Check with Polaroid. They make electrostatic ultrasonic transducers,
that cna be driven fairly high in frequency with good output levels.
Requires 200V bias+driving though.

Mike Pedersen
The MEMS Exchange

Kyu Tae Yoo wrote:

> Hello,
>
> I am trying to measure the frequency response of a capacitive
> micromachined microphone. I am using a super tweeter as a sound
> source upto 40KHz, but I am also interested in the frequency response
> beyond the audio frequency ( possibly upto 150 KHz). Does anyone know
> which transducers are capable of generating low ultrasound (40~150KHz)?
> and Who makes them?
> Thank you, and have a wonderful day