A Bit of History First…
In 1933 , Harvey Fletcher and W. A. Munson from Bell Telephone Laboratories presented loudness level contours (curves of equal perception of a sound). These curves were obtained statistically following listening tests carried out using listeners with hearing considered to be normal.
The loudness level contour of 0 corresponds to the threshold of hearing, the one of 10, 10 dB above the threshold of perception and so on. Taking all these curves into account is complex and cannot be integrated into a simple measurement tool. The loudness curve of 40 was chosen to determine the A weighting. This weighting is therefore valid for low amplitude sounds but is less well suited for higher sounds.
The story goes that the final dBA curve was defined from a calculation performed on a corner of a restaurant paper tablecloth. It represented the frequency response of an analog filter that most closely match this loudness contour.
Fletcher and Muson’s work was taken up by Zwicker and led to the writing of ISO 532-1, Measurement of loudness calculation – Part 1: Zwicker’s method.
 H. Fletcher & W. A. Munson: Loudness, it’s definition, Measurement and Calculation, JASA, Vol. 5, october 1933.
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This article takes place in our Acoustic 101 serie!