FREQUENCY DEPENDENCY OF THE EAR
Within the limits of the hearing range from 16 Hz to 20 kHz, the sensitivity of the auditory system is dependent on the frequency. The figure above shows curves of equal loudness in which the dependency is particularly pronounced at low sound pressure levels.
The curves of equal loudness are sinusoidal waves adjusted in their intensity to match the perceived loudness of a 1 kHz tone. This subjective reference value is measured in phon.
Looking at the curves, it is apparent that in order achieve a loudness of 20 phon at 40 Hz, a sound pressure level is required that is 40 dB higher than it would be to achieve 20 phon at 1 kHz. Increasing the loudness decreases the frequency dependency. At a loudness of 120 phon, the difference between 40 Hz and 1 kHz is reduced to only 10 dB difference in sound pressure level. Finally, the human hearing is most sensitive at 4 kHz.
