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A sound level meter is used to measure sound levels, usually in terms of subjective loudness. This involves the use of a weighting filter, usually to the A-weighting standard.
The use of A-weighting is historical, and not really justifiable on the basis of current understanding of equal-loudness contours and the fact that these have been shown not to apply for random noise. The use of an rms rectifier, also standard, is also hard to justify.
Sound level meters are commonly used in noise pollution studies for the assessment of roadway noise, industrial noise, and aircraft noise. Strictly speaking the A-weighting was only recommended for quiet sounds in the region of 40 dB SPL but today it is used at all levels, and the other weightings are little used, except for D-weighting which is used specifically for Aircraft noise at very high levels.
Sound exposure meters use the same principles, and A-weighting, to measure total exposure to noise over a period of time. The result is indicated in terms of LAeqT, which means the equivalent level (A weighted) over a time period T (often 8 hours). The equivalent level is derived by storing a large number of sound pressure values in pascals and then calculating the mean square before converting to dB SPL. High levels thus count more in the final figure. Exposure meters are usually used to assess high level industrial noise, in terms of potential for hearing damage, though they use the A-weighting curve which was derived from equal-loudness contours at very low level(40 dB SPL).
The ITU-R 468 noise weighting curve was designed specifically for noise measurement, along with a quasi-peak detector, and though used mostly for professional audio equipment measurements, would actually produce more meaningful results if incorporated into sound level meters for low level noise measurement.
The following standards define sound level meters and exposure meters: