Image: Gif from video by Cameron Geddes
SoundmodoIn this Gizmodo series, we find out what things sound, sounded, and would sound like.  

If you’ve ever watched a science-fiction movie, you might think you know what lasers sound like: some variation of a noise you could write as “pew.” But, you’ve used a laser pointer, right? Did it go “pew”?

Powerful lasers do make sounds—but they’re not “pew” and they don’t come from the light itself. Instead, the noise comes from the equipment that generates the laser light or the interaction between a laser beam and an object.

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First, let’s discuss what a laser really is. Matter is made of many atoms, around which there are electrons. Electrons can only exist in certain locations around those atoms, called energy levels. If you excite an electron with energy, it will jump to a higher energy level. Some time later, it might spontaneously jump to a lower energy level, causing the atom to emit a particle of light, called a photon. But rather than wait, you can also stimulate the emission yourself by hitting the laser with more properly tuned photons. The result is a tight beam of photons with synced-up electromagnetic fields. Lasers are devices that produce light based on this principle.

Modern lasers typically consist of some electrical source providing energy to a crystal placed between a mirror and another mirror that allows some light through. Light bounces back and forth between the mirrors and through the crystal, stimulating the emission of the crystal’s photons, which exit through the partial mirror. Other optics, power sources, and more crystals further tune the shape, duration, and power of the laser pulse.

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But sound is produced by vibrations through air, not by light. A beam of laser light itself does not make any noise.

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Producing laser beams can still be a noisy operation, though. The high-voltage power supply to laser pulses can make clicking noises, as shown in this video of scientists producing pulses using the powerful BELLA laser in Berkeley, California. The European XFEL laser, the brightest source of x-rays in the world, is extremely loud—but visitors are actually hearing the whirring of machinery and the flow of water through the setup, which cools the equipment.

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Additionally, European XFEL is driven by a particle accelerator, which is further cooled by liquid helium. That requires compressors in order to reach cold temperatures, generating a loud machine sound.

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The beam interacting with various mediums can also create noises. In the first half of the video above from the BELLA laser team, you can hear a static clicking sound. In this case, a laser is traveling left-to-right across the screen and is focused into a 20-micrometer-wide beam, generating an electric field strong enough that it forces electrons off of atoms in the intervening space. This generates a plasma that expands with a faster velocity than the speed of sound in air, creating a shockwave and the accompanying sound. It also changes the optical properties of the air, creating the colored ring and flashes projected onto the far wall.

A high-energy laser pulse striking material can also produce a loud noise. At the Biomedical Laser and Optics Group of the University of Basel in Switzerland, researcher Ferda Canbaz shines a powerful laser against bone, generating vibrational energy and noise as it chips away material. And in the video of the BELLA laser from Wim Leemans, you hear a loud boom—this is a shockwave produced by blasting energy into an unexposed black polaroid.

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So, no, today’s lasers typically don’t make “pew” sounds. But perhaps the deafening shock wave is a more realistic way to represent today’s lasers’ incredible power.