Hi,
So how will it detect the difference between no one being there and someone being there if you want both instances to be the same non- reflection?
Why don't you just scan the room and wait for a CHANGE in reflection?
If everything in the room is stationary, your reflections will be constant, when someone moves into the room the signal will change. BINGO you have detected a body...
What are you trying to make?
What is the application of your project?
Have you got an Ultrasonic unit to experiment with?
Can you please tell us your electronics, programming, arduino, hardware experience?
Thanks.. Tom... 
