Physics Dept. Demonstration Manual

DVW15 - Sound Transmission in Vacuum


To show that sound waves require a medium for transmission.

  • Bell jar with speaker and microphone
  • Vacuum plate
  • Vacuum pump
  • Audio generator
  • Audio amplifier
  • Speaker
  • Leads


As the Bell jar is evacuated, the amplified sound dies away rapidly to nil.

If the air is then re-admitted slowly, the sound gradually returns, to its original level.


Use a frequency of about 700 Hz, due to transducer behaviour. Also, use fairly high gain setting on amplifier, and a low gain setting on signal generator.