Pew Pew Lasers! What would directed energy weapons actually sound like?

Question

Anybody who has consumed a space opera movie, tv show, or video games knows that energy weapons go pew-pew. I however want to do something more creative for my setting, and with truth being stranger than fiction I thought to that reality might provide a better answer than anything that I could just make up.

So I ask what sort of sounds would accompany the firing of an energy weapon? In particular weapons that produce energy/plasma bolts. The closest phenomenon to that classic space-opera trope is ball lightning, so I imagine that they might sound a little thunder & lightning.

The weapons that I plan on using in my setting are closest to being plasma weapons, that fire the space opera standard glowing blob of light.

Answer 1

In space, no-one (else) can really hear your weapon fire, unless their ship's sensors can detect the weapon firing (by its electromagnetic signature) and provide an artificial audible feedback. This could be anything from "Pew" or "Zap" or "crack" to "Energy weapons fire detected bearing...".

In an atmosphere, the audible signature of the weapon depends on the power of the weapon. Where the power of an energy weapon is high enough to ionise the air it passes through, the cessation of the beam results in the low-pressure ionised path it carved in the atmosphere collapsing with a crack or a bang, the loudness increasing and the pitch decreasing with the diameter of the ionised path. Dye-lasers in an atmosphere typically make a distinctive crack or bang with each pulse.

The weapon itself may be entirely silent, or it may make a quiet, high-pitched whine similar to the charging capacitors in a camera flash. Indeed, many solid rod lasers are pumped using flash tubes, so this high-energy capacitor whine would be entirely in-character.

It is possible that a weapon might whine quietly, the pitch increasing as charge accumulates, holding at a high pitch when full charge is reached and maintained, followed by a crack as the weapon is fired and a quiet, low, rising tone as the weapon charges again.

Answer 2

A laser weapon fired in the atmosphere will essentially create a crack of thunder as the beam heats the air in its path. The sudden expansion of the air will create a low pressure channel, which will be rapidly refilled (creating the thunder sound effect) when the beam no longer fills the channel.

Since "real" laser weapons will most likely be pulse beam weapons to minimize atmospheric effects and to pack as much energy into a short a time frame as possible (to defeat enemy countermeasures and ensure that each strike on target delivers as much damage as possible), then strangely the effect of a military laser weapon firing will be somewhat like being downrange of a machine gun. Many years ago, a common training event was going to the "crack-thump" range. The trainees would be in protected space like a bunker or battle trench, and bursts of machine gun fire would be delivered overhead. The student would hear a "crack-thump" sound, with the "crack" being the bullet passing overhead, while the "thump" was the actual sound of the shot being fired. Since the bullet moves at supersonic velocity, the crack of the sonic boom arrives long before the sound of the shot, so by carefully listening, the distance to the enemy weapon can be estimated (the greater the separation between the crack and thump, the farther away the enemy weapon is).

So there will be a "snapping" or "cracking" sound as the beam passes through the atmosphere, with the rate being determined by the pulse rate of the weapon (probably more than 100rounds per minute, but maybe not as fast as a mini gun, due to other considerations). The sound at the target will be determined by things like what the target material is, how the material is affected by the beam and even if the material is under stress (if it breaks because of the energy delivered by the beam, then you might get anything from a "twang" like a guitar string snapping to the shattering of ceramics). Spallation effects (bits of material tearing off from the far side of the impact) will also have a sound of its own and change the sound of the overall target (imagine beating on the outside of a metal garbage can and each time you hit it, the sides of the garbage can are thinner).

The sounds at the weapon itself will vary, depending on "how" the laser is driven and used, but certainly there will be sounds from the generator, voltage and electrical handling systems (which convert electrical energy into the short impulses needed to drive solid state lasers), sounds from the optical train (the mirror's servomechanisms for pointing and tracking, and other sounds if it is an adaptive "rubber" mirror being flexed rapidly to overcome atmospheric activity), and most importantly, the sounds of the cooling system. Most lasers are horribly inefficient in converting electrical energy into laser light, so a 100Kw battle laser might have up to 20% efficiency and 5X that amount of energy being dissipated as waste heat. Even a Free Electron Laser (FEL) has a theoretical efficiency of @ 65%, so large radiators or heat sinks are a must.

(Edit: I somehow managed to get the efficiency and waste heat mixed up when writing the answer. corrected now).

Answer 3

I pulled aside an old friend of mine who has a PhD in this stuff. He promptly explained that a laser this powerful will typically produce a phenomenon called self-focusing.

Self-Focusing

This is both good and bad. It's good because the intensity of the beam at the focus grows until something gets in the way and stops it. It's bad because the location of the focus is not easy to control.

Typically, you get the focus only a short distance away from your optics. When you're trying to use a flashlight of doom against your foes, having a range of inches isn't desirable.

Pulsed vs. Continuous

High-powered lasers are always pulsed. You want to cram as much energy into the target in as small a time as possible. It also has the benefit of helping to bypass any countermeasures you may find.

Pulse periods are short. Very, very short. Down around the dozens of femtoseconds. Assuming you have adequate cooling and an ample supply of power, you can fire many hundreds of pulses per second.

Theory vs. Practice

In practice, your self-focus will be in the air, where it gets so hot that it turns a small volume of air to plasma. Plasma sounds like a great thing to have, but in photonics it's not. Plasma loves absorbing laser light - which it then radiates with a blackbody curve.

The combination of blackbody radiation, and rapidly heated air does one thing - scatters and defocuses the beam. Your high-powered energy weapon has become a very expensive, very complicated torch.

Each laser pulse will make a tiny explosion at the focus as it re-heats the air/plasma. Firing thousands of pulses per second will create a tone equal to the rate of pulses.

If you're privileged enough to be at a university with a photonics laboratory, you can witness all of this yourself.

https://www.youtube.com/watch?v=_AfRc4gLwI8

https://www.youtube.com/watch?v=jBjqT3AQkH0

Answer 4

This topic was a favourite amongst "fellow Nerds" at Be'er Shiva University, in Israel, many years ago. It was agreed almost universally that a laser-type or maser-type "weapon" would produce an audible "Bang," like our contemporary projectile weapons. A VERY audible "bang," by all estimates.

Admittedly the photon behaves as though it is "massless," but remember elementary Physics: *Photons/light behave as BOTH a wave, AND a particle.

Photons or microwave energy at sufficient amplitude to serve as a weapon would obliterate everything in the beam's path. At the literal "speed of light." The virtually-instant obliteration of all air molecules along the beam path would produce the same sound as a lightning bolt produces, passing between clouds or clouds and the ground: A really big "Bang."

The impingement of the energy-beam upon the target would initiate the obliteration of the target's structure at the site of the impingement. As a so-called "weapon-grade" beam-weapon would have to produce very rapid vaporization of the material of the target for it to be effective, there would be a definite audible "signature" to the impingement: The "Bang" of destruction.

This noise would be augmented by the out-rushing of ionized vapors consisting of partly-combusted elements of the target's construction or makeup at very high velocity--- at what amounts to an "explosive velocity." Liberation of gasses---ionized or not---from an explosion-site produces a "Bang."

At the moment the beam broke through the "skin" or "armor" of the target there would be a very rapid in-rushing of super-heated ionized particles and associated vapors that would serve to flash-consume any atmosphere or vapors within the target as well as flash-ignite any combustible materials (plastics, fuels, explosives, flesh, etc.) within the target. This very rapid combustion of the contents of the target would constitute an "explosive event," resulting in a "Bang."

When the beam is discontinued, the ambient atmosphere would rush back to occupy the void created by the annihilation of the atmosphere by the activation of the beam. Air rushing back into a void or area of extreme-low-pressure produces a "Bang."

An energy-weapon would, evidently, produce an audible noise similar to an explosion At Every Phase Of Its Operation. In all practical respects, these "reports" would probably overlap a bit, unless the beam impinged upon a very, very hard---or thick---target, which had slowed down the penetration of the beam into the core.

Then, there might be multiple distinct explosive noises: The first "Bang" would herald the firing of the weapon, and the second (or secondary) "Bang" would be the destruction of the target's contents. This would be much like the sequence of "reports" when a direct-fire artillery-piece is fired at a tank--- one report at the moment of discharge of the weapon, and a later report when the target was destroyed.

There doesn't seem to be any way around the at least theoretical production of "gun-like discharge sounds" during the deployment of an energy weapon such as a very powerful laser or maser.

Answer 5

If the shot is supersonic and mass-less (laser or energy), then no sound would be made by the projectile.

If the shot is supersonic and has mass, then the projectile will break the sound barrier at some point (likely immediately upon emission) and be deafening.

Addendums:

• The weapon may make a noise separate from the projectile (a reaction, explosion, or discharging battery).
• The impact may cause a noise (super-heating flesh, rapidly melting metal, good old fashioned explosions).
• Burning the air as the projectile (or laser) passes may cause a variety of effects, from nothing, to a slight sizzling noise, to an explosion, depending on how reactive the environment is.