These demo speakers can easily be made from old or unwanted speakers and connecting leads.
The speaker is often connected to a signal generator where clean sounds can be created. Passing a specific frequency through the speaker can result in the balls bouncing around. The louder the volume, the higher the balls will bounce, the higher the pitch the faster the balls will bounce. With the frequency generator putting out a low frequency (around 1Hz or one cycle per second) the cone can easily be seen to oscillate in and out. Increasing the frequency makes the cone move at a higher rate. The balls are just there to make this movement more obvious.
This equipment is also useful for showing how electrical signals can be turned into sound and therefore can also be used in some 'energy' lessons.
The loudspeaker consists of a movable coil attached to a cone. The coil loosely surrounds a magnet. If a current flows one way through the coil, the cone will move out, if the current is reversed, the cone will move in. It is this movement that creates sound.
The loudspeaker rating should roughly match that of the signal generator or Hi-Fi. These ratings are in Ohms and will be written on the magnet of the speaker and the output of the amplifier. Signal generators usually have a low output so should not cause any problems to most hi-fi type speakers.