Custom-Built Sound Laser Can Fire Music Directly at Someone

Maker Electron Impressions created a really useful contraption that can spit out sound in a very tight beam, similar to how a laser fires a beam of light. The name sound laser is quite fitting, as you basically point it at someone, and they will hear music or conversation, while everyone else is completely unaware.

This device is based on simple physics, which most speakers simply disregard. Normal speakers just blast air in all directions in order to provide sound to everyone nearby. This one takes a different approach, however. It generates ultrasonic waves at frequencies much higher than human ears can detect, roughly 40 kHz, and uses them to transport the actual audio stream. The goal is to integrate the audio onto these high frequency waves using a technique known as modulation. That’s all handled by a straightforward circuit that contains a 555 timer to generate the carrier wave and a Bluetooth audio amp to mix in the music or voice.

Air ends up playing a critical part in converting the inaudible information back into something humans can hear. As the ultrasonic beam travels through the air, it causes significant pressure changes. This natural response in the air just strips away the ultrasonic carrier, leaving only the real audio frequencies; it’s a pretty cool method to make music appear out of thin air, almost as if it’s coming from nowhere.

The key to keeping the beam as tight as possible and on target is to use a compact array of 73 small piezoelectric transducers. Each of these transducers translates electrical impulses into mechanical vibrations that generate the critical ultrasonic frequencies. When they’re all wired in parallel and driven in sync, constructive interference reinforces the forward motion while destructive interference cancels out the sound on the sides. Spacing them out correctly, using the half wavelength rule for the 40 kHz frequency, keeps the effective beam width nice and narrow, making it significantly smaller than what you’d get with ordinary audio frequencies.

The device draws power from a 12V battery pack that has been stepped up using a boost converter, after which the signal is amplified just enough to drive the array with an H-bridge amp. There’s a little potentiometer that lets you control the amplitude on the fly, and the whole shebang fits inside a handheld, gun-shaped contraption that the developer acknowledges looks like it was slapped together quickly during a frantic undergrad finals period.

When put to the test, the results are really interesting, as you can bounce the sound off walls indoors and maintain the audio traveling along the route of the bounce, and outdoors, you can receive clear reception over 100m while keeping the music confined in a narrow column. When you point it at a microphone or a camera, music will only play if you’re exactly aligned; if you move slightly, quiet will return as quickly as you can snap your fingers.
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