Interesting, I bought a parametric speaker (from Kickstarter of all places [1]), it too uses piezoelectric emitters.

My understanding: DML speakers directly emit audible sound by vibrating at the resonance frequency of a material. Parametric speakers emit ultrasonic waves and control how those waves interfere using phased-array techniques, the interference is what produces sound.

It produces completely different results compared to DML: The sound quality is not good, interference just can't produce the full spectrum of audible frequencies.

In return, you gain extreme control over directionality: the interference only happens in midair or when the ultrasonic waves hits something solid, the speaker itself is mostly silent. It's actually "too directional" for me to use, the beam is still audible after multiple bounces off of solid objects: I was hoping to use it in a office as a sort of "personal speaker", but after the beam strikes my head and I hear sound, a part of it is reflected and bounces off the ceiling and floor repeatedly, causing the speaker to be audible at the opposite end of the floor.

[1] https://www.kickstarter.com/projects/1252022192/focusound-th...