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Humans can detect sounds in a frequency range from about 20 Hz to 20 kHz. Then ultrasound is above 20 kHz

Occupational exposure to ultrasound in excess of 120 dB may lead to hearing loss.

Source

How can ultrasound hurt human ears if it is above the audible range?

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    $\begingroup$ I am not sure if it is a physics question. Try Biology SE $\endgroup$ – Bhavay Dec 21 '20 at 5:01
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    $\begingroup$ Do you have a clear idea of what decibels actually are? $\endgroup$ – Mastermind817 Dec 21 '20 at 5:16
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    $\begingroup$ Imagine you're a species that can't see ultraviolet light. Nevertheless, I point a 1000W ultraviolet laser at your eyeballs. It damages your eyeballs anyway because it's a 1000W laser. $\endgroup$ – user253751 Dec 21 '20 at 13:28
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    $\begingroup$ Vases have no means to detect sledgehammers. $\endgroup$ – Emilio M Bumachar Dec 21 '20 at 18:54
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    $\begingroup$ i think this is definitely a physics question. The hearing nerve [and/or brain] can only process 20-20khz frequency, based on the ear drum being a physical vibrating membrane. If the frequency is at some ultra level (above 20khz) it is the amplitude (above 120dB) where the sound [pressure] wave will rupture your ear drum you'll just never hear it happening. Similar to a ultraviolet (or infrared) laser to your eyes that you would never see but would definitely damage your eyes. $\endgroup$ – ron Dec 21 '20 at 20:38
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Ultrasound is used to break kidney stones and "burn" brain tumors. So no surprise that it can damage body parts, including inner ear. The effects of ultrasound on human body are multiple. One of them is heating due to absorption of the mechanical energy in the tissue and conversion to heat. This is used in physiotherapy as a desirable effect but it can be damaging if too much power is absorbed. There is also a mechanical effect. Parts reached by ultrasound vibrate so if the amplitude is too high they may break. The effect is used in ultrasound cleaners as well in medical field, to break apart stones and blood clots. Again, too much can do bad things. For this reason there are strict rules on maximum power allowed in any ultrasound imaging system. These effects have nothing to do with the fact that our ear can or cannot hear the sound. In similar way, your eyes can get burned by microwaves even though you don't see them.

Edit

Richard Tingle mentions an interesting aspect of this problem. I focused my answer above on the possibility of physical damage to the tissue due to ultrasound. In order to prevent these effectss, the limits for the maximum power used for ultrasound imaging is for example around 750 $mW/cm^2$ for sturdy organs like heart, liker, kidneys. For more sensitive areas like baby ultrasound or eyes the limits are much lower but stil of the order few (or tens of) $mW/cm^2$ . Now let's compare these with airborne ultrasound at 120dB level. In terms of intensity this means 1 $W/m^2$ or 0.1 $mW/cm^2$. And this is the same no matter if the sound is audible or not. The sensation we get depends on the frequency range but the physical intensity is the same.

This shows that the damage to the ears is of a different nature, and usually is a long term effect. One efect is an over-working of the sensitive hairs in the inner ear which in time can result of death of the cells in these hairs. The hairs are tuned to frequencies in the audible range. This does not mean that they do not vibrate when the frequency is off-resonance, just that the vibration amplitude is lower. Indeed, the limits established by the International Non-Ionizing Radiation Comitee for exposure to ultrasound are 110 dB for frequencies between 20 and 100 kHz (ultrasound) and just 75 dB for audible sound. This indicates that the effect of airborne ultrasound is less pronounced than that of sound. The difference between 110 dB and 75 dB is significant in terms of intensity. Hovewer the effect is here, as proved by many studies used by INRC to established their limits. The effects are not just possible hearing loss but also nausea, dizziness, headache and other things. So indeed 120 dB can be damaging even for ultrasound. You don't need to hear it to hurt you.

