# How to make a Sound Gun

I have this fictional sci-fi galaxy (similar to Mass Effect) in which a race of aliens is blind, but has superhuman hearing. I want to make these aliens interesting and somewhat scary on the battlefield, so the first thing I thought of was a sound gun. That is, a weapon that fires a directed beam of sound at a target. The original plan was that they could use the harmonic frequency of the human brain to blow up people's heads, but at this point I'd be happy if they could use it simply to stun or disorient foes.

The main problem I'm dealing with is that I don't think sound can be fired in a beam like light can. From experience, sound tends to be omni-directional or spreads out almost immediately. However, I know my knowledge of science is lacking, so I was hoping I could come here for some definitive answers.

So, in summary, can there exist a weapon that fires a directed beam of sound? How might it work, or if it's not possible, how might a similar sort of weapon work to get somewhat loud results?

You shouldn't be affected by the sound as it goes past, at least not as much as you would if you were in its path. I'd be hoping for the ability to temporarily or permanently deafen foes with such a weapon, but a smaller effect that can be scaled up is fine. As this is a sci-fi setting, there is a lot of room for technology and power, but I would like something that's based in real science.

• This question is more suited for physics any way here you go😉 – user6760 Aug 4 '15 at 13:24
• Related: worldbuilding.stackexchange.com/questions/11610/… the first answer may be helpful to you. – James Aug 4 '15 at 13:51
• In order for sound to be well directed, the speaker has to be larger than the wavelength. Given that sound travels at about 333m/s, that means that a 1 metre diameter speaker is needed to direct sound of 333Hz (approximately the E above middle C.) Achieving directed sound with speakers smaller than the wavelength is difficult. – Level River St Aug 4 '15 at 18:49
• race of aliens is blind, but has superhuman hearing. I want to make these aliens (...) scary on the battlefield, so the first thing I thought of was a sound gun. Wouldn't that hurt THEM more than it hurts us? Wouldn't they have a massive blinding light cannons which makes it impossible to see in the battlefield and permanently damage retinas? Just like we would use massive EM pulses against robots (like Matrix's ships do) because EM are pretty much inoffensive against ourselves (so no friendly fire!) – xDaizu Oct 18 '16 at 14:25
• @xDaizu well, part of the reason I asked this question was to see if a sound gun was at all feasible. I agree it might not be the best option for these aliens, but I wanted to know what could be done with sound, after which I would be able to decide if it would be useful. As it turns out, I think you're right, but I didn't know until I'd asked. – DaaaahWhoosh Oct 18 '16 at 16:07

## Ultrasonic Gun

It's well known that with the correct frequency applied that sound, and sound alone can shatter a wine glass. These demonstrations are always spectacular, so let's build a weapon on the same principle.

Assumptions

Since we are working with sound, above a certain threshold, a sound wave will just pulverize the target. The CDC lists 10psi of overpressure as

Most people are killed

so let's use that as our upper bound for power requirements for this weapon since we want to liquify the person, not instantly pulverize them.

Further, we assume that we can overcome the high natural elasticity of skin to cause it to liquify.

Safety Concerns

The OP states that this will be human mounted weapon so safety precautions need to be made to ensure the weapon carrier doesn't get liquified too. While ultrasound is still sound, it is "aim-able" in that pointing the ultrasonic transducer sends sound in that direction and not anywhere else. As long as the weapon bearer isn't "looking down the barrel" then they should be okay.

Power Requirements

The 10psi overpressure threshold set out in the Assumption section is 700 times the limits of human hearing (130db). Speakers that generate 110db of sound often require thousands of watts. Granted, the energy from normal speakers is spread across a much wider area than an ultrasonic transducer would but I think that sets a decent lower bound on the power requirements for this this weapon. Given that also don't know the power requirements to achieve destructive resonance in human tissues, the power supply may be asked to provide megawatts of power but since we don't know how much power it takes to liquify human flesh, let's just hand-wave and call it good.

Regular speakers just don't have the power output required or the directionality that we want, so let's go with ultrasonic transducers. Let us assume that a megawatt class ultrasonic transducer can be created and matched to adequate heat dissipation tech.

