[help] Looking for an ultrasonic sound emitter with variable volume dial?

I’ve been doing research on this but I don’t fully understand sound waves or the hardware that it takes to produce them.

I’m looking for an ultrasonic sound emitter that emits frequencies of 25-30 kHz for my dog.

I want a variable volume switch because I hear that the dogs will eventually discount the sound as they associate it with no negative effects. If I have one that I can emit at a low volume and gradually turn it up if the animal doesn’t respond to give the sense of effect to not responding to it.

It goes without saying that I don’t want anything that would hurt my beloved animal. I’d like info on safe decibel levels at those frequencies for dogs(?) I don’t know how it works. Thanks!