QUOTE (TheOOB @ Dec 7 2008, 08:20 PM)

It all depends on the sophistication of the unit and how anal the settings are. The more sensitive you make, the more false positives you get and thus the more time and resources you waste. The important point is that there are ways to sneak past the sensor, if you pin yourself against the wall with a sheet and move slowly it may just not notice you(infiltration vs device rating anyone?) and you can do the same to radar, generating an ECM to screw it up, which considering the amount of electronic devices won't cause to much problems unless you do it for a long time or they are really jumpy about security.
The basic mechanism on which is reflection of sound. Sound is emitted, travels trough air, hits a surface that reflectes it and retrnes to the source; by taking note fo the direction from wich comes the echo and the time that passed betwen the sound being emited and being recived (at lest its reflection) you can deduce the direction and the distance; from the distortion you can attempt to determine the shape of the surface being hit.
The sound also is not as precise as electromagnetic waves, it is based on vibrations that propagates due molecular collision of the gasses, there is much more dispersion and distorion involved, so making out shapes is hard. At best the sensor can track the evolution in the signature of the enviroment setting of the allert in the form of a feed sent to the spider who reviews it and decides what has to be done.
I was pointing out that you can't just pretend to be a wall in the midle of nothing and that you might be able to fool a sensor but if the other side of it there's a (meta)human things can still go wrong (computers work in matematical terms and so they lack the flexibility of human mind, what is fiendishly hard for them to pinpoint could be obvious to human mind).