I know it's supposed to be silent, but I still have extremely exceptional hearing (at least in one of my ears...) I can still hear up to about 17-19k quite clearly, which seems to be about where the "ultra sonic" audio resides. It's not actually silent, it's just too high for most peoples ears to actively perceive. 
I probably play the ultra sonics at a tenth of what I comfortably play other music at.
It sounds nearly the same to me as a CRT TV. And I'm not talking about tuning into a channel with no signal and playing white noise. Mute the audio on a TV, and a CRT still puts off audible noise at about 15.5khz that most people can't hear, but tends to really bug me.
Either way, I've gone back to the ultrasonic mostly for the consistent volume compared to the ocean sound. I just turn it down to the point where I can barely hear normal music, and that works fairly well.
I probably play the ultra sonics at a tenth of what I comfortably play other music at.
It sounds nearly the same to me as a CRT TV. And I'm not talking about tuning into a channel with no signal and playing white noise. Mute the audio on a TV, and a CRT still puts off audible noise at about 15.5khz that most people can't hear, but tends to really bug me.
Either way, I've gone back to the ultrasonic mostly for the consistent volume compared to the ocean sound. I just turn it down to the point where I can barely hear normal music, and that works fairly well.
