Sound Waves and Doppler Shift

1. Measuring the Speed of Sound

1. Set up: two speakers facing one another on an optical bench. One speaker should be plugged into a function (signal) generator on one side of a BNC tee, with the other side of the BNC tee connected to channel A on the oscilloscope. The second speaker should be plugged into channel B in the oscilloscope.
2. Turn on the signal generator and oscilloscope, and adjust the dial on the generator to produce a 5 kHz wave. The speaker connected to the function generator should produce a steady pitch that sounds like an alarm and two waves should appear on the oscilloscope.
3. Slide the speaker that is connected to channel B along the bench until the two waves are in phase. Record the distance between the two speakers.
4. Slowly slide the channel B speaker backwards so that the waves are out of phase. Continue sliding backwards until the waves are in phase again. Record the new distance between the speakers.
5. Subtract the final distance from the initial to find the wavelength of the sound. Use this value and the frequency to calculate the observed speed of sound using Equation 2.
6. Repeat steps 1.3-1.5 for 8 kHz and 3 kHz frequencies. Notice the inversely proportional relationship between wavelength and frequency.
7. Compare the experimental speeds to the expected speed using the classroom temperature.

2. Doppler Effect with a Tuning Fork/Doppler Apparatus

The video demonstrates an experiment using a Doppler apparatus, but this same experiment could be carried out using a tuning fork. The protocol using a tuning fork is described here:

1. Tie a 1 m long piece of string to the end of a tuning fork. When held at waist length, the tuning fork should get close to but not touch the floor.
2. Connect a microphone to an oscilloscope channel and place the microphone at a fixed distance (roughly 1.5 m).
3. Hit the tuning fork to create a sound and hold in place at 1.5 m from the microphone. Notice how many waves appear on the screen.
4. Hit the tuning fork again and start swinging the fork around in circles at a steady speed.
5. Those observing the swinging tuning fork will notice that as the fork swings towards them, the frequency, or pitch, gets higher. Simultaneously, the oscilloscope should show slightly more waves on the screen. As it swings away from them, the pitch gets lower and the oscilloscope should show slightly less waves on the screen. See Figure 1 below for an example of an oscilloscope view.

Figure 1: Depiction of a tuning fork's sound waves undergoing the Doppler Effect as captured by an oscilloscope. As the fork swings toward the microphone, the sound waves are emitted at closer distances and create the illusion of a higher pitch. Note: The change in frequency of waves traced on the oscilloscope monitor may be subtle, and the amplitude of the waves will also change relative to the tuning fork's position as sound wave amplitude is proportional to volume (or 'loudness').