The term tonpilz or "acoustic mushroom" may refer to a certain type of underwater electro-acoustic transducer. By sandwiching active materials between a light, stiff radiating head mass and a heavy tail mass, the transducer can effectively operate as either a projector (source) or a hydrophone. The transducer's size, odd shape, and acoustic projection capabilities have earned it the moniker "tonpilz", from the German words Ton (tone) and Pilz (mushroom) and from the figurative similarity.