what is the quietest sound that can be heard
Absolute threshold of hearing The threshold of hearing is generally reported as the RMS sound pressure of 20 micropascals, corresponding to a sound intensity of 0.98 pW/m2 at 1 atmosphere and 25 °C.[3] It is approximately the quietest sound a young human with undamaged hearing can detect at 1,000 Hz.[4] The threshold of hearing is frequency-dependent and it has been shown that the ear's sensitivity is best at frequencies between 2 kHz and 5 kHz,[5] where the threshold reaches as low as −9 dB SPL.[6][7][8]