What is that for?
Why was it the first thing I built on a Sunday morning?
What does it have to do with an empty battery?

Answer: Just 2 microphones to measure the time delay between the arrival of a beep. A beep that is generated from a smoke detector with an empty battery. The source was not even in my flat, and that was why I was unable to locate the complaining smoke detector with my ears in the first place.