Man it been years since Ive been down this rabbit hole.

You can also make it work with ultrasound. Im just going off memory. However take two ultrasound transducers. One will play a 200khz signal. The other the same signal but modulated with what ever audio input you want. Aim at targets skull.

I remember back in the 80s a guy 'rediscovered' this himself and had a similar device built into a pipe/tube to create an invisible emitted beam. He was showing it to the reporter and they went to the shopping mall to demonstrate it. He had it connected to a radio playing music and then would aim this invisible beam at people. You could see the people at the mall freak when suddenly they heard music in their head that no one else could. By walking in and out of the beam they could stop and start the music in their head.

The guy was trying to get a paten on it and was going to use it for things like air travel where your seat would beam whatever music or movie you wanted to listen to into your head. Thereby allowing each passenger to listen to whatever they wanted without needing headphones. Similar uses could be for cars, busses, or schools, etc.

Was very interesting at the time, but all of it suddenly went secret and nothing ever came of it. You can imagine the gov saying hey here's a million dollars piss off this is our stuff now or something. But still everyone saw and experienced it at the time so it does exist.

I always scream; I make a point of it. Feedback raged in my skull. The pain switch is an auxiliary circuit in the bonephone implant, patched directly into the pain centers, just the thing for cutting through a surrogate's barbiturate fog. It took a few seconds for my life to fall together, icebergs of biography looming through the fog: who I was, where I was, what I was doing there, who was waking me.