While browsing for info on perforated metal today, I came across what may be the ultimate reference, at least for our purposes. Provided by the Industrial Perforators Association, (who'da thunk it) there is a wealth of data on acoustical properties and applications, strength, formability, and a slew of other appropriate stuff. This goes way beyond grill material, and in my initial once-over, I realized the potential here could really turn on some bulbs for some of our more inventive members.
Here's a teaser;
Maximizing percentage of open area may NOT be the key factor in maximizing acoustic transmission.
It's all here;
http://www.iperf.org/