Bell in a Vacuum

The sound from a mechanical doorbell inside a bell jar diminishes as the air inside the bell jar is removed.

Turn on the alarm and put it on the platform and put the jar on it, ensuring that the platform and jar are properly greased and that there aren't any openings (ie, center the jar properly). You will hear the sound slightly muffled. With one of the valves closed and the other open and connected to the vacuum pump, suck out the air. As you do so, you'll hear alarm sound get weaker and weaker, until you almost can't hear it. As you open the valve that was closed, the alarm sound will get stronger as air is let in and sound is allowed to travel.

L2, vacuum pump on cart by workbench

PIRA DCS 3B30.30