Rough estimate of how close a human speaker is to a microphone? Is there a known technique for estimating how close a human speaker is to a single microphone? I'm hoping that there is a known technique that can analyze an incoming audio signal that contains a human voice with typical ambient room noise, and give a rough estimate of how close the speaker is to the microphone. Is there an algorithm/formula that can analyze such a signal and look for early reflections/reverb and then give a quality measurement back? Note, I am not looking for an estimate in units of distance (3 feet, etc.), just something that would tell me if the speaker is right on top of the mic, near the mic, far from the mic, or really far from the mic. Also, if it simplifies things, I can safely make the assumption that the speaker's voice will be significantly louder than any other human voice in the room.

Any papers or especially open source libraries would be fantastic.

PREREQUISITE: I would assume that an algorithm/formula that can detect if there is a human voice of significant volume present in an audio source would be a prerequisite for making the above quality measurement. Any information or resources along those lines would also be greatly appreciated.
Here is one possible suggestion: use a second order microphone (e.g. cardioid microphone). The human voice is a point source and has strong reactive velocity near-field component which velocity mics are sensitive too. In plain English this means you get substantially more relative bass when the speaker is close by. You could leverage to compare the energy in the 500-1kHz Band to the energy in the 100-200Hz band. With some reasonable calibration you ought be to able to tell whether the speaker is close or far.