Page 28 - The Master Handbook Of Acoustics

3
FUNDAMENTALS OF SOUND

Paper motion

W

Time
FIGURE 1-2
A ballpoint pen fastened to the vibrating weight traces a sine wave on a paper strip
moving at uniform speed. This shows the basic relationship between simple harmonic
motion and the sine wave.

Elasticity and inertia are two things all media must possess to be capa-
ble of conducting sound.

Sine-Wave Language

The sine wave is a specific kind of alternating signal and is described
by its own set of specific terms. Viewed on an oscilloscope, the easiest
value to read is the peak-to-peak value (of voltage, current, sound
pressure, or whatever the sine wave represents), the meaning of which
is obvious in Fig. 1-3. If the wave is symmetrical, the peak-to-peak
value is twice the peak value.
The common ac voltmeter is, in reality, a dc instrument fitted with
a rectifier that changes the alternating sine current to pulsating unidi-
rectional current. The dc meter then responds to the average value as
indicated in Fig. 1-3. Such meters are, however, almost universely cal-
ibrated in terms of rms (described in the next paragraph). For pure sine
waves, this is quite an acceptable fiction, but for nonsinusoidal wave-
shapes the reading will be in error.
An alternating current of one ampere rms (or effective) is exactly
equivalent in heating power to 1 ampere of direct current as it flows
through a resistance of known value. After all, alternating current can
heat up a resistor or do work no matter which direction it flows, it is
just a matter of evaluating it. In the right-hand positive loop of Fig. 1-3
the ordinates (height of lines to the curve) are read off for each marked