Speed of Sound

Aim: To investigate the speed of sound in air using a two-beam oscilloscope and the phase difference between two microphones.

1. Set the frequency to 1 kHz and start with a small microphone distance. Watch the CH1 (speaker) and CH2 (microphone) traces on the oscilloscope — they should be in phase.
2. Increase the microphone distance slowly, using the −1 mm / +1 mm buttons for fine control, until CH2 comes back into phase with CH1 for the first time. Record this distance as x₁.
3. Keep increasing the distance in the same direction until the traces come back into phase again. Record this distance as x₂. The difference x₂ − x₁ is one wavelength, λ.
4. Calculate the speed of sound from v = fλ and compare it with the accepted value of about 340 m/s at room temperature.
5. Change the temperature control and repeat the measurement. Does the wavelength you measure, and the speed you calculate, change in the way you would expect?
6. Why is it more accurate to measure the distance between traces coming back into phase over several wavelengths, rather than relying on a single wavelength shift? What effect does the microphone noise control have on how precisely you can judge "in phase"?