Noise measurement

(acoustics) The process of quantitatively determining one or more properties of acoustic noise.
(electronics) Any of a wide range of measurements of random and nonrandom electrical noise, but usually noise-power measurement.

The process of quantitatively determining one or more properties of acoustic noise. In noise assessment and control studies, knowledge of the physical properties of the undesirable sound is the initial step toward understanding the situation and what should be done to reduce or eliminate a noise problem.

The most common measures of noise are of the magnitude and frequency content of the noise sound pressure, time-averaged or as a function of time. Of increasing interest are metrics of sound quality (that may include both physical and psychoacoustic factors), such as loudness, pitch strength, and fluctuation strength. To characterize the noise output of a source, sound power level may be determined. To locate a source or to quantify propagation paths, sound intensity level may be measured.

Essentially all noise measurements are performed using electronic equipment. An electroaoustic transducer (a microphone in air and other gases; a hydrophone in water and other liquids) transforms sound (usually the sound pressure) at the point of observation into a corresponding electrical signal. This electrical signal is then operated on by analog or digital means with devices such as signal conditioners, filters, and detectors to determine the value (or values) of interest. This value is then given on an indicating meter or digital display. See also Electric filter; Hydrophone; Microphone; Transducer.

For noise measurements in air, a sound-level meter is the most commonly used instrument. The simplest sound-level meter comprises a microphone, a frequency-weighting filter, a root-mean-square (rms) detector, and logarithmic readout of the sound pressure level in decibels relative to 20 micropascals. Standard frequency weightings, designated A and C, were originally developed to approximate human response to noise at low and high levels, respectively, but now are used as specified in standards and legislation without regard to their origin. More sophisticated sound-level meters incorporate standardized octave-band or fractional-octave-band filters, and provide additional metrics and analysis capabilities. See also Decibel; Loudness.

Selection of a noise metric and measurement instrument generally depends upon the application. Four classes of application are: (1) discrete source emission, (2) hearing conservation, (3) outdoor environmental noise, and (4) indoor room noise. Within these classes there are found additional subclasses, measurements for enforcement of legal limits often being done differently than those for comfort, aircraft sonic boom being measured differently than power plant noise, and so on.

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