Passive sonar systems have a practical difficulty: you must place many microphones far apart and run wires to all of them (or have nearby computers to transmit their signal over e.g. radio.)
An alternative is to use laser vibration-detection systems, such as those used for remote snooping on speech. These use Doppler shifts in laser light reflected from an object (such as a window or wall) to observe changes in the distance to an object, changes much smaller than a light wavelength. A set of such lasers could put many "virtual microphones" hundreds of meters apart, with a consequent dramatic improvement in spatial resolution, without having to distribute equipment over a large area. I suspect that even a single laser beam scanning, say, a wall, could improve spatial resolution significantly, analogously to synthetic aperture radar. Some possible uses; maybe not all of these are plausible: - scanning three-dimensional human body shapes by their sound reflections, for recognition or to find concealed weapons; - scanning environments on the other side of a wall, by the way they shape sound wavefronts that impinge upon that wall; - measuring macroscopic distances and shapes such as buildings, caves, or terrain, for instance for building planning or inspection; - spatially localizing sound sources in order to filter out background noise, for example when recording music; - identifying, for example, vehicles by sound (having filtered out background noise) and measuring their position and velocity; - characterizing sound wavefronts some distance away in order to have time to precompute a canceling waveform for active cancellation; - wireless communication with people without requiring the people to possess any particular device; think of the user interface to the Star Trek ship computer. The spatial resolution of a passive sonar system is limited by the wavelength of the sound. Supposedly cats and bats can hear sound frequencies of 75kHz-100kHz (http://online.cctt.org/physicslab/content/phy1/lessonnotes/Sound/lessonsound.asp) which means air must be able to carry such sounds. http://www.massa.com/fundamentals.htm claims that attenuation grows linearly from about 0.5 dB/foot at 50kHz to nearly 5 dB/foot at 250kHz --- which would seem to limit 250kHz sounds to less than 25 feet, and 50kHz sounds to less than 250 feet. 250kHz is around 1.3mm, 50kHz is 6.6mm, and 10kHz is 3.3cm, so e.g. face recognition depends strongly on ultrasonics and close range. Really good spatial resolution probably requires the use of light rather than sound.
