HUMAN EARS ARE SENSITIVE TO SOUND in different degrees along the frequency range. We are most sensitive to mid-frequencies and it is not surprising that our ears are built to better understand sound in the range that we talk. The sensitivity to different frequencies also varies by the sound pressure level. As the level increases the differences flatten out. Figure 3.1 shows the human hearing range, from threshold of audibility to threshold of pain.
The human hearing range goes from approximately 20 Hz to 20,000 Hz. Below and above that range sound is inaudible for humans, but it might be audible for some animals, for example, the frequency used by bats to locate themselves (echolocation) or the frequency used for dog whistles.
3.1 Human audible range
Table 3.1 Permissible noise exposure in hours per day at a certain sound pressure level (OSHA 1919.95)
Duration per day | Sound pressure level (slow response) |
h:mm | dBA |
8:00 | 90 |
6:00 | 92 |
4:00 | 95 |
3:00 | 97 |
2:00 | 100 |
1:30 | 102 |
1:00 | 105 |
0:30 | 110 |
0:15 or less | 115 |
Humans also have an audible range of amplitudes that goes from very faint around 0 dB to painful at about 130 dB. Above the threshold of pain, our ears can be damaged permanently in a matter of seconds. Long exposures at lower amplitudes can also be damaging to our ears. Table 3.1 contains the limit time exposure at each SPL level per day dictated by OSHA. Note: Permissible levels in Europe are more stringent, with 6 hours at 80dB, 2 hours at 95dB, or 45minutes at 90dB, being sufficiently high exposure times to trigger requirements for noise assessment and hearing protection.
LOUDNESS IS THE SUBJECTIVE IMPRESSION of level (what we commonly call “the volume” of a sound). Our ears are created to respond differently to sound at different frequencies, so two sounds at the same pressure level will not necessarily have the same loudness to our ears.
3.2.1 Equal loudness contour
Figure 3.2 defines the loudness of a specific sound. Each curve is associated with a loudness, defined in Phon. It is easy to see that the shape of the curves corresponds with the threshold of audibility, becoming flatter as loudness increases. It can also be observed in the figure that the human ear has very low sensitivity at the low frequencies. For example, a pure tone at 31.5 Hz with 95 dB of amplitude will sound as loud to our ears as one at 500 Hz with 60 dB. Both are 50 Phons.
3.2 Equal loudness contours (based on ISO 226:2203)