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Posted by David Aiken on July 3, 2009 at 13:15:55:
So, at roughly 200 Hz, ie 250 Hz, your ceiling is only 65% effective yet you said previously "If less than 100% is suffucient, fine. It all depends how much you want to absorb." Seems like a fair bit less than 100% is quite sufficient which was exactly my point.
As for the recommendations in your link, the recommendation in table 1 is simply for <-10 dB. I skimmed it quickly and found no reference to a -15 dB level nor any mention of that figure being based on speech, nor an 18-25 dB reduction being necessary for music. The recommendation I've previously seen is the graph from Olive and Toole's research which suggests a -10 dB level for the early reflections and you've previously been rather scathing about my use of that reference.
As for commercial products, the RealTrap Mondo Traps I use have NRCs of 1.52 at 200 Hz when mounted on the wall and 1.39 at 200 Hz when corner mounted. The higher than 1.0 specifications result from the fact that the traps are double sided and absorb sound from both sides. Those figures are for a 4" thick trap. You can find data at other frequencies and for a number of other commercial products at the link below.
Commercial products are certainly capable of meeting the recommendations you refer to and are used in studio situations regularly for that purpose. What would be the point of publishing a standard that no one could meet?