virtually zero) with the amount of air displaced (virtually none) by the movement of the soundboard (infinitessimal) when the same wire is vibrating in a piano. See <http://www.du.edu/~jcalvert/waves/radiate.htm> I have added some more URIs below. >Why wouldn't the compression wave remain a compression wave? Why >wouldn't it travel through the bridge, into the board, to the rim >and back all the while remaining a compression wave? I've ordered a >couple of books on wave propagation. I'll see what the people who >are supposed to be authorities have to say on the subject. Why not indeed! That is just what does happen, and it is the propagation of the sound through the whole system in all directions that causes the pressure differences at the surface of the soundboard which set the air in vibration. Particles at the interior of a solid body, when affected by the _longitudinal_ sound wave (as all sound waves are) are restricted in their oscillation by the surrounding particles and the forces between them. At the surface of the body, inter-particular forces are acting in some directions but airwards there are no such restrictive forces. Instead of the particle passing its energy on to its neighbours, it passes it into the air. Since the sound is being propagated throughout the system, particles all over the surface of the board are radiating the sound into the air. Suppose the whole bridge/soundboard system is hollow and full of water and that the soundboard is perforated all over with little holes. If I now pump water into the system with a bicycle pump at the string termination by tap-tap-tapping the handle of the pump at a certain frequency, a pressure wave will flow through the whole system in all directions, but little spurts of water will shoot upwards from the soundboard at the same frequency that I tap the pump. The water pressure is released into the air only at the surface, of course. The sound we hear is due to minute pressure differences in the air which originate in minute pressure differences caused by the oscillation of particles at the surface of the instrument. JD <http://www.treasure-troves.com/physics/Sound.html> Sound is a longitudinal wave (P-wave) <http://www.treasure-troves.com/physics/P-Wave.html> created by compression and expansion of gas molecules in the propagation medium (such as air). Any action which compresses or expand a gas creates sound waves but, like all P-waves they cannot propagate in a vacuum. The method used by telephones and speakers to produce the pressure changes is modulation of a vibrating membrane. <http://www.harcourt.com/dictionary> acoustic radiation Acoustics. sound wave energy that travels from one point to another by exerting pressure on the medium, creating a pattern of compressions and rarefactions that can be described as a three-dimensional pattern. acoustic radiation pressure Acoustics. the total pressure of a sound wave on any surface of interface. <http://www.encyclopedia.com/articlesnew/10737.html> Acoustic radiation, propagated as sound waves, may be sonic (in the frequency range from 16 to 20,000 cycles per sec), infrasonic, or subsonic (frequency less than 16 cycles per sec), and ultrasonic (frequency greater than 20,000 cycles per sec). <http://www.aist.go.jp/NIRIN/People/kozuka/>When propagation of ultrasound in a fluid is interrupted by an object, there appears force to push the object in the direction of sound propagation; this is called acoustic radiation pressure. Although this force is very small, it is converged easily up to the order of acoustic wavelength using focused ultrasound and may be applied to manipulate micro objects without contact.
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