Soffit Construction - Final Design

Hey Guys. The wood for my soffits has arrived so I'm just posting two "final designs" for review. The right soffit uses a "stand" inside the soffit which I thought would decouple the speakers from the soffit frame. This would probably use thick sewage pipe filled with sand. The left soffit is built along the lines of the attatched picture. The soffits are angled at 45 degrees rather than 30 due to the dimensions of my room (3m x 3.1m x 2.6m) Which design would be better? I'm assuming the one with the stand inside as then the speaker would almost be completely decoupled from the frame? Is there any need to add a layer of rockwool in between the two sectons? Would the effect be detrimental to the structure? It would make construction easier as I could screw the timber to the MDF bases from underneath. The insides of the frame will be lined with rockwool, and the bottom section will be made into a bass trap. Any tips/comments would be really appreciated, as I'd like to start work on them on Friday. Thanks, Gareth

you should line it with insulation. including the separating plate - absorption and damping. the speaker needs to be isolated from the front panel of the soffit - rubber, 703 packed, etc. or alternative framing to support it from the back frame. or putting it on a stand... all options.

Thanks Glenn, you've been a big help:) Yeah, the plan was to line it all with rockwool. RE: the speaker being isolated from the front panel, is the left design okay providing the panel doesnt touch the speaker? The speaker box would be sat on rubber strips or similar. Is that enough to decouple it from the soffit frame? I also need to think about how to go about mounting the front panel onto the frame? is it just a case of glue and screw to the supportting frame? Thanks, Gareth

the rubber pad should be ok assuming its firm enough to support the speaker but resilient enough to decouple it. you could tap on it with a mallet to see how well it decouples. the front panel (bezel) for the speaker (the finishing touch) woud be attached to the front frame/panel using glue/screws/nails etc... as desired for a good finish - but you want to leave just a very small gap around it so it doesn't touch the speaker. if you can cut the bezel down to around 2-3mm gap, you're probably in good shape, 1mm would be nicer...

gullfo wrote:

the rubber pad should be ok assuming its firm enough to support the speaker but resilient enough to decouple it. you could tap on it with a mallet to see how well it decouples.

Thanks, I'm fairly sure I know what I'm doing now:) Just out of interest though, how can you tell if a speaker is "decoupled"? Also, would this cavity wall insulation be any good to decouple the speakers? I have some spare: http://www.celotex.co.uk/walls/masonry.php I noticed Paul Woodlock used Sylomer P. Does anyone know where to get something like this (perhaps a less dense one) or neoprene blocks about 50mm x 50mm x 50mm? Any suggestions of materials would be very appreciated. Thanks, Gareth

How about these? http://www.tracysworkshop.com/p-4790-hi-density-neoprene-block.aspx?currencysetting=gbp&affiliateid=10051 Would these hold concrete blocks up? the plan now is to construct a "speaker stand" as I think its probably better to decouple it as much as possible. P.S The 125kg weight for the concrete filled box is just a note - thats what Paul's weighed.

Just an updated picture of the lower half fitted together before I go to bed. Should I use some thin rubber pads on the bottom of the stands, or would they be better left with the wood flat on the floor? http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=33242&doy=19m8 Also, should the speaker be slightly off centre along the horizontal axis, or should it be right in the middle of the soffit? Thanks, Gareth

that looks like some substantial speaker stand 8) generally speaking you can center it on the soffit but if you can offset it a bit, like many other tactics, it can help to eliminate possible common resonances in the frames and plates (ie you're not going to get the same two frequencies resonating because you have different sizes...) on the bottom section - on the inside of the framing, add a plate that extends down to about 1' above the bottom. this extends the soffit so you don't get a weird cutoff on the speaker response. pad the front of that plate with cloth covered insulation to stop reflections back from the console. http://www.johnlsayers.com/phpBB2/viewt ... 15c6#64862

Just some mini ideas. At studiotips Brian did tests in function of making stands heavier with filling stuff. As consensus standard fine dry sand proved to be a good damping material (was known already but confirmed by testing executed by Brian). That concrete shrinks in that box. But easier in your case I think is just using a thick steel plate cut at the size of your speaker. You can order that in any metal reworking company. And combine that construction into one (now you make a stand and build this corner structure around).

Thanks eric, I'll definately look into that idea as there's a guy who does metal just down the road. To be honest I dont *really* know how heavy the weight ontop of the pads needs to be. Half because I don't actually know what I'll be using yet, and half because I'm just pretty new at this lot :oops: As I understand it, you need a certain weight to compress the material enough to get the correct resonant frequency. I know Paul Woodlock used Sylomer P - and he put 163Kg ontop of it!! Hopefully I'll be able to get my hands on some of this for a reasonable price: http://www.armadillovsc.com/products/products_neoprene.htm Maybe I could just fill the box with sand? I have plenty of that lying round from the building work.

that looks like some substantial speaker stand

Haha yeah probably a bit overkill for some Event 20/20's! Could anyone point me in the right direction to calculating the mass neaded ontop of the pads - could probably simplify it a bit then!

on the bottom section - on the inside of the framing, add a plate that extends down to about 1' above the bottom. this extends the soffit so you don't get a weird cutoff on the speaker response. pad the front of that plate with cloth covered insulation to stop reflections back from the console.

The picture shows what the plan was.. is that okay? The inner of the frame will have rockwool inside, but should I add rockwool to the front and use the inside of the supports for the Ply? I'd have thought it better to keep the soffit front as big as possible. What thickness of Ply should I use, or is it a case of thicker is better? I was planning on 3/4" (18mm). Any thoughts? Thanks for the tips guys! Gareth

Sand is a good filling material, better than using concrete here. But the sand has to be dry and fine. In order to work as intended it must flow. I like the idea better of a steel plate (keeps gravity point of speaker more logical). You just have an elegant thin steel plate cut on the exact size, rather than that large box. Sand ca 2 tons/m3 + the wood ca 0.7 tons/m3, steel ca 8 tons/m3. Paul calculated this stuff, but if you cut whatever elastic material in pieces which deflect 3 mm under load you're OK. I've somewhere a post with related formulas and more explenation (it's all the same if it refers to rizers, floating floors, whatever). I once read that Genelec advices 4 Hz for their large speakers but they don't describe how they obtain that. This calls for a very large deflection which makes such speakers very wobbly (or you must solve that by increasing mass and supporting surface (depending on type of course). I think you can do with less, hence the 3 mm (<10 Hz) I still think that you can make one integrated corner construction in which you position these speakers rather than making independent constructions (much more stable and simpler). But people as Gullfo are much better than I am to work out such ideas. And my reply only is valid when you think to need decoupling for whatever reason. What I mean is: decoupling is something to be applied when you expect problems of whatever nature if they're not decoupled. Decoupling is no quality label for good sound.

someone around here used a stack of blocks with rubber conveyor belt below the speaker as the internal stand ;-) to Eric's point, a single structure is easier to build. if you're going to extend the soffits low, remember to be aware of desk reflections hitting off the front so put some absorption on the lower bit. 1" (25mm) MDF is common for the front soffit plate because its heavy. 29/32 (21mm) plywood can be used. 3/4 (19mm) can be used as well but may be more prone to resonating so ensure you've made it very rigid and applied insulation well to dampen it.

Wow, thats certainly a lot to think about :shock: ! Well my original idea was to make an integrated construction, but I read from others (kendale being one - I think) that they felt they should have made the speaker stands more rigid, hence the idea to build a seperate stand. I think due to erics suggestions I'll hold fire on the concrete idea. Using a steel plate sounds an attractive option - I can see why this would work. I think now I need to decide whether to use internal stands or an integrated construction. Eric, any suggestions on where to look for your spreadsheet - Would be interesting to play about with. What would you guys suggest to make.. The integrated construction or an internal stand? Thanks for the help! Gareth

If you should use this design (leave it to others). I should make a big round hole in the board beneath and between the resilient supports. The air also acts as a spring, with higher resonance than the rest. That way you neutralize that spring (better than only counting on the open edges). I think you better make these vertical supports (underneath speakers) stronger (and lower supporting horizontal board as in original design) if you make that speaker with steel that heavy. That support should be stable and stiff, the decoupling will take care of the rest. For me looks good. If decoupled right, it won't (or hardly) excite rest of structure. Here are the formulas to relate deflection with resonance. http://www.johnlsayers.com/phpBB2/viewt ... ght=#13425 Should be better if integrated in some sticky, since it relates to a lot of resilient applications as speakers, rizers, floating floors etc. (had to find it back now).

Thanks eric, I'll deffinately give those formulas a look! Perhaps you should post them in a dedicated thread and hopefuly they will get stickied? Which horizontal board do you think I should lower? I assume you mean the one the speaker sits on? If so, then wouldnt that drop the speaker too low in relation to the mix position? Do you mean make a hole in the place shown in the picture?: Thanks for the help - Again!:) Gareth

gareth.h.rees wrote:

Which horizontal board do you think I should lower? I assume you mean the one the speaker sits on? If so, then wouldnt that drop the speaker too low in relation to the mix position?

Gareth, I must have expressed myself unclear. I didn't mean lower as a verb, but referred to the lowest of both boards. In fact, however you make this, as long as strong, stiff, stable and as simple as allowed is OK for me. I even (without studying the previous things), don't see why you use this double horizontal board. But that doesn't matter for the part of the question, I responded to.

gareth.h.rees wrote:

Do you mean make a hole in the place shown in the picture?

Yes, another solution could be putting these resilient pieces on equal sized pieces of wood in order to increase that air-gap somewhat to say 50 mm (that weakens the air spring). Both solutions are OK.

Ahh sorry Eric, I'm with you now:) Yeah I agree the design would need to be strengthened if usinging a steel plate:) I'll probably just use some 2x4's or similar.

I even (without studying the previous things), don't see why you use this double horizontal board.

Thats just so that I can make it in two sections, as my "studio" is yet to have a roof, so I need to build it in my garage:P Ideally I would have built it straight into the room but as I'm in university during term time I wanted to get things started so that once the room is finished I can slot them straight in:D

Yes, another solution could be putting these resilient pieces on equal sized pieces of wood in order to increase that air-gap somewhat to say 50 mm (that weakens the air spring). Both solutions are OK.

Yes I did think about firmly attatching the two sections actually, which I'll probably do as its only a few screws. I'll still use the hole though:) I've now cut out the horizontal sections (8 in total) so will get some pictures up once they start to look like something a bit more significant:D Also need to have a look round for rubber pads - I'll report back with any prices and info i get! Thanks for all the help:D Gareth