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I learned that the human ear doesn't hear sounds outside the range of 20-20,000 Hz.

I can understand that sounds below this range are so weak that they don't affect the ear.

But why do sounds above this range have no effect on the human ear?

The reason I'm asking is that I'm going to teach a class on hearing and sound waves, but I'm confused about the direct reason of the lack of perception of ultrasonic waves - is there a barrier in the ear or what is the direct case of blocking of hearing for these waves?

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    $\begingroup$ Welcome Abd-Elaziz Sharaf. This is not simply a Q&A site, here we are expected to show the results of our own investigations into a subject here, you can edit to tell us what your searches found and where the sticking points are. See our section on how to ask then you can edit your question to include your researches. $\endgroup$
    – Jiminy Cricket.
    Mar 18, 2022 at 15:02

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It is perhaps easier to answer the question by rephrasing it as 'What determines the frequency range that an ear can hear'?

In order for a sensory organ to respond to sound, it must be set into motion by the sound. This is usually achieved through some support structures connected to the ear. In mammals, this includes the outer ear, the ear canal, the eardrum, the middle ear and complex structures in the inner ear (the organ of Corti). Generally speaking, when these structures are very small, relative to the wavelength of the sound, they will not be moved sufficiently by the sound and therefore not respond to the sound. When they are very large, again relative to the wavelength of the sound, they cannot follow the rapid oscillations of the sound due to their inertia and therefore do not respond to the sound. Therefore, in general, large ears on large animals tend to mean better hearing at low frequencies, whereas small ears on small animals tend to mean better hearing at higher frequencies. The upper hearing limit of humans is determined by the mechanical properties of especially the middle ear and is likely a reflection of specializations to improve hearing at lower frequencies, required for hearing and understanding speech. Many other mammals, typically smaller than humans, have much better high-frequency hearing than humans. In almost all cases, however, this is at the expense of poorer hearing at lower frequencies, where we have good hearing.

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    $\begingroup$ Welcome and thanks for your answer. You say: 'large ears on large animals tend to mean better hearing at low frequencies, whereas small ears on small animals tend to mean better hearing at higher frequencies.' I reckon you mean the inner ear, and specifically the length of the cochlea? This webpage from Van der Bilt uni doesn't support that claim. Further, why would mammals have evolved ears designed for low-frequency hearing? Bats are a big group that have exquisite high-f hearing $\endgroup$
    – AliceD ♦
    Apr 25, 2022 at 7:06
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    $\begingroup$ The length of the cochlea basically limits the the frequency range, assuming the frequency distribution is about the same across species (not entirely true either, given that the Greenwood map differs across species). The upper frequency limit is determined by factors other than the inner ears length and independent on the size of the animal, afaik. $\endgroup$
    – AliceD ♦
    Apr 25, 2022 at 7:20
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    $\begingroup$ There are several scaling phenomena at play here at the same time. In general, large animals have large sound producing structures and therefore are better at making loud sounds at low frequencies. At the same time, they also tend to move over larger distances than smaller animals and therefore benefit from the longer transmission of the low frequencies over the higher frequencies. They also have larger middle ears and inner ears, so it all goes hand in hand. In general. $\endgroup$
    – user69929
    Apr 25, 2022 at 15:58
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    $\begingroup$ There are always exceptions, and bats and dolphins, have extremely well-developed high-frequency hearing. For dolphins much better than what you would expect from their size alone. The reason is echolocation, because they benefit from the higher resolution they can get by using sound with small wavelengths (high frequencies) to 'see' finer details. Dolphins tell us that it is possible for mammals much larger than humans to have ultrasonic hearing. That we do not have ultrasonic hearing tells us that in our evolutionary history, ultrasonic hearing did not increase survival of our ancestors. $\endgroup$
    – user69929
    Apr 25, 2022 at 16:11
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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.

References
- 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

Further reading
- Can humans perceive sounds above 20 kHz?

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  • $\begingroup$ Wow... that's informative... is there any diagram descriping those 3 steps ? - for my note I was taking about physical wave energy but I'm happy with your notice.. I think @Jakob discriped physically and you did physiologically but I can't imagine your description clearly $\endgroup$
    – Abd-Elaziz Sharaf
    Apr 30, 2022 at 22:30
  • $\begingroup$ @Abd-ElazizSharaf which 3 steps? $\endgroup$
    – AliceD ♦
    May 3, 2022 at 7:25
  • $\begingroup$ stimuli - ear sensitivity factors - inner hair cells affect $\endgroup$
    – Abd-Elaziz Sharaf
    May 4, 2022 at 9:49

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