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Fletcher Rings the Bell

Hallo hoppy Reader,

Just a quick installment of the Bitstream regarding the perception of loudness and the origins of the Equal–loudness curves

I was trawling through some tech lit on the web this past weekend and came across a historical note of some significance, to me anyway! It was a copy of the Bell Laboratories Record, the house organ at the time, with an article by Harvey Fletcher. Specifically, the January 1935 issue had a piece by Fletcher, their Director of Physical Research, on the subject of Loudness and Pitch. His research, along with W.A. Munson, was first published in 1933 and lead to the development of their equal–loudness curves in 1937.

Published in the July 1933 Journal of the Acoustical Society of America, the program for the ninth meeting of the Society documents a lecture given by Monsieurs  Fletcher and Munson entitled, Loudness of a Complex Tone, Its Definition, Measurement and Calculation. “At the meeting of the Society held one year ago Fletcher presented a paper by this title in which was discussed the formulation of a promising method for computing the loudness of a non–fluctuating complex tone from the measured values of the intensity levels and frequencies of the components.” Talk about a run–on sentence!

The Journal entry continues, “Also at that time Munson presented a paper on the loudness of pure tones. The present paper combines the material of these two previous papers with the results of subsequent experimental work, describes some further modifications in the formulœ of this method of computing loudness and illustrates the application of the formulœ to some examples. In view of the complex nature of this problem, this computation method is not considered to be fully developed and further work is required to define its scope. In addition, it is necessary to determine experimentally to what extent the proposed computation method may be applicable to the types of fluctuating complex tones encountered in speech, music and noise.”

As you can guess, “ further work” has been done right up to the present, leading to our better understanding of aural perception and human hearing. A modern application stemming from this seminal work is the current palate of perceptual audio codecs, such as MP3 and AC-3, that are available to audio propellerheads everywhere. Ain’t history fun?

OK, that’s all for today. Everybody continue to geek!


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