Hey Thanks for that... im using ultrasound to transfer a code to a phone... So i actually do want to make sue humans cant here it.... 20k is used because the phone microphone has a die out beyond 22k... i been recording the ultrasound and using a fourier transform library on the phone that give be a power output. So i can measure if the sound is being generated. I will try using the transducer you'v mentioned.
Coming back to the amplifier question, how do i connect it to an amplifier. For the time being i am using an 8w 8 ohm buzzer that i got from a set of altec lansing speakers..