parametric array

A loudspeaker that projects a narrow beam of sound rather than filling a room. It emits an ultrasonic carrier — typically around 40 kHz, well above hearing — modulated with the audio signal. Air is slightly non-linear, so as the beam travels the ultrasound self-demodulates and audible sound appears along its path.

Its behaviour is the closest documented match to descriptions of a voice only one person can hear. MIT’s Media Lab, where the commercial version was invented, describes it plainly: aimed at a listener it “allows only that person to hear it”, and aimed at a surface it “causes the sound to appear to originate from there, creating something of a ‘virtual loudspeaker’”.

Documented real-world example: Holosonics has sold the Audio Spotlight commercially since 2000. The AS-24iX is rated at 95 dB SPL at 1 kHz and listed by resellers at about $2,299. Off the beam axis, level falls below a tenth of the on-axis amplitude.

Its limits matter as much as its capabilities. Demodulation loss is around 60 dB, air absorbs roughly 1 dB per metre at these frequencies, bass is poorly directional, and anything blocking the beam blocks the sound. The audible result is real sound in air: a second person standing inside the beam hears it, and a microphone inside the beam records it.

Related terms: structure-borne audio, the Mosquito, microwave auditory effect, Voice-to-skull

Sources: Audio Spotlight — MIT Media Lab; Products — Holosonics; Sound from ultrasound — Wikipedia