Measuring sound waves with lasers I've recently been learning about recording techniques for music production, and I was surprised to learn that there are only three types of microphones in wide use (dynamic, condensor, and ribbon), all of which are essentially just different ways of measuring mechanical vibrations.  The downside of this is that the microphones have a non-trivial variation of responsivity vs. frequency, which leads to different mics being used depending on what frequency range you are interested in.  
Question: Are there techniques for measuring sound waves non-invasively, using e.g. laser interferometry?  How do such techniques compare to mechanical measurement of sound waves?
 A: All of the references I've been able to find on the subject of laser microphones describe detecting miniscule motion of surfaces due to impinging sound waves.
In principle, it should be possible to directly detect sound waves in air using laser interferometry - but in practice it is very difficult.  The variation in refractive index due to pressure differences in different parts of a sound wave is very small, partly because the refractive index of air is itself very close to 1.  But what makes "direct" interferometric detection particularly difficult is the fact that unless the laser beam is propagating at precisely 90 degrees to the direction of the sound wave, the laser beam will cross multiple crests and troughs of pressure whose effects will just cancel each other out.
That said, it would still be possible to make a direct laser interferometric microphone using an array of fast optical sensors and light moving in all directions, so that somewhere in the array would correspond to a beam moving perpendicular to an incoming sound wave.  This microphone would detect not only the sound amplitude, but also its direction.
An analogous system would use an acoustic lens to form an acoustic image from the incoming acoustic field.  Each point in the image would correspond to light coming from a fixed direction, and the image could be used to form an interferometric image of the incoming acoustic field.
