Page 39 - The Master Handbook Of Acoustics

14 CHAPTER ONE

ships between harmonics or between harmonics and the fundamental
are quite different from this. Remember how one revolution of the
crankshaft of the automobile engine (360°) was equated with one cycle
of simple harmonic motion of the piston? The up-and-down travel of
the piston spread out in time traces a sine wave such as that in Fig. 1-12.
One complete sine-wave cycle represents 360° of rotation. If another
sine wave of identical frequency is delayed 90°, its time relationship to
the first one is a quarter wave late (time increasing to the right). A half-
wave delay would be 180°, etc. For the 360° delay, the wave at the bot-

90¡

180¡

270¡

360¡

Time
FIGURE 1-12

Illustration of the phase relationships between waves with the same amplitude and fre-
quency. A rotation of 360 degrees is analogous to one complete sine cycle.