BB absorber , air gap, treatment

Member for

20 years 8 months
Submitted by Anonymous on Sun, 12/21/2014 - 06:32

Room dimensions - 14' 6" L x 12' 3" W x 7' 11" H
Construction material in room: old plaster walls, to my knowledge, no insulate material of any kind on the inside of the inside walls, OC R19 on the inside of the outside walls
2 windows,
Current treatment: (2) Roxul filled BB absorbers, both 2'x4'x4" deep, with a 4" air gap between the boundary and the Roxul, (3) ceiling to floor corner bass traps (Roxul-filled corner shaped)
(1) 2'x4'x3" OC703 cloud, suspended 3" down from ceiling above my mix position.
Location: Akron, Ohio

Hi guys.

I have a question regarding air gaps; is there an area of either neutral-benefit return - or even maybe diminishing return - when increasing the amount of space between a BB absorber and the boundary to which it is affixed?

Case in point... I have a two 2'x4'/4" deep broad band absorbers (Roxul 4") on the walls on either side of my mix position. The current air gap between the treatment and the boundary is 4". I also have one 2"x4'x3" cloud ( OC703) suspended 3" down from the ceiling directly above my mix position.

I guess I'm wondering if the current 4" gap on the wall-mounted BB absorbers is going to provide me with an attenuation that is as good as it is going to get in this particular room, or, if I increased the air gap to 6", or even to 8", would I gain further reduction of low/low mid frequencies - in both the frequency and the amount of db attenuation for those frequencies?

Is there a point where this air gap either stops being useful and becomes nothing other than wasted space, or, is it possible that it might even get to a point of any kind of diminishing/harmful return?

Or, would I be better off just adding another BB absorber? (I'd rather not, as wall surface space is at a premium with the current BB absorbers that are there now).

I've tried to do my own research on this first... looking in Rod's book, searching older posts from Space, and going through the notes that Andre sent me... I'm not able to find an answer on this (I'm not saying the answers aren't there, I'm saying I haven't been able to find the info); there is some information I've come across on the internet, but frankly, I don't trust it, as there are so many conflicting answers out there from people who I'm sure don't really know and are just guessing, so I thought I would turn to our resident experts here.

Thanks :)


A great friend and one time moderator on this forum has used gaps up the 2.3:1 ratio range. He was a man who would measure before building. Common ceiling tile is used with a nominal 16:1 ratio.

Merry Christmas!


ps: BBC had an absorber with a 7:1 ratio.

avare, post: 422633, member: 17005 wrote: BBC had an absorber with a 7:1 ratio.

Wow... well okay then... this answers my question!

Thanks a bunch, Andre. ;)

And a very Merry Christmas to you as well! :)