Discrete Incoming Intruder detector

I wish to discretely detect incoming intruders approaching a doorway 24/7. I want to ignore people leaving the building from the doorway and walking away. I think this may be done using an acoustic detector, where I can access the phase of the reflected wave? Easiest might be some software to work under windows XP for example, but I would consider building a small microprocessor controller, if it would do the job. I see a number of such devices are available, but I don't know if I can access the raw device interface to get a script written, to enable this to be done, using detectors on the market just now? I'd expect to be able to pick up intruders at a distance between 2 and 4 meters from the doorway. Has anybody any experience of this requirement please, who might suggest a suitable device and/or drivers? ## Deliverables I'm hoping to find someone with experience of this application.