6 gates are better because the output impedance is lower so it's louder.
Normal audiable piezo transducers have a sharp peek at about 32kHz, it's easy to find if you know how. You'll need an oscilloscope with a 5mV range, a signal generator, two transducers and a frequency counter (optional). Connect one piezo to the signal generator set at 25kHz 50% squarewave connect the other transducer to the scope and frequency counter, hold it close to the transmitting xducer, tune the sig-gen until the waveform on the scope is perfectly sinusoidal and has the highest amplitude, this is the resonant frequency.