Hydraulic Trandsducer?

Possibly a high-powered underwater speaker.

Just a word of caution.

Putting a transducer into water that contains human bodies will directly couple the energy from the transducer to the body.

This might cause damage to internal organs and skeletal joints.

You might want to do some research before subjecting your friends to this energy.

I agree, kind of dangerous experiment. - The same technique using adjustable sound waves used in lithotripsy, where internal calculi, kidney stones, etc. are broken down into small enough size to enable its normal passing into the urethra for excretion..

Could be dangerous... I would simply use a pump, and pump the water through the nozzle, like all spa's have done for decades... it works well, is simple, and won't hurt people.

What a bunch of chickens. If you do it right, you will get full-immersion Magic Fingers. I'm not sure about the 1800Hz; bone-thrumming organ tones might be better.

I agree - depends on the power; even a few hundred watts @ the desired frequency are likely to be just a tickle. I would think it would be more of a challenge to increase the power to get the desired effect.

HOWEVER, I reserve the right to be wrong.

Speaker came to my mind too.

What does OP want to clean in the bathtub?

Lubell Labs made the underwater speakers for at least the 2000 Olympics. They cover the range you want and have pretty good power.

In place of electronic/electrical device, actual pump and nozzle will serve better. May be you do not like that.

Maybe they want a large sub-ultrasonic cleaning tank. Always thought of that for car engine blocks.

Just make sure to avoid the "Brown Note" frequency, or you might end up cleaning the system prematurely.