Direction detection
It is surprisingly simple to have a mic only react to a certain sound pattern.
In school, we work on a project that is basicly about measuring the overall room climate and see how one can optimize it.
For that, we basicly use a Raspberry Pi to plot the data collected by multiple sensors (light, sound, temp, humidity, CO₂) connected to an Arduino Uno. (Actually, the Pi is connected to a touchscreen, and all is placed inside a - hopefully by tomorow done - 3D-printed housing. At least our contest we attend in is on Friday...)
Anyway we bought a wrong sound sensor. It only had a range of 10cm. So, anytime you clapped a meter away from it, or just on the opposite site of the sensor, the reading was towards 0.
So, I guess, this system should be pretty reliable as it's simple and probably well thought through.