Measuring the speed of sound 3

Demonstration

An oscilloscope is used to find the time of travel of a pulse of sound generated by its time-base.

Apparatus and Materials

• Demonstration oscilloscope
• Microphone
• Amplifier and loudspeaker

Health & Safety and Technical Notes

Read our standard health & safety guidance

This demonstration is only possible if the oscilloscope has an output terminal from its calibrated time-base. This is referred to as the sweep output, and may be on the back.

As the time-base sends the spot on the screen back to the left at the start of its trace, a pulse comes from the sweep output and the amplifier generates an audio signal which is sent to the speaker.

Procedure

1. Connect the sweep output and the earth terminals of the oscilloscope to the input of the amplifier. Set the volume control of the amplifier very low, and connect its output to the loudspeaker.
2. Start with the time-base set to 100 ms/cm.
3. Connect the microphone to the input and earth connections of the oscilloscope and set the Y-gain to 0.1 volts/cm.
4. Position the microphone and loudspeaker so they face one another with an initial separation of about 0.1 metres.
5. At 100 ms/cm the signal received by the microphone should just be seen. At 10 ms/cm it should be clearer, and better still at 1 ms/cm.

6. As you move the microphone away from the loudspeaker, its signal will be seen to move across the screen. This is because of the time taken for the signal from the loudspeaker to travel through the air to the microphone.
7. Record the distance (d) moved by the signal on the screen when the microphone is moved 1 metre. Using the corresponding time, the speed of sound can be found.

Teaching Notes

• The value of starting at 100 ms/cm is that students can see that the click from the loudspeaker corresponds to the start of the sweep.
• Since the sweep output is feeding the Y-input, the picture is automatically synchronized. However, the input signals can influence the starting of the trace and could produce erratic pictures. Set the time-base to run independently of any input signal.
• The accuracy of the result obtained for the speed of sound will depend on the accuracy of the calibration of the oscilloscope.

This experiment was safety-tested in July 2006