Here’s a simple hack for DJs wanting to headline major parties: spend years building an underground party scene with your best friends and also be the guy who brings the sound system. Running audio for raves gives one a practical education on the physics of audio, particularly how to contain it so the cops don’t show up and confiscate your gear. I came across LRAD, Long Range Acoustic Devices that look like big spotlights. They create a military-grade beam of shockingly loud and piercing high-frequency audio that can tell a crowd to go home, or focused to ruin one specific person’s day, from 300 yards. Extremely impractical for concert audio.

Kicking around an electronics surplus store, I found a bin of ancient 20mm cellphone speakers for $10. My muse delivered a concept for a low-tech, low-expectations DIY LRAD that I could use to make unsuspecting hippies hear voices in their heads.

What It’s For

Make a porta-potty deliver a performance review. Direct squishy sounds at a couple from thirty feet behind them to make them think they’re walking on wet ground. Create an invisible zone of guitar solos on a hiking path that confuses local runners as they pass through it. The moral ceiling on all of this is left as an exercise for the reader.

THE DEVICE

The bike-mounted HellaRAD is forty-seven inches wide, an inch and a half tall, eleven inches deep, and just over five pounds. At 50 feet the 2–6kHz audio beam is 4–6 feet wide, meaning it can be used to play higher-frequency sounds — baby laughter, bells, flowing water, birdsong — directly at one person, maybe two if they’re standing close and deserve it equally. Anyone outside that 4–6 foot wide zone won’t hear a thing. On a city street it cuts clearly over ambient noise at 50 feet. Across a park, it reaches out to 150 feet with authority. Fifty surplus cellphone speakers, a block of foam, a tiny 50W amp and a drone battery.

How It Works

I’ve taken a swing at designing speakers before, based on the Quarterwave Theory, which you can go read about. Using a combination of all the major LLMs, I worked out the physics to construct a line array, the same designs used to make sure every seat in a stadium has great sound and minimal echo. An array is a bunch of speakers precisely aligned so the audio waves reinforce each other straight ahead and cancel off to the sides, focusing into a narrow beam that tightens as frequency rises, making them perform as a single acoustic source.

The 50W Bluetooth amplifier limits the frequency range of the sound from 2–6k; the volume adjusts the projection distance. Each speaker fires into its own narrow tube. The tubes compress the sound like cupping your hands around your mouth, which enhances speech frequencies and makes the sound travel farther. External horn flares provide vertical control and additional forward projection. Useful for offering unsolicited feedback at a distance!

I call it the HellaRAD. Premiering this summer!