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    $\begingroup$ ESWL is 1000s stronger ultrasonic pulses guided to intersect, compared to imaging. That said 70-80% of the time I have an ultrasound for kidney stone diagnosis it will start to move and obstruct within 24 hours. It's far beyond random- it is as if the movement of the probe or the direction of the instrument is enough to irritate the tissue inside me, causing blockage and then excruciating pain. I'm told it's coincidence, however... 73 kidney stones I've gotten pretty good at knowing triggers. $\endgroup$ – J.Hirsch Dec 22 '20 at 13:06
  • $\begingroup$ But ultrasound cannot damage organs. Even you can break the tumors if you had the power to break them at the exact points. Tumors require very less energy to break. $\endgroup$ – user283021 Dec 23 '20 at 3:23
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    $\begingroup$ @user91594 Very less compared to what? The absorption of ultrasound in soft tissue is about the same. It's easy to damage healthy tissue too. Heating the living cells to temperatures between 43 to 80 degrees kills them. This is the mechanism used usually to kill malignant cells. In order to avoid killing healthy cells they use ultrasound focused to the volume of the target or multiple beams overlapping over that volume. "Ultrasound cannot damage organs" is what they tell you about imaging ultrasound. And this is true due the power regulations imposed on the equipment. $\endgroup$ – nasu Dec 23 '20 at 4:44
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    $\begingroup$ An interesting question though, is if for the same dB level a sound you can't hear is less damaging to your ears than one you can. Since, as I understand it the hair cells used to hear a frequency have the same resonant frequency as the sound they will detect. And so are presumably most vulnerable to those frequencies? The same way a building is most likely to be destroyed by an earthquake at the buildings resonant frequency $\endgroup$ – Richard Tingle Dec 23 '20 at 14:42
  • $\begingroup$ Indeed. I edited the questin to discuss this aspect. $\endgroup$ – nasu Dec 23 '20 at 20:13
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Sound, independent of frequency, is (or is caused by) periodic changes in local pressure. These periodic pressure changes can be turned into periodic movements, e.g. it‘s moving the eardrum and the other parts of the inner ear. Even when the sound can not be perceived by you (as sound), the eardrum and the other parts are still moved and they may be moved so strongly that they may get damaged.

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An experiment to try (at your own risk):

Get a hold of one of those "cool mist" humidifiers -- the ones that use ultrasonic transducers, rather than just a wick and a fan. Fill the basin with water and turn it on. Put your finger over the transducer. It won't be long before you pull your finger away from the pain (personal experience).

Now imagine that happening to your ears.

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Sound at particular high intensity (not frequency) can harm your hearing.

Even though ultrasound cannot be detected by humans ears, at high intensity (as for all sound) it can still cause damage to human ears. Ultrasound with intensity in excess of $120 dB$ can cause hearing damage.

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If you have occupational exposure to 120 dB acoustic energy above 20 kHz, then either your ears are within a meter of, or in solid or liquid contact with the energy source. Through ambient atmosphere, ultrasound dissipates in much less distance than lower frequencies. At typical conditions, compared to 20 kHz, 100 kHz is attenuated 4 dB more per meter, i.e., its power drops a bit more than twice as quickly as you move away from the sound source.

at 25 deg C, 100 kHz drops 4.5 dB/m, but 20 kHz drops only 0.5

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    $\begingroup$ Your graph does not seem to support these values (6 to 10 dB) but I agree with the general idea of attenuation increasing with frequency. $\endgroup$ – nasu Dec 22 '20 at 12:51
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What I feel that ultra sound and infrasound are not audible because infrasound generates small to and fro motion of our eardrum which is un observable and our ears cannot detect it (biologists should tell you the exact range)

Some reason can apply here. The to and fro motion is so fast that it becomes uninterpretable

Regarding the damage. It is like using a device with higher voltage and lower voltage than the rated voltage

Fast to and fro motion can result in damage of eardrums

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