I also participated at a school lab about vacuum physics. There is vacuum in a closed ambient when the gas density inside it (mostly air) is much less than the gas density around it or when its air pressure s lower than 300 mbar. This causes several unusual effects, caused by different factors, such as the absence of air and the difference of  air pressure between inside and outside or between inside an object in the containter and outside that object, but in the container.

For example, a balloon in vacuum slowly enlarges and finally bursts, because the air in the balloon „pushes“ the balloon's rubber skin and finds no resistence (no air outside that „pushes“ the rubber skin back).

The sound propagates through a medium, in our case, air. Without air, a bell in vacuum may ring but we can't hear it, as the sound can't propagate.

Something not effected by vacuum is a magnetic field, that needs no medium.

A bell in vacuum.
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