Page 119 - Master Handbook of Acoustics

conveys considerable information on how well it is running. An audio playback system can produce
sounds deemed very desirable by the owner, but to a neighbor they might be considered intrusive. A
loud ambulance or fire truck siren is specifically designed to be both objectionable and to carry
important information. Society establishes limits to keep objectionable noise to a minimum while

ensuring that information-carrying sounds can be heard by those who need to hear them.
Our evaluation of noise is very much a subjective response. Generally, high-frequency noise is
more annoying than low-frequency noise. Intermittent noise is more annoying than steady or
continuous noise. Moving and nonlocalized noise is more annoying than fixed and localized noise. No
matter how it is evaluated, noise intrusion may be a minor annoyance, or it may cause serious
consequences such as hearing damage.

Noise Measurements

Defining noise as unwanted sound fits many kinds of noise, but noise is also an important tool for

measurements in acoustics. This noise is not necessarily different from unwanted noise; it is just that
the noise is put to a beneficial use.
In acoustical measurements, pure tones are often difficult to use, while a narrow band of noise
centered on the same frequency can make satisfactory measurements possible. For example, a studio
microphone picking up a pure tone signal of 1 kHz from a loudspeaker 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 kHz is radiated from the same loudspeaker, the level from position to position
would tend to be more uniform, yet the measurement would contain information on what is occurring
in the region of 1 kHz. Such measuring techniques make sense because we are usually interested in
how a studio or listening room reacts to the complex sounds being recorded or reproduced, rather
than to steady, pure tones.

Random Noise

Random noise is generated in any analog electrical circuit and minimizing its effect often poses a
difficult problem. Figure 5-13 shows a sine wave and a random noise signal as viewed on an
oscilloscope. The regularity of the one is in stark contrast to the randomness of the other. If the
horizontal sweep of the oscilloscope is expanded sufficiently and a capture is taken of the random
noise signal, it would appear as in Fig. 5-14.