Hidden Door is a research and engineering studio working on perception technology, founded in 2018 and headquartered in Hong Kong. We turn laboratory physics and code into hardware and software systems that performers, healers, and creators can actually use.
A narrow beam of audible sound, delivered to a single listener and inaudible to anyone outside it. Hardware, hand-assembled and numbered.
A voice is modulated onto an ultrasonic carrier. A transducer matrix fires it along a dead-straight path — ultrasound stays tight instead of spreading like normal sound. When the beam meets your target, the air itself demodulates it back into audible speech. The sound isn't travelling from the device; it is born at their ear.
“I tested it across a noisy bar and the person opposite me heard every word. Two feet away, nothing.” — Tomás Rivera
“Setup took about ten minutes. Aiming is the whole skill, and it's easier than I expected.” — Priya Raghavan
A bone-conduction system that plants information directly in a spectator's mind — concealable, wearable, and portable. Where the whole system began, 2018.
Instead of pushing sound through the air, ECHO sends it through bone. A transducer against the body turns audio into fine vibration that travels the skull straight to the inner ear, bypassing the eardrum entirely. Nothing sits in the ear, nothing is visible, and the room hears nothing at all.
“Echo is scary good. A powerful effect with unlimited potential.”
“Great item, and impeccable service. Highly recommended.” — Andrew Morris
“Very nice! I recommend everything from Hidden Door.” — Natalie Jones
“The first time I used it in a real room, the spectator physically stepped back. Nothing else in my case gets that reaction.” — Marco Ferreira
“It disappears under a shirt and it just works. After two months of steady gigs, it has never once let me down.” — Daniel Okafor
ECHO and SONAR aren’t two separate products. They’re two chapters of the same system. E for ECHO. S for SONAR. From covert receiving to directional transmission — a complete system for invisible audio control.
The underscore is still empty. We’re not done.
In Mission: Impossible – Ghost Protocol, a device projects sound at a single person with pinpoint accuracy. Special effect, or something the laws of physics allow?
We make the key. You open the door.
We don’t call ourselves masters. We’re gatekeepers. Our job isn’t to open the door for you — it’s to hand you the key. The key was always yours. Everything Hidden Door builds exists to help you use it.
“The goal was never to be the size of a deck of cards. It was to fit inside one.”
Founded in 2018, Hidden Door is a research and engineering studio working on perception technology. We study how sound reaches a person — and how it reaches only that person. From bone conduction to ultrasonic directional audio, we turn laboratory physics into tools performers can actually use.
“Reality is coded in language. Perception can be engineered — which means it can be rewritten. Power lives in that unseen architecture. What we build opens a crack in its surface.”
Billy Chin — Founder & Lead Engineer
