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Which gives me this idea to train my loud music playing neighbor to behave. A sensor listens for his music to go beyond a certain level and triggers my Pi to play loud music back to him. Then stop and check if he has lowered his volume, else rinse and repeat. I know its evil, but apartments in Montreal are so bad.

Thats awesome! I have a related system for pausing music, but never thought of using an ultrasonic sensor. I might need to add that in!

https://news.ycombinator.com/item?id=14151439

I have something similar, linked above.

If you're in a cube I wonder if you'd be better suited to an IR-beam break? If you have to walk through an "entrance" you could detect that happening pretty easily - although you'd want to avoid flaps to handle the case of somebody leaving too.

Here's my version of that, via an ESp8266, and MQ:

https://github.com/skx/esp8266/tree/master/d1-distance