Ultrasound loudspeaker choose

Good morning gentlemen, I would like to purchase a loudspeaker for ultrasounds with a frequency greater than or equal to 20khz. But I'm here to ask you which loudspeaker to choose, piezoelectric transducers like those of ultrasonic cleaners or magnetorestrictive transducers like tweeters, I would like a lot of power 100W or more, and frequencies greater than 20khz. I hope you can help me, thanks