“Turn any surface/wall/table etc into a speaker with a surface transducer. This type of speaker does not have a moving cone like most speakers you've seen. Instead, a metal rod is wrapped with the voice coil. When current is pulsed through the coil, the magnetic field causes a piece of metal to expand and contract - and if it's pressed against a surface it turns the transducer into a speaker!
Often these size transducers are pressed up against a flat surface to turn it into a sounding board. The effect is a surprisingly clear sound that comes from the surface itself. The body of the transducer is heavy so that it can be placed with the top down on a table. And for another fun trick, put the transducer at the end of a large plastic cup to point sound in a particular direction.”
Essentially a weird speaker, the transducer is useful if you want to turn a specific flat element (wall! Table! Ceiling!) into a speaker. There are lots of fun applications of this.