The sound ratings (NRC) on mineral fiber & fiberglass ceiling tiles are
pretty close, just depends what model tile you buy. Armstrong's model #3201
has a NRC of .95 (one of the highest) & is fiberglass. This type of tile is
not the flexible stuff you see in the big box stores. Their model#2983 has a
rating of .70 & is flexible.
Most of the decent mineral fiber tiles have ratings between .50 - .75.
Take a look at the following for a little more info:

http://www.armstrong.com/commceiling..._acoustics.jsp


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Dave Lavelle
D.E.L. Interiors, Inc.
Wilmington, DE
www.delinteriors.com