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.                    9
water is actually moving towards us, and it is difficult at the time to get rid of this notion. If, however, we examine a little more closely, we see that the boats and other objects floating about, do not travel forward with the wave, but simply rise and fall as it passes them. Hence we conclude that the water of which the wave is composed, is not moving towards us. What is it, then, that is being transmitted ? What is it that is moving forwards ? The up and down movement—the wave motion. Now the vibratory movements which give rise to sound are, as we shall presently see, transmitted through the air much in the same way; and therefore, although a sound wave is not exactly analogous to a water wave, yet a brief study of the latter will help us more easily to understand the former.
Let ADECB (fig. 7) represent the section of a water wave, and the dotted line AEB the surface of still water. That part of the wave ADE above the dotted line, is termed the crest, and the part ECB below, the trough. Through D and C, the highest and lowest points, draw DH and EC, parallel to AB, and from the same points draw DF and CH perpendicular to it. Then the distance AB is termed the length of the wave, DE or CH is its amplitude, and the outline ADECB is its form.
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These three elements completely determine a wave, in the same way as the length, breadth and thickness of a rectangular block of wood, determine its size; and further, as any one of these three dimensions may vary independently of the other two, so any one of the three elements—length, amplitude, and form—may vary, the other two remaining constant. Thus in fig. 8 (1), we have three waves of the same amplitude and form, but varying in length ; in fig. 8 (2), we have three waves of same length and form, but of