Where to place a PZM-11LL to prevent picking up duct noise?

I am currently working on a psychology department tenant improvement of spaces studying child behavior. The ceilings are 9'6" above a finished floor and the space is a long rectangle of 700 square feet. Two ceiling-mounted microphones per room will be installed close to low-pressure mechanical fixtures. How far away from a ceiling diffuser should a ceiling-mounted PZM-11LL be placed?
Chris Knorr, LEED AP
DWL Architects + Planners, Inc.

Reply: The farther the better, but I'd say two feet minimum. Try to divide the room in thirds with the two PZMs. If you can temporarily duct-tape the mics to the ceiling, make a recording of quiet voices and see if the air-handling noise level is acceptable.