It is generally agreed that 20 kHz is approximately the upper human acoustical hearing limit through air conduction. The reason for this is debated, but the transfer function of the ossicle chain in the middle ear is a suspected to determine the upper limit of audible sounds to 20 kHz (Hemila et al., 1995). Indeed, by means of bone conduction we can hear up to 50 kHz, and values up to 150 kHz have been reported in the young (Pumphrey, 1950).
Others, however, have reported that it is not only the middle ear structures, but mainly the tonotopic organization together with the finite length of the inner ear (the cochlea) that determines the upper frequency range. In mammals, tonotopy arises from a number of components, namely (1) the spatially graded BM stiffness that determines passive tuning of the basilar membrane (BM) and (2) the active frequency tuning of the BM due to auditory-nerve fibers innervating adjacent inner hair cells that sharpen up the tuning curve. In turtles, however, tonotopicity results largely from (3) the spatial gradient of the electrical properties of hair cells. Whatever the mechanism, tonotopy restricts the audible frequency bandwidth by limiting the characteristic frequencies of the auditory-nerve fibers innervating the extreme apical (low frequency) and basal (high frequency) regions of the cochlea (Rechiro & Temchin, 2002)
Your notion that
I can understand that sounds below this range are so weak that they don't affect the ear.
Is partly inaccurate, because you mix up sound level and frequency. It is, however, also partly true, because (1) large-amplitude, low-frequency stimuli may become audible at high intensities due to the high-frequency tail of the frequency tuning of inner hair cells (Smith et al.,1987) and (2) because the human ear is most sensitive to mid-range frequencies, increasing the amplitude of low-frequency sounds through cochlear curvature can affects low-frequency hearing limits.
- Pumphrey, Nature 1950; 166: 571
- Hemila et al., Hear Res 1995; 85: 31-44
- Smith et al., AJA (1987); 29: 125-38
- Rechiro & Temchin, PNAS; 99(20): 13206-10
- Can humans perceive sounds above 20 kHz?