How Sound Is Measured

The intensity of sound is measured in units of decibels (dB). The unit decibel is a ratio of any two components of sound, power, sound pressure, intensity, etc. The human ear has the capability of distinguishing a wide range of sound intensity and frequencies. The range of sound pressure that an individual might be able to distinguish from the threshold of hearing to causing damage to the ears is over a million. Because the decibel scale is a ratio, to measure a single source of sound a base line called a filter is used. Three standard filters are used. They are identified as A, B, and C. Filter A is less sensitive to very high and very low frequencies. The C filter is commonly used for high frequencies. The B filter falls in between the A and C. It is seldom used. When using a sound meter it is important to identify which filter is used. This is commonly done by using the units of dBA or dBC after the reading.

To simplify the measurement of sound, decibel readings are based on an exponential scale of sound pressure levels. Remember that the decibel is not an absolute measure of the sound pressure, but rather is a ratio of a measured sound pressure to a reference sound pressure. Also, because this sound scale is exponential, a sound of 10 dB has an intensity 10 times greater than a sound of 0 dB, and a sound of 20 dB is 100 times more intense than 0 dB. Thus, a 10 dB difference in sound pressure level changes the intensity by 10 times, a 20 dB difference changes the intensity by 100 times, a 30 dB difference changes the intensity by 1,000 times, and so on.

The basic instrument used to measure continuous sound is the sound-level meter. It consists of a microphone to pick up the sound, an amplifier, one or more frequency weighting networks, and a meter to display the sound level. Sound-level meters usually are self-contained (they operate on internal batteries) and are small enough to be hand-held although tripod mounting generally is preferred.

Proper sound-level meter use involves careful placement of the microphone to avoid sound reflections that influence the readings. In measuring the noise exposure of an individual (his or her work environment), the microphone is placed as close as possible to the subject's ear (or where the ear would be in the work environment). Prior to use, the instrument must be calibrated against a known stable sound source.

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