Measuring the speed of sound 2

Demonstration

An oscilloscope is used to time a pulse of sound travelling from a speaker to a microphone.

Apparatus and Materials

• Oscilloscope

• Microphone
• Ticker-timer
• L.T. supply for the timer
• Cell, 1.5 V
• Loudspeaker, small (about 4 ohms)
• Resistor (about 2 ohms)
• Drawing pin
• Scrap of kitchen foil

Health & Safety and Technical Notes

Read our standard health & safety guidance

The ticker-timer acts as a pulse generator: making 50 pulses per second if the timer is polarized.

The spike on its blade hits the drawing pin with a scrap of kitchen foil underneath, making brief contact that completes a circuit to give a pulse through the loudspeaker.

The resistor inserted in the loudspeaker circuit provides a small potential difference to be taken to the oscilloscope. The potential difference triggers the time-base as the speaker emits a pulse of sound.

If that pulse fails to appear on the screen, try reducing the value of the resistor.

The microphone receives pulses of sound from the loudspeaker and shows them on the oscilloscope trace.

The CLEAPSS Lab Handbook (Section 12.14} has several pages of guidance for teachers and technicians about using oscilloscopes.

Procedure

1. Set the gain of the Y-amplifier of the oscilloscope to 0.1 volts/cm and the time-base to about 0.5 milliseconds/cm.
2. Start with the microphone about 0.25 metres from the loudspeaker and move it steadily away. Students should see the microphones pulse moving away from the loudspeakers pulse and growing smaller.
3. Measure the distance that the pulse moves when the microphone is moved, say, 0.5 metre. Find the time corresponding to that from the time-base calibration. From this, calculate the speed of sound.

Teaching Notes

• Concentrate on the pulse picked up by the microphone, rather than looking for the time between the emitted pulse and that received. This avoids any uncertainty in the position of the emitted pulse.
• The accuracy of the result obtained for the speed of sound will depend on the accuracy of the calibration of the oscilloscope. If the calibration is doubted, check by connecting the output of the 1000 Hz oscillator to the Y-input.