[With apologies -- no, it's not cryptography.] Considering the wide variety of gun noises and urban noise artifacts, a lot of false alarms would be expected. On the other hand some of them might be worth responding to from the police perspective. It might be preferable to respond to firecrackers early in the evening and discourage accidental or deliberate arson later. On Sun, 29 May 1994, Sandy Sandfort wrote:

There aren't many sounds that are really that similar to gun shots. For one thing most rounds are supersonic unlike firecrackers and car backfires. There really is no such thing as a "silencer" outside of the movies. *Sound supressors* work marginally well for subsonic rounds. They are pretty much useless for supersonic rounds.

The above is is a bit of an oversimplification. Most of what these gadgets would be listening for are .22/.32/.38/9mm/.45 etcetera. 9mm and 22LR could easily be supersonic, but the other common ones tend to be slower. .45ACP, for example, is usually in the 900fps range (or slower), and won't give any sonic signature other than the shot itself. Regarding automatic shoot-back>

Please. This is the purest nonsense. The microphone system only works well enough to get the cops to the general vacinity of the shooting. However the real issue is legal presumptions and liability. Ain't gonna happen here pardner.

S a n d y

As several people pointed out, arming a mechanical device would be litigationally unacceptable. Worry about it when private security firms are allowed to employ mine fields. Furthermore, how long would an audio monitor last in the company of a stealable weapon? 15 minutes? A more reasonable concern would be future "upgrades" of the system for greater sensitivity and a wider range of uses. Regards, JMJ