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R/E/P => R/E/P Archives => Acoustics in Motion => Topic started by: maxime on October 02, 2008, 06:27:37 am

Title: bass traps, which is most effective?
Post by: maxime on October 02, 2008, 06:27:37 am
Hello,
I am about to make some work on the acoustics of my control room.
I was wondering about the differences in performance between membrane basstraps and conventional porous absorbers bass traps. other than the fact that the porous absorber is wide-band by design
and that the membranes take less space, I know very little of the advantages of each and which is adequate in specific cases.
maybe some have nice links to some technical info and numbers?
my studio has a high ceiling (4.5m), with very few bass damping material for now (concrete all over the place).
one last question: is it ok to hang a membrane absorber a few inches from the wall?
I guess that both the surfaces act as membranes in that case and should be of different thickness to provide larger bandwith absorption.

thanks

maxime
Title: Re: bass traps, which is most effective?
Post by: Ethan Winer on October 02, 2008, 02:14:26 pm
Maxime,

There are two types of "membrane" bass trap. The older style is tuned, and made with a wood (or sometimes plastic) panel that responds to wave pressure like the traps shown in THIS article. The other type is made by bonding a thin membrane to rigid fiberglass, and this type responds to wave velocity and is more broadband. You can see the effect of a bonded membrane in my Density Report that compares three different densities of rigid fiberglass with and without the FRK paper facing.

--Ethan
Title: Re: bass traps, which is most effective?
Post by: maxime on October 02, 2008, 03:38:47 pm
very intersting!
i'll think about it.
any kind of foil or paper will do?
even if it's glued afterwards?
do you think the design will loose in performance if it's is covered with fabric against the paper?
I know andy munro uses revac (deadsheet) glued to plywood  as a membrane on his bass traps. but I he uses the sealed cases..
Quote:

"The trap we use now is based on the traditional panel trap, but the damping factor is different. There is another term in the formula, but basically it's still a mass on a spring. You have a certain mass of material, and a certain air stiffness in the box behind, and it creates a natural resonant frequency. The advantage of this design is that at that natural frequency, the movement is maximum, and that results in a loss of energy. The stiffer the panel, the narrower the Q; but with this very dead material, we can create a low-Q trap that works over quite a wide frequency range with a Sabine co-efficient of about 0.7. We use three types: the deepest, which is around eight inches deep, has a maximum absorption at about 40Hz. The others work at about 80Hz and 100Hz, but we tend to stagger them to try to give overall absorption over a wider range. We can also make them with a higher Q by bonding the Revac onto very thin plywood. That works quite well for tuning out room modes."



best
maxime

Title: Re: bass traps, which is most effective?
Post by: Ethan Winer on October 03, 2008, 12:10:43 pm
maxime wrote on Thu, 02 October 2008 15:38

any kind of foil or paper will do? even if it's glued afterwards? do you think the design will loose in performance if it's is covered with fabric against the paper?

Some materials are better than others, but almost anything can be used if it's not too thick and heavy. Paper plus cloth can be okay.

--Ethan
Title: Re: bass traps, which is most effective?
Post by: franman on October 23, 2008, 06:45:07 pm
Interesting test results with the FRK vs non faced...
Title: Re: bass traps, which is most effective?
Post by: Thomas Jouanjean on October 24, 2008, 12:46:27 pm
maxime wrote on Thu, 02 October 2008 14:38


do you think the design will loose in performance if it's is covered with fabric against the paper?


I would go for something where I'd have more control over which freq I'm working with - so a design/system or product which can provide you with either rather reliable lab info or a good theoritical model to estimate that.

Below 90Hz, I usually go for large tuned systems.

But it is not possible in every situation...

Your room seems pretty big, do you have more info about it?