Imagine sonar and other passive acoustic detection systems. Multiple microphones, and computers to process the information. An object emitting a device can be pinpointed with great accuracy with just 2 microphones. Imagine many.
Now, flip it around. Imagine many speakers laid out. Each one could be powerful enough to hurt (rock concert-level). With the correct software, one could generate acoustic waves that reinforce at any point in the range of the speakers. Reinforce so that at only one point, the energy put into the air by ~50 big speakers all collide.
I plan to market it as a hunting weapon, like AR-15s.
It's not my original idea, I read about the concept in Scientific American. No, not the killing deer part. They were discussing the submarine detection angle. | 
