threshold of audibility

The level above which sound intensity, at any specified frequency, must rise in order to be detected by the average human ear.

This threshold value decreases with increasing frequency from a value of about one microwatt m^-2 at 50 Hz to slightly less than 10^-6 microwatts m^-2 at about 2000 Hz, and then increases with increasing frequency to about 10⁴ microwatt m^-2 near 30 000 Hz. As the sound intensity rises above the threshold of audibility at any frequency, continued increase will finally raise the intensity to a second type of limit, the threshold of pain (threshold of discomfort, threshold of feeling). This threshold is rather insensitive to frequency, being found at about 10⁶ microwatts m^-2 for nearly all frequencies in the audible range. It represents the limit above which appreciable increase in sound intensity will lead to sensible pain in the average human ear.
See decibel.