Shot Spotter

The city of Oakland, in California, has recently installed a system called "ShotSpotter" that uses acoustic sensors scattered (and hidden) throughout the city to pinpoint the exact location from which gun shots are fired. Within seconds, the system automatically issues an alert over the police network, including information such as an aerial photo of the area, the time and place the incident occurred, and the number of shots fired.
This may sound like a complicated system, but in fact it is based on an extremely simple (yet brilliant) idea that cleverly applies a couple of concepts that have been known for hundreds of years: triangulation and acoustic location. Basically, whena gunshot is fired, the three closest sensors detect the sound, the direction it's coming from and the precise time, and the information is used to triangulate the shot location. The figure above illustrates how ShotSpotter works (click image for more details).

The key components of the sensors comprising the system include a microphone, a GPS receiver and a network connection that constantly sends information over to a central server. More information about the sensors is available here, as well as in this article. The system was developed by this company, and is currently being implemented in several cities across the US. For a flash animation (with sound) illustrating how ShotSpotter works, click here.

Just for the sake of curiosity, you may like to know that Leonardo da Vinci is believed to be the first person ever to propose the use of sound for object detection and location purposes. In 1490, he observed: "If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you." Incidentally, that was about 200 years before the first mathematical theory of sound propagation was published (by Newton in 1687) and more than three centuries before the speed of sound in water was accurately measured (in 1826)! [more info here]
Later acoustic location became a very important tool and found application in several different areas. One of the most famous examples of acoustic location is the sonar. This site presents an interesting overview of the history and application of acoustic sensors, including several "curious" devices like the ones illustrated below... :)