Question

(The question has its origin because I asked myself in how far frequencies outside our perception can harm our hearing.) First of all, the energy of a mechanical wave (in this case, the sound wave, which stimulates periodic movements of a gas) is proportional to both amplitudeandfrequency. Often, I read that it is written that energy depends only on amplitude, but when I have two waves with the same amplitude, and one has a higher frequency than the other, it apparently carries more energy. It has a shorter wavelength and moves the molecules much faster over the same distance. SinceE=F∗sE=F∗s, a wave with a higher frequency must carry a greater force (and therefore energy) to overcome the inertia of the molecules more quickly. I'm not entirely familiar with how the ear works, but I do know that it has three sections. Waves enter through the outer ear, continue to vibrate the eardrum with their frequency, the middle ear amplifies this, and then somehow it goes into the inner ear, where there's a fluid-filled cochlea with hair cells. Certain frequencies can only reach specific spots due to the cochlea's geometric structure, allowing differentiation of pitch. Specific hair cells vibrate more strongly for certain frequencies, opening ion channels and sending signals to the brain, or something like that. However, what I'm wondering now is whether it's really about the geometric structure or the fact that the hairs are of different lengths, thus having different natural frequencies, and frequencies that match their natural frequencies simply stimulate the right hairs through resonance? If that's the case, one can consider it similar to a forced oscillation, for example, when ultrasound reaches the hairs, we're far from the resonance case, so the hairs vibrate at ultrasound frequencies, but the displacement is extremely small, is that the reason they don't transmit signals? How can I understand this? And if ultrasound stimulates the hairs to tiny vibrations, where does the rest of the ultrasound wave energy go? Do the hairs heat up, or do they simply reflect it (since in non resonance case the energy transfere between an exciter and oscillator isn't really good... try to excite a spring with a weight attached to it with a significantly faster frequency than its natural frequency from above, and you'll see that even when you wiggle it with high amplitude, the oscillator hardly moves from its position but onlytwitchesfaintly at your specified frequency, so same frequency but lower amplitude means the energy almost stays in the exciter itself)? TLTR: Are vibrations in the ear stimulated at frequencies beyond our perception, and what is the reason we don't perceive them even when we increase the amplitude (decibels)? (And I'm referring to vibrations that don't originate through bone conduction, I've also read about this possibility.)

Answer 1

Human ears perceive sound through hair cells resonating at specific frequencies. The energy of a sound wave is affected by amplitude and frequency, but humans cannot perceive ultrasound due to extremely small vibrations at non-resonant frequencies, resulting in energy dissipation rather than sensory stimulation.

Step-by-step explanation:

The perception of sound in humans is a complex process that involves the conversion of sound waves into signals that the brain can interpret. The energy of a sound wave is influenced by both its amplitude and frequency. A higher frequency results in more energy being transferred because more waves pass through a point in a given timeframe. In the ear, the mechanics of hearing involves the outer ear funneling sound waves onto the eardrum, the middle ear amplifying these vibrations, and then the inner ear's cochlea, where hair cells on the basilar membrane respond to particular frequencies.

Hair cells are tuned to resonate at specific frequencies and have a role in the place theory of hearing. High-frequency sounds make the part of the basilar membrane near the entrance portal (the oval window) vibrate, whereas low-frequency sounds affect areas farther along the membrane. Most humans can perceive sounds between 20 and 20,000 Hz, but sounds outside this range, such as ultrasound or infrasound, don't produce noticeable vibrations of hair cells or are not picked up due to the place theory.

When ultrasound waves encounter these hair cells, they cause extremely small vibrations and little to no energy transfer, because they are far from the resonance frequencies of the hair cells. The non-aligned vibrations may be dissipated as heat or reflected, meaning the energy does not significantly stimulate the hair cells and therefore doesn't result in perception.