The Piezo buzzer is excited by the sine wave from WG1. Choose the frequency that will give maximum sound intensity (find out this from the frequency response experiment). We need to adjust the distance between the buzzer and the microphone accurately. We have used a magnetic metal scale and mounted the microphone on a magnet, using cellophane tape ( This is not the best way to do this, sound reflected from the scale also reaches the microphone).
Toggle the enable measurement button. Adjust the distance to get both the traces in phase as shown in the figure. Note down the position. Now move the microphone until the traces are 180 degree out of phase. This happens when you move by half wavelength. Multiplying the wavelength by frequency gives the velocity.The microphone was moved from 13 to 17.5. Velocity = 2 x 4.6 x 3700 = 333 meter/second