knowing that humans have a hearing frequency of about 10hz up to 20kHz, what happens when a sound of 30kHz is heard? Will we just ignore it because it’s out of bounds, or will the sound shatter our ears?

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I got curious about wavelengths lately, so I’m curious about how our eyes and ears work 🙂

In: 1

Well, what I do know is, we have these ultrasonic cleaners (some sort of bath to clean metal parts, also used for jewelry) and we are not allowed near it when it is operated because although you can’t hear it they say it will make you deaf. Or so I was told.

You can’t hear it, but depending on the frequency and how strong the sound is, you may feel a pressure or discomfort or pain in your ears.

If you have *really good speakers* on your device, that can actually play all the way down to 16 Hz and all the way up to 20,000 Hz, you can use an [online sound generator](https://www.szynalski.com/tone-generator/) to see how well YOUR hearing is. Not everyone can hear the full range, and actually age has a big effect on hearing ([older people lose some](https://decibelhearing.com/hearing-loss-overview/high-frequency-hearing-loss/) of their high frequency hearing).

Ultrasound can damage your hearing in two ways: resonance at sub-harmonic frequencies, and heat. Let me explain.

Normal hearing happens because vibrations in the air are transmitted through the ear drum, via a few bones and other in-between stuff, to the cochlea. The cochlea is a stiff tube rolled up into a spiral. The tube is wide at one end and then tapers in width along its length. All in all it looks a lot like a snail’s shell.

The differing width along the length means that different parts of the cochlea have different *resonant* frequencies, i.e. frequencies they like to vibrate at. E.g. the base of the cochlea, where it’s widest, likes to vibrate at very high frequencies, up to ~20 kHz. Whereas the tip, where the tube is narrowest, like to vibrate at low frequencies, down to ~200 Hz.

All along the cochlea are tiny “hair cells”. Cells that look like hairs. They turn vibration into electrical signals, which are sent to your brain. If you play a sound at 3 kHz, the cochlea will vibrate mostly somewhere in the middle, and so mainly the hair cells in that part will be stimulated and sending signals. The brain thus can interpret the location of the cochlea that’s responding most, to figure out what frequency (or more likely, frequencies) are present in the sound that’s being heard.

Permanent hearing damage occurs when these hair cells are harmed or destroyed. This happens when the vibrations get too intense, tearing the cells apart.

Ultrasound is outside the resonant frequency domain of the cochlea. No part of the cochlea vibrates along with those frequencies. But that doesn’t mean that those frequencies don’t cause *any* vibration. In particular, they can cause vibrations at *sub-harmonic* frequencies, i.e. frequencies that are exactly half, or a third, or a quarter, or some other even fraction of the fundamental frequency. So e.g. if you play a sound at 30 kHz, this may cause vibrations in the cochlea at 15 kHz. Through this mechanism, therefore, a 30-kHz tone can still cause damage if it’s played loud enough. Your tolerance in terms of decibels is higher for ultrasound, though, since these sub-harmonics don’t contain as much vibrational energy. Also, since they cause vibrations in your cochlea, **you will hear those vibrations as normal sound**. In other words, the “damaging part” of the ultrasound will be audible, so you can’t damage your hearing that way without being aware of a loud noise.

The above applies to ultrasound that’s pretty near the range of audible sound. Ultrasound used in e.g. medical applications has a far higher frequency. For instance, high-intensity focused ultrasound (HIFU) treatments, which can be used e.g. to destroy tumors, uses ultrasound that’s around 20 MHz, i.e. 1000 times higher in frequency than the highest we can hear. At these frequencies, resonance starts to happen at a molecular level, rather than at the level of tissues or cells. So you’re causing molecules to vibrate faster, which means you are heating them up. And that heat can obviously cause cell damage. In the case of cancer treatment, that’s exactly the point. But if you directed this kind of high-energy ultrasound at your ear, you could damage your hearing that way, without hearing any sound. Of course, this isn’t specific to your hearing – any tissue that you target this way can be damaged and it doesn’t really have much to do with conventional conceptions of sound or hearing any more. Also, it’s worth pointing out that the energy delivered by these ultrasound waves is hundreds of times more than even the loudest audible sounds you are likely to encounter in your life.

Sound is a physical wave of pressure in the air. Compress air, and it’ll send a ripple of pressure through it. That pressure can affect us in a variety of ways. For example, an explosion, which creates a wave of very high pressure, can knock us over or even crush our organs. We happen to have some special cells in our ears than can detect a range of these waves and send an electrical sjgnal to the brain, which the brain then interprets as a noise.

A sound that falls outside this range won’t get detected, so no signal will be sent. However, this isn’t us ignoring the wave, it’s us lacking the cells to detect it. Theoretically, it could be outside our hearing range, but still powerful enough to shatter our ears. If that happened, we wouldn’t hear anything, we’d just suddenly go deaf.

Sound is pressure, the louder the sound the more pressure it has… a sufficiently loud sound can damage you whether you hear it or not…

An explosion basically a huge pressure wave, it is technically a sound

As for the range, your ear is made up of many things, ear drums, the little bones in your middle ear and the hearing follicles in your inner ear…. every part has a range of vibration it can carry… if the sound is lower or higher than the limit the sound just doesn’t make your ear things move.

It is just accustics and mechanics, how hundreds of thousands years of evolution finetuned our hearing, having a range that picks sounds in nature potentially harmful to us… or the range of human speech to understand each other