Now, perforated metal is an excellent way to modify bad acoustics, if you can reatin the air cavity above the dropped ceiling.  The small holes, if small enough, almost disapper, and the surface can be painted decoratively or plain.

As someone who believes architects never think about how things they create will be cleaned - like Richard Dattner's huge, dusty "decorative" round air ducting 70 feet above the Asphalt Green Swimming Pool on East 90th Street - I have always wondered >>>how you are supposed to spec a repainting job<<< on these perforated ceilings.  The ones I'm thinking about - with 1/8 to 1/32 holes (maybe smaller than what Ilene has in mind) - are frequently paint clogged. 

 

Of course, if, every 15 years or so, you just rip out the interiors of your concert halls, then it's not a problem.  We do that here:  the hall opens, the acoustic reviews are uniformly glowing, then 10 years later (maybe when the ceiling needs repainting?) the critics and owners say the acoustics were always terrible.  Actually, kind of efficient.

 

Best,  Christopher