The sound of a laser

An acoustic laser? Well yeah, they exist, they're called a SASER. They emit in the MHz to THz, so they are acoustic, but far out of the audio range. Thinking about it, though, the standard means of sound generation is pretty much the same as a laser. A laser is a kinda nifty device. The fundamental part is something which can emit light, but doesn't really want to. This is typically a population of atoms in an excited energy state, where the transition to emit light rarely happens spontaneously, kept there by "pumping" with another frequency. The trick comes when attached to a tuned cavity. Then the light coming from the tuned cavity can exite the source, allowing it to release the pent-up energy, by emitting more photons at exactly the same frequency as the light from the cavity. This sets up a feedback loop, and you get light amplification by stimulated emission of radiation (one of the best acronyms in the history of science), resulting in very narrowband emission (a single tuned precise frequency) and coherence. The acoustic saser folks have figured out materials allowing similar effects to happen with high-frequency sound.

Actually, though, I think every musical instrument is a saser. Consider a violin. The bow, moving across the string, builds up energy as it stretches the string, attached to it with rosin. At some point it will slip, releasing the energy, and a pulse of sound into the string. Without tuning, a scratchy, high frequency squeak results. But coupled to the tuned instrument, the timing of the next slip is fixed by the returning pulse down the string. Stimulated emission. A highly tuned strong sound is emitted, with fixed phase relationships between the harmonics (coherence?). I'm no acoustics expert (though I have skimmed some acoustics literature), but this seems like a saser to me. Wind instruments are similar, with the vibration of the reed (whether cane, for reed woodwinds, or lips, for brass) being controlled by the return wave from the tuned resonator. (Flutes have a vortex shedder rather than a reed, but I think it's pretty much the same deal.)