Which ultrasonic speaker will you use?

I am using and old tweeter from a hifi.

Was worried about frequency response, so connected it to my signal generator at 22khz. A micophone was detecting it no problem, about 50% of voltage that i got at 10khz, so will give it a go!

There are many speakers on rapid electronics that have -10db at 20kHz, will obviously be ĺower output at 22khz.

If my tweeter (4ohm 20W) doesnt work, will order a different one.

 

A tweeter is fragile. A continuous high frequency tone will quickly destroy it.
-10dB is 1/10th the power. Half the voltage also reduces the current to half so the power is 1/4.