Page 134 - The Master Handbook Of Acoustics

109
SPEECH, MUSIC, AND NOISE

bad noise interfering with our listening to a favorite recording, it is just
that the noise is put to a specific use.
In acoustical measurements, the use of pure tones is often very dif-
ficult to handle while a narrow band of noise centered on the same fre-
quency would make satisfactory measurements possible. For example,
if a studio is filled with a pure tone signal of 1,000 Hz from a loud-
speaker, a microphone picking up this sound will have an output that
varies greatly from position to position due to room resonances. If,
however, a band of noise one octave wide centered at 1,000 Hz were
radiated from the same loudspeaker, the level from position to posi-
tion would tend to be more uniform, yet the measurement would con-
tain information on what is happening in the region of 1,000 Hz. Such
measuring techniques make sense because we are usually interested in
how a studio or listening room reacts to the very complex sounds
being recorded or reproduced, rather than to steady, pure tones.

Random Noise

Random noise is generated in any electrical
circuit and minimizing its effect often
becomes a very difficult problem. Heavy
ions falling back on the cathode of a
thermionic vacuum tube produce noise of a Pure
relatively high amplitude and wide spec- sine
trum. Furthermore, the introduction of some wave
gas molecules into the evacuated space will
produce even more noise. Today a random
noise generator is made with a silicon diode Amplitude Time
or other solid-state device followed by an
amplifier, voltmeter, and attenuator.
In Fig. 5-15 a pure sine wave and a Random
noise
random noise signal are compared as

viewed on a cathode ray oscilloscope.
The regularity of the one is in stark con-
trast to the randomness of the other. If the
FIGURE 5-15
horizontal sweep of the oscilloscope is
Cathode ray oscillograms of a pure sine wave and of
expanded sufficiently and a snapshot is
random noise. Random noise may be considered
taken of the random noise signal, it made up of sine waves, which are continually shifting
would appear as in Fig. 5-16. in amplitude, phase, and frequency.