Do PZMs need boundaries to reject sound?

I have several Crown PZM-30D mics which I am thinking of mounting in a theatre type environment. Instead of placing them on a table top around the auditorium, I was thinking of attaching them 10 feet above the audience on a metal grid system. The grid holds projectors and tube speakers and is suspended from the ceiling with the HVAC duct work in between. The grid lines are 5 feet apart and hangs down about 10 feet from the ceiling. How important is it to have a solid surface behind the PZM-30D to pick up the hemispherical radius below? Without a solid surface will ambient noise from equipment on the roof of the building be picked up by the PZM-30Ds?
Kevin Gecsi
Creative Director of Media,
The Ohio Supercomputer Center

Reply:
Each PZM needs a boundary at least 2 feet square to reject sounds behind the boundary from about 80 Hz and up. If you need to reject lower frequencies, I'd recommend a Plexiglass boundary 1/4" thick about 4 feet square for each PZM.

An alternative is to use unidirectional hanging mics without boundaries, like the Crown CM-30 or CM-31.