Well its clearly absorptive
http://www.thermo-hanf.de/upload/pdf...sche_Daten.pdf Quote:
Bewerteter Schallab- sorptionsgrad (Müller-Prüfbericht Nr. 52297/1)
DIN EN ISO 354 Nach ISO 11654 bei
Klasse A 160mm w = 1,00
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There is no break down per frequency band... but it did get a 1.00 coefficent on a "Klasse A" curve so that looks good. Look here for a general description
http://www.rockfon.co.uk/sw1165.asp?PID=116
This info is not broken down per frequency - but even absent assurance of how well does low in the band - keeping in mind that these Dope Traps are about a foot thick - I'd bet they are excellent broadband absorbers all the way down to the bottom.
Look at the performance of some other natural fiber products... at 4" / 100mm thick:
http://www.bobgolds.com/Absorber/home.htm
So at a guess, I'd reckon these 240mm thick panels are truly broadband... no need to conseder "upgrading" them with a membrane. That would just introduce peaks and dips in the absorption curve. Why would you want that?