Bell in Vacuum with Plate & Bell - ElectricBell in Vacuum Experiment Propagation of Sound. A science experiment using a ringing alarm clock under the bell jar. As the air is pumped out of the sealed bell jar, the noise of the alarm clock fades, thus demonstrating that the propagation of sound is mediated by the air. In the absence of their medium, the sound waves cannot travel. Use with Vacuum Pump, SER1014149. Units: Each.