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Sonic weapons are frequently used by military circles as non-lethal irritants to deter opponents. My question is; could a naturally evolved organism kill, or at least incapacitate, small prey using sound of some sort, and if so, how would it do this?

I'm assuming a focused beam of sound, likely ultrasound, would work best for a biological weapon intent on killing rather than irritating. Other than that, I don't know what to expect.

Edit: I forgot to mention, the organism I have in mind is terrestrial. I think the pistol shrimp's cavitation bubble and all that wouldn't work in air, at least not without a hugely more powerful snap.

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Bombs kill by sound, if you want to look at it that way

Bombs kill via a shock wave. A compression of air due to the expansion of whatever that defines the bomb. If strong enough, things can be carried by that shock wave — like shrapnel. But, if the shock wave itself is strong enough, it kills and destroys. A nuclear bomb is a great example. After the radiation (in the first gazzillionth of a second), what happens next is a shock wave that flattens everything for some distance from the detonation.

And another word for a shock wave is sound.

What we hear with our ears is nothing more than a series of very gentle shock waves. Some, a single sharp wave (like the clap of hands); others, a continuous susurration of sinusoids (speech, or a tuning fork).

So, can you kill with sound? Yup, happens with bombs all the time.

but, what's doing the killing and what is being killed?

GrandmasterB and Burki correctly points out that Synalpheus pinkfloydi (known as the "Pink Floyd Pistol Shrimp," whomever named the little bounder must have just returned from a concert) can kill by snapping its claw near its prey. Very small prey. We humans can kill things that small, too, by clapping near them. Gnats, for example, can be killed via clapping concussion.

If, on the other hand, you want to kill a human (by means other than biblical elephant stomping and lion eating, different kinds of sounds were involved in that incident, like tummy growling and angry elephant honking... I love that story) then we need to create considerably more sound.

Fast and Furious

Part of the problem is that you need both amplitude and frequency. And you need a lot of both. A sharp explosion can be thought of as half a sine wave (it's more than that, but I'm simplifying). The amplitude is magnificent, but so is the frequency, demonstrated by how fast the sound level climbs from zero to its maximum amplitude. The lower the frequency (a slower climb to that maximum), the easier it is for a body to absorb it and the greater the amplitude must be to kill.

This is why bombs can kill but you little brother's stereo can't.

Possible but Impractical

Can sound kill you? The short answer is “yes” — and, rather shockingly, the European Space Agency says that it now has such a sonic weapon in its arsenal that, if it was so inclined, could kill you. (Source)

What the ESA has is a huge air horn used to test satellites against the noise (read: vibration) caused during launch. Stand in front of it and you die. But if you visit that site you'll see the horn is as tall as a human. And that's the problem. There's a lot of human to kill and we can often withstand a baseball bat to the head — and it takes a honking (pun intended) lot of sound to exceed the impact force of a baseball bat.

Conclusion

  • As the size of your target animal decreases, the likelihood that the source animal can create noise of high enough amplitude and frequency that it will kill increases.

  • As the size of your target animal increases, the likelihood that the source animal can create noise of high enough amplitude and frequency that it will kill decreases.

My guess is that the sound energy required increases exponentially with the size of the creature you intend to kill. Meaning you very quickly get to the point that the source animal can't create enough sound to kill the target. I suppose you could suggest that the foot of the elephant that killed that poacher constituted a shock wave of a sort — but consider the size and weight of the elephant compared to its prey! On the flip side, Synalpheus pinkfloydi might give us a nasty pinch, but the sound it uses to kill its prey simply sounds cute to a human.

Therefore, I'm going to say no. It's implausible that an animal could cause enough sound to kill a target animal. Unless you artistically consider the impact of an elephant's foot to be "sound." But if you want to use this as a super power, I might point you to The Spleen.

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    $\begingroup$ If you stand too close the launch of a big rocket, the sound can kill you even if you are protected contact with from the hot exhaust gasses. I've read that one of the challenges of designing a launch site for a big rocket is, preventing the soundwaves from reflecting off the ground, damaging re-usable equipment, and damaging the rocket itself. $\endgroup$ – Solomon Slow Apr 10 at 19:00
  • $\begingroup$ @SolomonSlow, that observation about reflected sound waves is literally the reason the ESA built their sound horn - to test those effects. $\endgroup$ – JBH Apr 10 at 19:04
  • $\begingroup$ "another word for a shock wave is sound" This is mistaken. Shockwaves are supersonic by definition. The terms are not interchangeable. $\endgroup$ – Max Barraclough Apr 10 at 22:13
  • $\begingroup$ @MaxBarraclough, From Merriam-Webster Dictionary... Shock Wave: a compressional wave of high amplitude caused by a shock (as from an earthquake or explosion) to the medium through which the wave travels. Just because "sound" is something that someone or something "hears" doesn't mean that it's not a compression wave. And just because a human can't hear a sound (due to frequency or amplitude) doesn't mean it's not sound. You're suggesting that convention supersedes fact. The point my answer makes is that it does not. $\endgroup$ – JBH Apr 10 at 22:16
  • $\begingroup$ @JBH Shockwaves are a different physical phenomenon from sound waves. britannica.com/science/shock-wave $\endgroup$ – Max Barraclough Apr 11 at 21:10
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As with all questions like this, its good to first research what actually exists in our world of wondrously diverse life. Synalpheus pinkfloydi (yes, actual name) is a shrimp that hunts by snapping its claws so loudly it kills its prey.

