Speed of Sound

In this lab, you are invited to measure the speed of sound. Because the sound wave is not visible, traditional measurement methods that were used in mechanic do not apply directly. I chose the resonance method for this laboratory exercise.

The method, as well as the theory behind it, is explained step by step in the video below.

Equipment

1. Glass cylinder with water

2. A glass or plastic pipe with a graduation

• Notice that the water container and the water itself is not important in this exercise. Water was chosen to provide a flexible ending at the bottom of the pipe.

3. A tuning fork

Laboratory Procedure

1. Select a tuning fork. Note its frequency.

2. Move the pipe up and listen carefully for the increase in volume (the sign of the resonance)

3. Note the height of the column of air L

4. Calculate the wavelength λ

The problem with calculating λ is that we do not know which situation occurs (λ/4, 3λ/4, or 5λ/4) simply because the sound is not visible. One must then make an informed guess. Here are some suggestion as how to do that:

• Assume it is the first model (λ = 4L). Calculate the wavelength and multiply by the fork frequency. If the result is roughly about 340, this is the correct model.
• If the result is NOT even close to 340, assume the second model (λ = 4L/3). Calculate the wavelength and multiply by the fork frequency. Compare to 340. In most cases, that is the end of investigation.
• Sometimes, however, even the second model does not fit. Try the third model then (λ = 4L/5). Calculate the wavelength and multiply by the fork frequency. If the product does not compare to 340, repeat the measurement.