Bell in Vacuum

It is common knowledge that for sound to travel, there has to be a medium for it to pass through, be it air, water or a solid. It follows that in a vacuum sound can not exist. This is something that is ignored in most science fiction movies.

The bell in a vacuum chamber is a classic demonstration that sound can not travel through a vacuum. Strangely though, it was something that I had never witnessed, so I decided to set it up in the vacuum rig.

The bell is somewhat improvised and uses a spring and a solenoid powered from a half-wave rectified 50Hz low voltage supply. It is important not to use a mains powered bell as at reduced pressure the air would break down, probably blowing the supply fuse. The bell also has to be supported by rubber bands to stop the sound being conducted through the metalwork. I also found I had to start out with the chamber evacuated and switch the pump off, then let the air in. The pump was too noisy and drowned out the sound of the bell.

In this movie, the bell-jar is in place and starts out evacuated with the bell operating. If you carefully you should be able to see the bell shaking. After two seconds, a bang is heard as the solenoid air admittance valve opens. Air can then be heard rushing into the chamber. About 12 seconds into the movie, the bell can just be heard. At the end of the movie the chamber is at atmospheric pressure and the bell at full volume.