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

The distance is again changed to make the driving signal and microphone signal out of phase, as shown below. The change in distance 'd' is measured and the velocity of sound is given by v = 2 * d * 3500

Distance adjusted for zero phase difference	Sine wave driving the Piezo and the microphone output