Synalpheus pinkfloydi

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    $\begingroup$ Would a similar strategy work in air? $\endgroup$ – SealBoi Apr 9 at 18:02
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    $\begingroup$ Is it physically possible? Yes. It'd obviously need to be orders of magnitude louder. $\endgroup$ – GrandmasterB Apr 9 at 18:44
  • $\begingroup$ The shrimp's attack leverages cavitation to transform the snap motion into a concussive shock wave. The cavitation phenomenon specifically occurs in a liquid where a bubble forms out of the liquid itself containing the liquid in the gas phase, then subsequently collapses. The collapse happens very quickly, which is what transforms the energy that formed the bubble into a sharp concussive shock wave that concentrates the energy so that its impact is delivered over a shorter time. Cavitation can't occur in air, as the air is already in a gas phase. $\endgroup$ – Dan Bryant Apr 10 at 18:17
  • $\begingroup$ wow that shrimp must have some really comfortably numb claws from all that snapping..... $\endgroup$ – Efialtes Apr 10 at 20:34
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Bats use echolocation in air to locate and eat insects.

Toothed whales and dolphins used echolocation in water to located and eat fish and other prey.

And since it isn't understood how sperm whales manage to eat enough fish and squid for their needs there is a theory that they use sonic waves to stun, or maybe even kill, or maybe even start digesting, their prey.

So sperm whales are not only the largest and strongest predators known at the present, they might also be armed with sonic death rays.

Of course the sonic death ray theory is unproven.

https://scienceline.org/2008/05/ask-locke-whale/1

And as others have said, pistol shrimp use sound to kill their prey.

Added April 10, 2019:

And here is a creepy story from history. As I remember the story, after The Battle of the Nile 1 August 1798 or the Battle of Trafalger 21 October 1805 someone on a British ship saw a boy sitting on a cannon very still and when touched, the boy fell to the deck dead, without any visible wounds. The story was that the boy had somehow been killed by concussion from the British cannons being fired or maybe a French shell exploding nearby. Believe it or not.

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All the marine critters listed in other answers are good examples. Water is non-compressible, so underwater transfer of sound is much more efficient than in air.

In air, transmission losses make ultrasonic power transmission a non-starter. This is basically why you can't charge a phone using ultrasonic power without cooking stuff and going deaf. Perhaps a really big critter could shear apart a weak ~centimeter sized critter but it would be working pretty hard. This could change based on what your critters are breathing, basically the more dense and polar the fluid the better it will transmit sound.

Large bombs that kill things with the concussion wave (e.g. everything in the modern era) are kind of a technological "pistol shrimp that works in air". The concussion wave is not so much a continuous wave as a single pulse-- The shockwave pulse "accumulates" tons of energy because the energy can't move faster than the speed of sound in air. Presumably a critter that can move faster than the speed of sound can generate shockwave pulses in air, just like a pistol shrimp's cavitation bubble makes a

However, many insects and mammals use hairs or whiskers to sense things. Sound could in principle "jam" whiskers and whisker-like hairs that detect air movement. As human hearing is also implemented as hairs whose movement couples to sound, this would be the equivalent of making it too noisy to hear, except it would probably impact sense of place/movement more than loss of hearing affects humans. This could be incapacitating. Apparently really loud sound can couple to mammalian inner ear proprioceptive sensation (also a bunch of little hairs).

The mammalian sense of touch is comprised of several independent systems, and some of them sense frequency more than displacement even though they seem to be sensing "spatially localized" stuff. See wikipedia "mechanoreceptors". One could imagine "overwhelming" light-touch receptors over the whole body, then slowly sliding up some poisoned tentacles that can no longer be felt by the victim. This seems a bit contrived though.

Also, some hunters that use echolocation will intentionally jam each other's pings.

TL;DR no "killing things with a soundbeam in air" but "blinding things with soundbeams in air" and "killing things with soundbeams in heavy fluid" are plausible

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Whilst not a weapon, many animals can use sound to incapacitate prey. More specifically, very loud and unexpected sounds.

This is likely not what you had in mind, but the fight or flight response has a third option, freeze. Instead of making a decision one way or another, an animal becomes indecisive and instead freezes up and feels as though they can’t move, preventing a creature from moving sounds an awful lot like incapacitation to me. This ‘freeze’ response is where the phrase “like a deer in the headlights” comes from. Rather than getting out of the way of the car, the deer is so scared to move that it doesnt and gets hit by the vehicle. This means that any creature that can make a loud noise has the ability to incapacitate another through fear.

Applying this more specifically to your question, if your creature can produce a sound so alien, so terrifying that hearing this noise often triggers the ‘freeze’ response, that could be utilised to its advantage. Using the power of fear, it can immobilise its prey long enough to catch and kill them. Whilst fear in unpredictable and everyone who experiences it may react differently, it still could allow for your creature to gain a huge advantage when it does work in their favour.

As pointed out in the comments, ventriloquism could also aid this creature, if they can throw their voice, their prey would not know where to run. Alternatively, an easier way to replicate this effect is to have multiple of these creatures surround their prey. Here is an example of a tiger’s roar being able to paralyse its prey. Supposedly the key is low frequencies

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The Wikipedia article on Sonic Weapons talks about low-frequency, high-amplitude sounds causing cavitation and tissue shearing (basically, like tiny shockwaves). It doesn't have to be super powerful if it's persistent; many small blows in quick succession can add up to significant damage.

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