The frequency waveform generator WG is set to the resonant frequency of the Piezo, for maximum sound output. The compressions and expansions of the Piezo generates traveling high and low pressure regions in front of it. The microphone digitizes the pressure and plots it as a function of time. The oscilloscope is triggered by the sine wave driving the Piezo. When the microphone is moved, the phase difference between the two waveforms changes. For half wavelength the phase difference is 180 degree. A connection diagram is shown below.
The distance is adjusted to make the driving signal and microphone signal in phase, as shown below.
|Distance adjusted for zero phase difference||Sine wave driving the Piezo and the microphone output|
|Distance adjusted for zero phase difference||Sine wave driving the Piezo and the microphone output