Using the Magic of Directed Sound to Do Epic Pranks

Human ears may be hot garbage when compared to the super-powered ears of other mammals, like bats and rats, but they’re still pretty remarkable. We could place you blindfolded in a quiet room and drop a pin. Not only would you hear that, but you’d be able to locate the location of the dropped pin with pretty decent precision. However, you are also susceptible to tomfoolery when sounds don’t follow the natural patterns humans evolved to register. Maurycy harnessed that power to create this “Rickroll laser” capable of epic pranking.

This device exploits the human brain’s processing of aural stimuli, which can’t differentiate between natural and unnatural sounds — assuming the latter is convincing enough. If you’ve ever played a horror video game while wearing good surround sound headphones, then you already know that to be true. Even though you know, on a conscious level, that you’re wearing headphones and that there isn’t a killer clown in the room behind you, the game can easily convince you otherwise with just a well-placed sound effect. But forcing a prank victim to put on headphones tends to spoil the surprise, so Maurycy’s device does the same thing unobtrusively from afar.

This is a practical, hobbyist-friendly implementation of a concept that has been around for a long time: focused sound. We’re used to sounds being somewhat omnidirectional, or at least very broad, because that’s how most natural sounds and most speaker drivers work. But it is possible to focus sound in a narrower “beam” that is only audible along that vector. It is even possible to make that beam audible only for a particular distance range, either by projecting overlapping soundwaves or by relying on ultrasound’s tendency to drop in frequency over distance until it reaches an audible pitch.

Maurycy’s device does the latter. It consists of 73 ultrasonic transducers arranged on a custom PCB in five rows of 15, 14, 15, 14, and 15. Those work together to act as one oversized ultrasonic transducer, which emits sound in a beam just a couple of degrees wide.

To drive the big transducer, Maurycy designed a circuit that uses a 555 timer to generate a square carrier wave of 40kHz — well into the ultrasonic range. An audio signal, input through an op-amp, modulates the amplitude to “embed” the audio into the carrier signal.

Point the transducer array at a crowd of people, and only those within the narrow beam and far enough away will be able to hear the audio. Bounce that beam off of a hard wall first and you can further obfuscate the source of the sound. As Maurycy says, you can Rickroll people by playing the Astley classic so that only they can hear it.