Bell Jar and Ringer. A modern replacement for the traditional Bell Jar and Ringer. An acrylic chamber houses a battery-operated, high output piezo sounder. When the end plates are attached and the unit connected to a vacuum pump no sound can be heard but the sound returns when the air is allowed back in. The apparatus features a piezo buzzer enclosed within a clear acrylic tube suspended between two posts limiting the transfer of sound, and is controlled by a switch that has a red LED attached to show that it is on. When the unit is connected to a vacuum pump, the air can be removed, and the effect of sound being unable to pass through a vacuum clearly demonstrated. Sound returns when the air is allowed back into the chamber. An effective demonstration with none of the complications of the traditional apparatus.

GENERAL NOTES
A motorized pump should evacuate the chamber in about 30 seconds to 1 minute.  Removal of air is rapid initially, but as the density of the air reduces, the amount removed is less.  Running the pump for 2 minutes does not remove twice as much air as for 1 minute. The sound level from the generator is very high in open air.  When it is placed in the chamber before evacuation, the sound level reduces significantly since transmission to the observer is now through the walls of the chamber which vibrate very little compared to free air. The effect is similar to closing a window to reduce noise from aircraft. Closed windows have much less effect on reducing road traffic noise since much of the transmission in this case is through the ground and the building structure.