5.3 - Measuring Wave Speed

Measuring the speed of sound with an oscilloscope

• A signal generator connected to a speaker produces sound of a known frequency.
• Two microphones and an oscilloscope are used to find the wavelength of the sound, and from that the speed of sound in air.

Method

1. Connect a signal generator to a speaker. Place two microphones in front of the speaker, each connected to the oscilloscope so the sound at each microphone shows as a separate trace.
2. Start with both microphones next to the speaker, then slowly move one away until the two traces align again - exactly one wavelength apart.
3. Measure the distance between the two microphones to find the wavelength, $\lambda$.
4. Use $v=f\lambda$ to calculate the speed of sound.

The speed of sound in air is around 330 m/s, so the result should be close to this value.

Measuring the speed of water ripples with a lamp

• Water waves in a ripple tank are made visible by shining a lamp down through the tank.
• The wave crests cast shadow lines on a screen below, and the spacing between those shadows gives the wavelength.

Method

1. Attach a signal generator to the dipper of a ripple tank to create water waves at a set frequency, $f$.
2. Dim the room lights and shine a lamp above the tank to cast shadows of the wave crests onto a screen below.
3. Measure the distance across 10 wavelengths (shadow lines) on the screen, then divide by 10 to find the mean wavelength, $\lambda$.
4. Use $v=f\lambda$ to calculate the wave speed.