ODSEVEN New Large Wireless Surface Transducer for Raspberry Pi
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!
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.
This transducer is about 45mm in diameter and has 4Ω impedance and should be run at about 5W.