Frequency Requirements

There is no one frequency that the human body resonates at, or perhaps there is but the power requirements to do so amount to getting hit by the blast wave of high explosives.

At a minimum, you only need to know two frequencies: muscle and skin to count as "liquifying the target". Muscle alone will just make the target fallover. Skin alone just make the target look naked. This leaves the horrifying prospect that the brain are still intact but the target's body is liquifying. If the nerve endings are still functional then.....Death shortly follows but looking down to watch your body turning to liquid.....that's nightmare fuel.

Killing someone may not be as effective as just maiming them by liquifying their skin. Wounded soldiers take significantly more resources to treat/transport than a dead soldier.

Time for Effect

I have no numbers but I think the effects would happen very quickly, just a few seconds. This makes it an effective battlefield weapon because you can't expect a target to stay still for a minute or so.

Weapon Design

Since an ultrasonic transducer by definition emits sound at greater than 20Khz, we will need a second transducer to use interference to generate the target frequency. Both transducers can be fitted to a gun-like mount linked to the power supply backpack. Heat dissipation from the power source and the transducers will be important considerations. Add a control to allow the weapon carrier to select a target frequency with some presets for skin, bone, etc. However, access to the raw frequency permits the carrier to experiment with finding the frequency for stone or metal.

Sound guns; also known as Long Range Acoustic Devices (LRAD) in the US military; exist, were developed by the military, weaponized, and deployed several years ago (circa 2005).

These weapons project a highly directional sound beam and have been used to "discourage" Somali Pirates when they attack cruise ships.

According to the manufacturer's specifications, the systems weigh from 15 to 320 pounds (6.8 to 145.1 kg) and can emit sound in a 30°- 60° beam at 2.5 kHz.

and some technical information about sound directivity:

The parameter "ka", which is the wave number multiplied by the speaker radius, is often used to characterize sound source directivity. For this source, ka=19 at 2.5 kHz, and according to the LRAD data sheet, the beam angle of about 30 degrees total is precisely what is predicted for a regular loudspeaker.]

Small spherical "point-source" acoustic devices follow the known inverse square law, which predicts the loss of 6 dB per doubling of distance from the source, solely due to geometric spreading. Large speakers (or large arrays), such as these, have an interference pattern in the nearfield which produces peaks 6 dB higher than the output pressure and nulls where the pressure is essentially zero.[5] The larger the speaker, and the higher the frequency, the longer the effective nearfield. The nearfield for this device is approximately 8m.

1. The Vortex Cannon is absolutely possible. It just needs refinement.

2. Additionally you might be interested in Kate Bush - Experiment IV

No technical details are shown but the idea is there. (Not suitable for nervous children!)

Song lyrics:

We were working secretly
For the military
Our experiment in sound Was nearly ready to begin ...

They told us, all they wanted
Was a sound that could kill someone From a distance

3. The idea of focusing sound has already been tackled here.

How can you focus sound?

• Could you expand on some of these? As soon as a link dies we'll lose every detail there. – Samuel Aug 4 '15 at 19:40
• A torodial vortex is not “sound”. – JDługosz May 4 '17 at 5:27

Parabolic dishes can be used to focus sound waves. With a pair of parabolic dishes facing each other, people are able to talk at more than 100m distance without straining. This is sometimes seen in childrens' play areas.

The gun in chasly's excellent answer burns 100L of air with the corresponding amount of acetylene: that's about 20L of oxygen and 10L of acetylene: about 60g of explosive material. I think a suitably reinforced 2-3 metre dish with 1kg of solid explosive at its focus would make a less bulky and probably more effective weapon. Admittedly, i've never built one.

• So do you mean you set off an explosion in the focus point of one dish and someone is destroyed at the focus of the other? That's very ingenious. However, persuading the target to stand still at the second focus point might be tricky! – chasly from UK Aug 4 '15 at 18:08
• @chaslyfromUK no, you just use one dish at the weapon end. – Level River St Aug 4 '15 at 18:45
• If you make it slightly ellipsoidal, you can have a point focus without any second dish, although further than twice the focal range it would be less powerful than just a parabolic dish. – AJMansfield Aug 6 '15 at 10:21

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