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Context: in a scenario where batteries suddenly became extremely energy dense due to some events between the main characters, I was thinking about which electromagnetic weapons I could use, such as rail/laser/microwave guns, and it came to my mind that sound waves could also be useful to propel projectiles (since they are artificially made with electromagnetism).

Some time ago I read that if you had a strong enough source of sound (like a "super-speaker"), the sound waves would start to act like an explosion and kill whoever is unlucky enough to be around.

However, the thing is that this sound wave would have to be made in a short period of time (like a normal firearm) and incredibly strong to make a "bubble" of air behind bullets and thus propel them.

And thus, it came to my mind that maybe this impact/sound wave could be strong enough to destroy its user's eardrums. But I can't tell for sure.

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    $\begingroup$ What do you suppose the difference is between a really powerfully intense soundwave, and the shockwave of an explosion? Now, would you consider a narrow metal tube into which a projectile fitted, and at one end of which some "bang powder" is deflagrated in order to generate a suitable bang and push the projectile, a kind of loudspeaker? $\endgroup$
    – Starfish Prime
    Commented Apr 1, 2022 at 15:15
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    $\begingroup$ A loudspeaker is already an electromagnetic device, in its core. If you would put the bullet on top of the loudspeaker cone, it would pop out when you put a drum sound. But then, you'd actually be constructing an electromagnetic gun which are based on the principle of electromagnetic accelleration, using several coils in series, and a bullet traveling through these coils. That is far more effective, it will require less weight and volume than having to put all this energy into a single coil, like a loudspeaker does. $\endgroup$
    – Goodies
    Commented Apr 1, 2022 at 16:29
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    $\begingroup$ @StarfishPrime: A sound wave is by definition periodical, which means that the sound pressure can never get above 1 atmosphere, or about 94 dB SPL. A shock wave on the other hand can be made as powerful as you want. $\endgroup$
    – AlexP
    Commented Apr 1, 2022 at 18:10

3 Answers 3

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Regardless of how a projectile is imparted with energy once it's no longer being propelled the kinetic energy of the projectile is all that's needed to determine performance.

If we take a 9mm round as an example when fired out of a pistol you're looking at about 630 Joules.

In an ideal system with no losses it will take a minimum of 630 Joules of energy to launch a round. In reality given all the losses it's going to take a lot more than that.

So how efficient can we make it? Sound radiates in all directions so there's going to be a lot of losses. It would be best to contain all the sound energy in a chamber with the only outlet being through the projectile. We also want to impart all this energy in a very small amount of time. 1 Joule can power a 1 Watt device for one second. If we want to impart all our 630 Joules of energy in a 100th of a second we're going to need 630,000 watts of speaker system, capable of dumping all that energy almost instantaneously, into a very small projectile, with 0 energy loss. Keep in mind that most home speakers are less than 20 Watts.

Or you could use a very small amount of explosive to do the job for you in a much smaller and more efficient package.

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    $\begingroup$ I'm assuming the OP is trying to get to an energy weapon/projectile hybrid, so is there anything to get around this? Can the sound energy be applied over more time in a barrel like in a rail gun with multiple coils? Could cool secondary effects (stunning, breaking rocks, whatever) make this a fun tool for an adventurer (practicality be damned)? $\endgroup$
    – DWKraus
    Commented Apr 1, 2022 at 17:05
  • $\begingroup$ @DWKraus Op asked for energy to propel a projectile with sound. The lower bound is going to be the same regardless of the mechanism. If OP wants something cool, they don't need to choose to run the numbers. Most cool weapons in fiction aren't well grounded in the hard numbers, and they're more badass for it. Lightsabers for instance are so much cooler than anything you could create if you allowed your fictional world to be bounded by the mundane. $\endgroup$
    – sphennings
    Commented Apr 1, 2022 at 17:32
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YOUR PROPULSION SYSTEM WOULD DESTROY MORE THAN JUST THE USER'S EARDRUMS

we're going to need 630,000 watts of speaker system, capable of dumping all that energy almost instantaneously, into a very small projectile, with 0 energy loss. Keep in mind that most home speakers are less than 20 Watts.

As @sphennings stated, we're going to need to make a massive amount of noise in order to propel the projectile. And that much vibrational force is going to destroy a lot more than the user's eardrums. In fact, it might even be enough to rip apart the gun, the bullet being fired, and the person pulling the trigger, along with everything else in a large area around the gun.

And honestly, your gun is less of a gun and more of a bomb at this point. A bomb that is prohibitively energy-intensive and requires the user to be holding it in order to trigger it, with no delay between the triggering and the detonation.

Needless to say, there are cheaper, quieter, and less destructive ways of committing suicide.

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The safe way (for the user of course)

After reading the previous answers, I have come up with a working solution that can't be heard by the human ear. This is a complicated answer based off of an Audrino lifting a piece of A10 paper with ultrasonic waves with 18 watts of power (9 volts at 2 amps). A piece of A10 paper weighs 0.056g grams, using this information I made a watt to gram ratio of about 1:0.0031 Watts per gram. Meaning, if we tried even LIFTING a 9mm round, it would take 34838.70 watts of power. Assuming those batteries are what you say about them, The force to fire a 9mm is 460 joules, after a bit of calculation that is 46738.1574 grams of force. So we can use the watt-gram ratio to find that 46738.1574 grams would need 15076824.9677 watts of power to fire a singular 9mm with ultrasonic waves. to lanch larger projectiles you would need to use the watt-gram ratio and divide the weight of the object(in grams) by 0.0031 to get the amount of watts needed to fire it. I really hope this helped and thank you for your time.

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    $\begingroup$ Side note: If the 9mm took 630 joules it would need 20648695.0645 watts to fire. Different results, $\endgroup$
    – WellActually...
    Commented Apr 6, 2022 at 17:55
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    $\begingroup$ Side Side note:This can also be used to lift and/or throw just about anything $\endgroup$
    – WellActually...
    Commented Apr 6, 2022 at 18:00
  • $\begingroup$ Please include a link to your references for the energy consumption, and what indicates it is safe. $\endgroup$
    – DWKraus
    Commented Apr 7, 2022 at 2:16

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