A mate of mine who has a hobby involving digging up old bottles (and other items) from old rubbish dumps, came across instructions for cleaning the bottles with ultrasound. The instructions he showed me involved using a metal tank, signal generator set to 40kHz and a 'transducer'. I expect that the transducer must be a loudspeaker of sorts with can be used under water, or is it? Could a plastic tank be used instead of a metal one? Where would be the best or cheapest place to purchase the 'transducer' and a signal generator capable of generating a signal of 40kHz?