An analogue of the laser but using sound.
The possibility of amplifying sound by stimulation in a crystal was first explored by Douglas Shields at the University of Mississippi in 1980.
The research was later taken up by Harold de Wijn at the University of Utrecht, using a 5 mm ruby crystal reduced to 1.8 K in liquid helium. He stimulated it with an optical laser, then used a magnetic field to cause the electrons to discard their excess energy not as photons but as phonons or sound particles. The reverberations of the phonons further amplified the electrons to produce more movement of kinetic energy.
Further reading. http://pauli.physique.usherb.ca/attracte/09-2000/saser.htm (in French, so I probably haven't understood enough of it).
There is also a SASER project from the Jupiter Space Station Group, in which the first S stands for solar. They call themselves "A project based on an experimental attempt to mix microwave signals with the solar radio spectrum and receiving these signals back on Earth."
They can be found at
http://www.jupiterspacestation.org/homepage.html