Our sensitivity to sound depends on its frequency and volume. Human ears are most sensitive to sounds in the midrange. At lower volume levels humans are less sensitive to sounds away from the midrange, bass and treble sounds "seem" reduced in intensity at lower listening levels.
Fletcher-Munson Curves for phon levels of 0, 10, 20, 30, 40, 50,...120
In 1933 Fletcher and Munson (two Bell Lab scientists) generated their Equal Loudness Curves. These curves compare sound at different frequencies and levels, and are plotted for different Phon levels. Each Phon level is defined by the decibel level at 1000 Hz. Hence the 40 Phon curve is a plot of the amplitude of single tones at different frequencies that sound as loud as a tone at 1000 Hz and 40 dB.
H. Fletcher and W. A. Munson, "Loudness of a Complex Tone, Its Definition, Measurement and Calculation", The Journal of the Acoustical Society of America -- July 1933 -- Volume 5, Issue 1, p. 65
Subjects: Noise & Vibration