|Physics Dept. Demonstration Manual|
To show that sound waves require a medium for transmission.
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.