HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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THE TRANSMISSION OF SOUND.                  19
evidently the distance through which the individual particles of air vibrate; that is, the extent of particle vibration. Further, it is plain that, the greater the swing of the prong A, the greater will be the degree of condensation and rarefaction in (ab); that is, the greater the extent of particle vibration, the greater will be the degree of rarefaction and condensation. Lastly, just as the particles in the case of the sand tube could perform their vibrations in an infinite number of modes, so the fork A, and therefore the air particles, may perform their vibrations in an endless variety of modes, giving rise to as many wave forms.
It is often convenient to represent a sound wave after the same manner as a water wave. Let fig. 15 represent the section of a
water wave, the dotted line being the surface of the water at rest. This may also represent a sound wave the length of which is (ab) distance above the dotted line representing extent of forward, and distance below extent of backward longitudinal vibration. For example, the air particles which are at the points I and / when at rest, would be at d and g, (ld = ce and hk=gf) in the state of things represented in the figure. The curve in the figure, related in this way to the wave of condensation and rarefaction, is termed its associated wave.
The velocity with which these sound waves travel is very great, compared with that of water waves. The method of determining it is very simple in principle. Two stations are chosen within sight of each other, the distance between them being accurately known. A gun is fired at one station, and an observer at the other counts the number of seconds that elapse between seeing the flash and hearing the sound. As light passes almost instantaneously over any terrestrial distance, the time that sound takes to travel over a