Fletcher and Munson of Bell Telephone Laboratories researched the ability of the humans to distinguish between different frequencies at different loudness levels. The telephone company was very interested in finding how humans heard and understood speech, in order to make more efficient telephone systems. Their paper, published in the Journal of the Acoustical Society of America in 1933 revealed, among other things, that the human ear is not linear, and is not capable of detecting all frequencies equally at all sound levels.
In studying the chart below, one is struck by how the human ear is well adapted through evolution to become "tuned" to capture the vocal utterances of---other humans.
By following the red response curves along the chart, you can detirmine the relative
Sound Pressure Level required to make different frequencies be perceived at the same
loudness.
Example: A tone at 1000 Hz is played at 40dB SPL. Following
the red line to another frequency, say, 50Hz, you notice that you would have to play the
50Hz tone at about 70dB SPL to make it sound as loud as the 1000Hz tone.
The ear's non-linearity is why your monitors seem to lack "low-end" when the
volume is turned down----and it points to why one should pick a reasonable studio
monitoring level and not change it during the day.
-Rich Rarey
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Reference: AudioCyclopedia by Howard M. Tremane;
Indianapolis, Indiana; Howard W. Sams & Co., Inc.;
1979
Further Reading: