Soffit Framing and Decoupling Clarifications

Hey. Work is underway on my soffits and before I add to my framing I just need to check a detail. Should the box that the speaker goes in be in contact with any of the frame or should it be attatched to the frame but not the bezel. Here's a picture of what I'm asking =) Also, the speaker box will be placed on 4 neoprene pads about 50mm x 75mm x 15mm. Will this be adequate for decoupling? Here's a link to the PDF datasheet of the material I currently have : http://users.cs.cf.ac.uk/G.Rees/Misc/Armacush.pdf However, if this is not suitable I've been looking around and found: http://www.polymax.co.uk/acatalog/Rubber_Anti-Vibration_Pads.html These guys have also said they can supply "35cm x 35cm x 2cm" neoprene sheets: http://www.kellysearch.com/partners/036 ... 005010.htm No data for the type yet though. Any thoughts/suggestions? Also, while I think about it - Is the box construction okay? For some reason I have a horrible thought that I've read somewhere its not a good idea to pour concrete into MDF? If that IS the case (which I bloody hope not! :oops: ), what would be good alternatives? Sand? Moulten Steel :twisted: ? Thanks, Gareth

the box can touch the frame since you're mechanically isolating the speaker from it. the selection of isolator material depends on the weight of the speakers since you want some compression of the material to achieve stability and enough spring left to reduce structure borne transfer.

Just to clarify, the isolator will be isolating the "box". Is it still okay for the box to be touching the frame? Thanks, Gareth

I think that you really should isolate the speaker from the box, otherwise the box will resonate...and you don't want that!! :wink: HOOK

HOOK wrote:

I think that you really should isolate the speaker from the box, otherwise the box will resonate...and you don't want that!! :wink: HOOK

Hmm. In all the pictures I've seen the speaker is built inside the box? Do you suggest it would work better like this: Also, I've assumed the box should be around 1mm larger than the speaker to allow a tiny air gap between the two? Is this correct?

gareth.h.rees wrote:

its not a good idea to pour concrete into MDF? If that IS the case (which I bloody hope not! :oops: ), what would be good alternatives? Sand? Moulten Steel :twisted: ? Thanks, Gareth

The MDF will swell at whatever parts of it come into contact with liquids in general but water in specific.

Hi, Just isolate the box from everything.Don't let it touch the frame or the bezel, and don't let the speaker touch anything apart from the box it's in. Make a box out of MDF or whatever, line it with 0.2mm builders plastic and then pour concrete in there.You can even dismantle the box after concrete is set and dried and have just bare concrete block instead... Hope it helps

Sen wrote:

Hi, Just isolate the box from everything.Don't let it touch the frame or the bezel, and don't let the speaker touch anything apart from the box it's in. Make a box out of MDF or whatever, line it with 0.2mm builders plastic and then pour concrete in there.You can even dismantle the box after concrete is set and dried and have just bare concrete block instead... Hope it helps

That was my original thinking... but now i've been given three bits of advice all of which contradict eachother! :( I think perhaps making the box part of the frame structure and then isolating the speaker from *everything* (ie, so that it touches nothing* is the best way to go as thats what the genelec site recomends? Could someone clear this up and point me in the right direction, possibly pretty quickly as I want to continue with the construction tomorrow. I allready have the outer box made but its not too late to change the dimensions of the innner one - but I'd like to get that done over the weekend :? Also, would it be okay to leave the wood around the box? If not then I'll have to change the dimensions anyway as currently it is designed so that the wood stays in place? Thanks, Gareth

What 's with the concrete? Why and where are you pouring it? I'm confused, but will the space behind the speaker not be a bass absorber?

What 's with the concrete? Why and where are you pouring it? I'm confused, but will the space behind the speaker not be a bass absorber?

If you go to the construction forum and check out "eruss" - he had good results building his speakers into concrete boxes, rather than the usual wood boxes people make. I can see the logic, as the concrete is thicker than the wood so helps to push the sound out of the front of the speaker. I'm making a "sandwhich" of an MDF/Concrete/MDF box. The sketchup drawings show this pretty clearly :) In my frame I have two sections - the top, which will be sealed off but filled with loose abosorbsion, which will deaden any sound bouncing round in the structure. The lower half will be a bass trap. Thanks, Gareth

you want the speaker to be isolated from the box. the box can contact the frame as longs as its solid and won't vibrate. if you're building the box to fit tightly around the speaker, then isolate the box. what you don't want is to isolate the speaker from the box and then the box from the concrete box since then you're building a membrane absorber... generally speaking you want the soffits/bezel to act as an extension of the speaker housing without significant diffraction, resonant or absorption effects. this means the face of the speaker is level with the soffit and the soffit is massive and rigid and damped. this also means the speaker should be isolated from the structure (structure borne transfer) and also not move (high frequency phase effects from movement). then to top it off you have to create some sort of step baffle compensation for the additional 3-6db bass increase as well as consider the transfer of heat away from a powered monitor (if it is one), the bass port on the back (if it has one), or just air flow between top and bottom cavities as part of any bass trapping system built-in there. so if you seem to be getting conflicting answers, its because you are trying to balance a variety of conflicting requirements. which is one consideration for not soffit mounting the speakers...