SOUND LEVEL METER

Typical Number in Hospital: 3

Cost Bands: 2,4

References: 3,8

A basic sound level meter consists of a microphone, an amplifier, rectifier and logarithmic scale meter calibrated in decibels above a reference level. Sometimes called (correctly) a sound pressure level meter it is used to measure sound level in rooms, levels of speech and (when coupled to an artificial ear or artificial mastoid) for calibrating audiometers . It may be used for environmental sound measurements such as in audiometric booths or in the audiology department for adjusting one's voice level during simple speech perception tests, or for testing the performance of apparatus.

More complex versions may include true r.m.s. averaging, variable time constant averaging of sound, peak sound level recording, and also filters or weighting networks. Filters may be used to measure the sound level in particular frequency bands as a method of analysing the frequency content of environmental sound or for measuring distortion of pure tones from the harmonic spectrum. The filters are often arranged in octaves or part octaves for this purpose.

Weighting networks may be used instead of filters to modify the frequency response of the amplifier so as to provide a reading which more closely approximates to the perceived loudness of sounds. The most commonly used weighting is the A-weighting, calibrated in dBA which is suitable for relatively quiet sound levels (about 40 dBSPL at 1 kHz). For higher sound levels the dBB and dBC scales are more appropriate. Other weighting characteristics are used for specific sounds such as jet aircraft noise. Without filters or weighting networks the meter is said to be in the linear mode, in which case the sounds are expressed in decibels above a standard level (dBSPL) of 0.00002 Pa, which corresponds to an average for the normal hearing threshold at 1 kHz.