At 10:42 AM +0100 1/4/02, Richard Brekne wrote: >OK... he does not expressly say that sound emanates from the assembly in this >fashion when it is the strings that are the source of input energy. >But he does say >that the strings energy reaches the soundboard by way of >longitudinal waves. That >certainly sounds more akin to the reasoning JD and Robin present >then what your >camp have said. Fair enough ? Then he goes on to cite an experiment which >demonstrates that sound is indeed produced when the bridge is coupled to a >longitudinal sound wave source. Am I to assume he is going somewhere >else with all >this ? The jist of his whole opening discussion seems to point in >this direction >pretty clearly, but ok. Richard, Never mind McFerrin. Look in any basic high school textbook on sound or any of dozens of web sites and you will be told that sound is propagated as waves in an elastic medium. These waves are ALL longitudinal. This is not some crackpot theory dreamed up by me and Robin; it is simply demonstrable fact. The vibrations of the string set up pressure at the termination, causing the molecules at this point to vibrate and these vibrations pass _through_ the bridge and _into_ the soundboard at a speed depending on the material in question. The sound propagates throughout the structure and is radiated mainly from the soundboard but also from other surfaces such as the rim and the key bottom. A piano with pine keys and a well-designed key bottom will feel quite different from a cheap piano with basswood keys. There follow just a few definitions and URIs. Just one point, to link to Robin's Chickering saga: The incisions in a fiddle bridge are there not purely for decoration but to lengthen the path of the sound on its way through the bridge from certain strings. The apron of a bridge is designed so that the sound will enter the soundboard at a more responsive point. The same goes for the canted long bridge in the treble etc. As to the relieved underside of the bridge in the tenor of a Steinway O (with patent acoustic dowels), one would need to read the patent. VIBRATION: A periodic motion of the _particles_ of an elastic body or medium in alternately opposite directions from the position of equilibrium when that equilibrium has been disturbed. (Webster's Third New International Dictionary) SOUND: Mechanical _radiant_ energy that is transmitted by _longitudinal_ pressure waves in air or other material medium and is the objective cause of the sendsation od hearing. (ditto) SOUND is a wave characterised by over-pressure - i.e. an excess of particles - at one point and a lack of particles at another point. It is a longitudinal vibration of the air that is propagated step by step. http://library.thinkquest.org/27178/en/section/6/1.html?tqskip=1 The Soundry http://library.thinkquest.org/19537/ WAVES: ...If the path be a straight line in the direction of the wave, we again have simple harmonic motion, but the vibrations are known as _longitudinal_. Longitudinal vibrations are of interest, since sound is transmitted by them through matter (Cassell's Encyclopaedia, 1903) ACOUSTIC RADIATION: A mode of coherent mechanical energy transfer, usually referring to the transfer of energy from the sound source to the surrounding medium. Sound PROPAGATION, on the other hand, is the movement of SOUND WAVEs through a medium. <http://www.sfu.ca/sonic-studio/handbook/Acoustic_Radiation.html> ACOUSTIC IMPEDANCE: The acoustic impedance Z of a surface or medium is the ratio of the amplitude of the SOUND PRESSURE r and the amplitude of the PARTICLE VELOCITY v of an acoustic WAVE that impinges on the surface or medium. By analogy to Ohm's law for electrical impedance. Z = r/v <http://www.sfu.ca/sonic-studio/handbook/Acoustic_Impedance.html> SOUNDBOARD: A piece of wood used in stringed instruments, including keyboard instruments, which acts as a coupling device between the string and the air in order to AMPLIFY the sound. Since the amount of radiation varies with the size of the board, large ones are usually used to increase the sound output. The board will have many RESONANCE frequencies which are activated by the vibrating string; however, soundboards are designed so that each FREQUENCY produced by the string will sound equally loud. <http://www.sfu.ca/sonic-studio/handbook/Soundboard.html> SOUNDBOARDS, AND OLD WIVES' TALES There is no physical movement in the board, as you think of physical motion. Only a vibration transmitted longitudinally through the board in all directions, a molecular wave, which might appear as raindrops in a pond. In other words, the soundboard conducts vibration just like the ocean. Does the ocean "move" as a result? Well of course, itıs molecules have to move, but that isnıt called physical movement, so you can say absolutely that the ocean doesnıt move to transmit sound. The sound transmitted in the ocean is spherical in nature, unless the transducer is directional, but the ocean quickly re-conducts that directed sound throughout, so itıs difficult to focus a ray of sound in the ocean. Double-ditto a solid like a soundboard. <http://www.player-care.com/cb/> http://www.acoustics.org/faqs.html The mathematical theory of sound propagation began with Isaac Newton (1642-1727), whose Principia (1686) included a mechanical interpretation of sound as being "pressure" pulses transmitted through neighboring fluid particles. http://asa.aip.org/pierce.html Sound is translated through a wooden bridge to sound board. http://library.thinkquest.org/C0120889/string.shtml Horizontal and Vertical Transverse Waves <http://www-ccrma.stanford.edu/~jos/waveguide/Horizontal_Vertical_Transve.html> When a key is hit, it operates a small felt-covered hammer that strikes and vibrates a corresponding steel wire. This vibration transmits pressure differences to the bridge attached to the soundboard inside the wooden frame. The soundboard then agitates the nearby air to produce the sounds that we hear. <http://www.oxy.edu/~hudson/paper19.html>
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