Factors affecting the speed of sound

Demonstration

This compares the speed of sound at different pressures and in different gases.

Apparatus and Materials

• Tube at least 1 m long and about 8 cm diameter
• Demonstration oscilloscope
• Miniature loudspeaker or earpiece
• Microphone
• Amplifier
• Vacuum pump
• Supplies of carbon dioxide and natural gas
• Cotton wool
• Balloon

Health & Safety and Technical Notes

Vacuum pumps and gas cylinders are heavy: the Manual Handling Regulations must be complied with.

A methane-air mixture, 5 litres in volume, presents a possible fire hazard. Reduce the risk by ensuring that there are no possible sources of ignition in the vicinity.

Read our standard health & safety guidance

This experiment uses the same method as:

Measuring the speed of sound 3

The output from the calibrated time-base of the oscilloscope is fed via the amplifier through the bung in one end of the tube to the earphone or loudspeaker. At the other end of the tube there should be leads running through the bung to the microphone, so it can be connected to the oscilloscope. One of the bungs should also be pierced by a glass tube, which is connected by pressure tubing to the vacuum pump.

Place a little cotton wool in the tube to dampen out any standing waves and make the display clearer.

Do not admit carbon dioxide directly from a cylinder to the tube: this may blow out one of the bungs. Either fill a balloon first, or blow the gas through the open tube.

Procedure

1. Set the Y-gain of the oscilloscope to about 0.1 V/cm and the time-base to about 1 ms/cm. The pulse received by the microphone should be displayed on the screen.
2. Pump some air out of the tube. The signal should not have moved across the screen, but it should have a lower amplitude. This shows that the speed of sound is independent of pressure.
3. After pumping out as much air as possible, admit natural gas and it will be found that the speed of sound is greater.
4. After re-evacuating, admit some carbon dioxide and the speed will show a marked decrease.

Teaching Notes

• You can ask students whether the speed of sound would change at different altitudes in the atmosphere. This experiment shows that it would not as long as it is only the pressure which changes.
• In step 2, there will be a drop in temperature if the vacuum pump rapidly reduces the amount of air in the tube. This will creep back up to room temperature with a corresponding movement of the signal on the screen.
• Up a mountain at 1500 m, the pressure will be about 25% less than at sea level. Mountaineers who estimate the speed of sound by hand clapping, find the speed of sound to be nearly the same as at sea level. There will be a small reduction due to the lower temperature.
• The reason for this is that as long as the temperature does not change, molecules will have the same average speeds regardless of their density. At lower pressures they are merely further apart.

This experiment was safety-tested in July 2006