Using a CRO to show acoustic waveforms

Class practical

A CRO can show waveforms from tuning forks, musical instruments and the human voice.

Apparatus and Materials

For each student group

• Microphone
• Leads, 4 mm, 2
• Tuning fork

• Oscilloscope

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance

Procedure

1. Connect the microphone to the input terminals of the oscilloscope.
2. Set the time-base to 10 ms/cm (10 ms/div).
3. Set the tuning fork vibrating by banging it gently on a knee or resilient pad. Touch its base onto the microphone.
4. Adjust the fine time-base control and the sensitivity control to give a good display.

Teaching Notes

• Using the base of the tuning fork like this often gives extra frequencies. The ideal arrangement is to hold the microphone near a vibrating tuning fork (preferably on a sounding board or box), but there may not be sufficient amplification.
• Once the microphone is introduced, students will want to make their own sounds either by singing, shouting or playing a musical instrument.
• The pulse shapes of pure notes from a tuning fork, sine waves, can be compared with the wave shapes for musical instruments.
• The frequency of the note can be measured from the screen by using the time-base setting.
• You can also show waveforms using a loudspeaker connected to an appropriately-set datalogger.

This experiment was safety-tested in October 2006

• A video showing how to use an oscilloscope: