I previously wrote about hacking voice assistants with lasers. Turns you can do much the same thing with ultrasonic waves:
Voice assistants — the demo targeted Siri, Google Assistant, and Bixby — are designed to respond when they detect the owner’s voice after noticing a trigger phrase such as ‘Ok, Google’.
Ultimately, commands are just sound waves, which other researchers have already shown can be emulated using ultrasonic waves which humans can’t hear, providing an attacker has a line of sight on the device and the distance is short.
What SurfingAttack adds to this is the ability to send the ultrasonic commands through a solid glass or wood table on which the smartphone was sitting using a circular piezoelectric disc connected to its underside.
Although the distance was only 43cm (17 inches), hiding the disc under a surface represents a more plausible, easier-to-conceal attack method than previous techniques.