This reminds me of when I played around with laser diodes as a kid (of course to burn random things).
I ordered a powerful green laser diode from eBay, wired it up and pointed it at some black paper, hit record on the camera, excitedly connected the battery and... nothing. I checked the wiring, all was good. I looked in the end of the diode, I could see a faint red glow inside, but nothing else. I must've got a dud unit.
Later I looked at the recording and my heart sank. When past me connected the battery, the room immediately lit up with a bright white glow. The diode emmitted an intense beam of infrared light... and I pointed that thing directly at my eye.
There wasn't a wider point to that, this just reminded me and I wanted to share. I suppose be careful of what you can't see.
I got lucky. That sort of thing can cause big problems. Especially those that stay unnoticed until old age. Hearing is also one of those.
Can the laser in OP’s laser burn things? No? Point of my comment is to console OP’s concern with long term damage to his eyes. Some Windows Hello’s IR emitter is powerful enough I can see bright red blinking.
I have a friend who built one of these as their thesis. IIRC they were telling me that the air through which the sound beam propagates acts as a low pass filter, so if you're at the correct distance from the device the high frequency energy should have dissipated.
Interesting stuff, I wish I had more time to learn about what they where doing.
Not sure if there are standing waves involved, or resonance. I presume it is very similar to a phased array [1] for beamforming in antennas, except that then anisotropic properties of medium may not be negligible to construct the wavefront (temperature gradients & wind), which is probably also why these devices do not sound great. To produce a high quality waveform at the receiving end the physics probably becomes quite involved rather quickly.
No, we don’t hear ultrasonic frequencies because our ears do not resonate at those frequencies.
We hear sounds when the cilia in the cochlea resonate with the incoming sound. We don’t have cilia of the length required to resonate with ultrasonic sounds, so there’s no danger of hearing loss.
Animals may get their hearing damaged, if they are in the path of the sound, are close enough that it’s still ultrasonic at their location, and are sensitive to the frequency used, I believe. Maybe someone who knows for sure can say for sure.
I'm not an expert on this, but there don't seem to be any reported cases of hearing loss from sounds above 30 kHz, but there are documented cases of unpleasant effects. In any case, I'd keep some distance, just to be safe.
