The frequency characteristics of sound are important; they describe its annoyance factor as well as its potential for hearing damage. They indicate to the noise control analyst probable sources of noise and suggest means for confining them. To the mechanical or design engineer, frequency analysis can show the source of machine vibrations which produce noise and contribute to damage. Noise is often a symptom of malfunctioning, and the frequency analysis can sometimes describe the malfunction. Spectral characteristics are also useful in describing the transmission of sound through a wall or its absorption by some material.
Octave-band analyses are not difficult to make, even in the field, and are adequate for noise control work, though not for machine design. They are usually made with sets of bandpass filters attached to a sound-level meter using the flat-frequency response. The filters are designed so that each passes all frequencies within one octave; but at the edge of the pass-band, the transmission falls off sharply. The transmission at one-half the lower band-edge frequency (or at twice the upper) is at least 30 dB below the pass-band transmission; and it is at least 50 dB below passband transmission at one-fourth the lower (or four times the upper) edge of the pass-band frequency.
In computing loudness from measured data, one-tenth, one-third, one-half, and full octave-band analyzers are used. In describing the noise-transmission characteristics of objects such as walls and doors and classifying noise environments for speech interference, analysts ordinarily use full octave-band analyzers. Table 6.4.2 lists the commonly used center frequencies and the frequency ranges for octave-band and one-third octave filters. Separate bandpass filter sets are often used for these analyzers; for one-tenth octave and narrower band analyzers, tunable electronic units are widely used. Narrow-band analyzers are used in studying the characteristics of machines.
SPEECH INTERFERENCE AND NOISE CRITERIA (NC) CURVES
Interference with the intelligibility of speech is a serious problem caused by noise; it impairs comfort, efficiency, and safety. The amount of such interference depends on both frequency and level of sound; and a family of curves has been developed to describe various noise environments. These are called NC curves and are shown in Figure 6.4.3.
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