Also, further to Trent's fine comments...

A piezo transducer has a resonant frequency, the further away from that frequency (or it's harmonics), and the piezo is rubbish. Use an Ultrasonic transducer (old style burglar alarm, antique TV remote, car parking sensors type), but even then, most don't go much beyond 40kHz. A pipistrelle Bat should be a quite good emitter... wire one up to a spare PIC I/O and you're in business (not checked a pipistrelle's Datasheet to see if it's 5v triggered)... trouble is you'll then have to budget for a conformal coating over the PCB to protect it from the Bat guano!