Limits of Hearing

Human hearing can detect small variations in air pressure, ranging from 10 µPa up to 100 Pa. The detection of these small variations occurs in the presence of atmospheric pressure, where 1 atm = 101.3 kPa. Furthermore, humans perceive loudness on a logarithmic scale. The international standard reference for sound pressure level measurements is 20 µPa (0 dB), which is the threshold of quiet. This is considered the nominal threshold of hearing, although approximately half of the general population can sense sounds at even lower levels. On the opposite end of sound pressure level measurements, humans experience discomfort and pain from sounds with sound pressure levels greater than 100 Pa (134 dB). Within this range in level, humans can typically discern changes as small as 1 dB.

Human hearing can detect frequencies between 20 Hz and 20 kHz. Frequency components outside this range are not generally considered to impact the human perception of sound, regardless of the sound pressure level. As explained in the previous section, there are physical reasons why human hearing is most sensitive to frequency components around 1 kHz and 4 kHz. Many studies have demonstrated the sensitivity of the ear as a function of frequency, which is typically plotted in equal loudness curves as in the following figure.

Besides the sound pressure level-dependent sensitivity of hearing, humans also can distinguish very fine changes in frequency. Below frequencies of 500 Hz, the ear can differentiate tone bursts with a frequency difference of approximately 1 Hz. Above 500 Hz, the barely-noticeable distinction is proportional to the frequency (0.002 x f ).