Bell jar is a term used in sciences. It refers to a kind of a jar that has the shape of a bell. It is a kind of laboratory glassware. It is used for doing the various types of experiments. The bell jar can be sealed. By sealing it, it can be taken to the class room also for the purpose of an experiment that involves an alarm clock or a vacuum pump. The air is pumped out of the sealed vacuumed jar. This allows the noise of the alarm clock to fade away which thus demonstrates the fact that the propagation of sound is arbitrated by the air. If it is deprived of this medium that is air, sound cannot travel.