Ultrasonic transducer for standing waves underwater

[fpickle]

Hi

I'm not too sure where to post this but since I am, after all, doing a project under the EE department, I thought this might be the best place.

I'm new to acoustics, so what I know is rather minimum.

I'm trying to look for ultrasonic transducers that can manipulate objects underwater. What I'm having trouble now is picking the right transducer. I was hoping to achieve standing waves using the transducer but I seem to have found out that all the piezoelectric transducers operate at a particular resonant frequency only. Does that mean that it can't run at any other frequency, since I would most likely need to adjust the frequency to achieve standing waves?

How about the pulses frequency? Is that the one that causes the standing waves? Or the oscillation of the piezoelectric?

Thanks so much in advance.

 

[anorlunda]

You could make a device with adjustable frequency, but the commercial ones don't need that, so they don't have that feature.