Exterior Drum Practice room build plan, Wales, UK

[Roguejackal]

Probably to late for you as iv not been on the forum for a long while but I was in the same situation as you being in the UK not being able to go higher than 2.5m in height, so I went down around 800mm down in fact

[Roguejackal]

sorry I seem to have manage to double post

[badass_mcfunk]

Probably to late for you as iv not been on the forum for a long while but I was in the same situation as you being in the UK not being able to go higher than 2.5m in height, so I went down around 800mm down in fact

Yes - I would have done that myself too - but going down further was not possible due to that area of my garden comprising very large rocks - went down as far as I could...

[badass_mcfunk]

.... Just wanted to check my baffle box design too... I'm using 125mm duct and will have 4 of these boxes; two on either end of inlet, and two in either end of exhaust.

baffle plan JV.JPG

Any thoughts?

[Soundman2020]

I just have a minute or so for a quick answer, so no time to check your math. But two questions:

1) Is the cross sectional area at all points inside the box at least twice the cross sectional area of the entry and exit ducts?
2) Did you allow for the thickness of the duct liner on all interior surfaces? That isn't show on your diagram, but its important that your calculations of cross sectional area are done from the surface of the duct liner, and not the surface of the box itself. (You do know that you have to line all of the interior surfaces of the box with 1" duct liner, right?)

- Stuart -

[badass_mcfunk]

1) Is the cross sectional area at all points inside the box at least twice the cross sectional area of the entry and exit ducts?

Yes - If the 'baffles' are not included in this calculation. The question slightly confuses me
The baffles divide the box into 3 compartments for air to run through, and each compartment I 'think' would be less than twice the cross sectional areas of the exit and entry

2) Did you allow for the thickness of the duct liner on all interior surfaces? That isn't show on your diagram, but its important that your calculations of cross sectional area are done from the surface of the duct liner, and not the surface of the box itself. (You do know that you have to line all of the interior surfaces of the box with 1" duct liner, right?)

- Stuart -

Yes, all interior areas will be lined with duct liner. The adhesive neoprene liner I have is approx 7mm thick.

[Soundman2020]

The question slightly confuses me ... The baffles divide the box into 3 compartments for air to run through, and each compartment I 'think' would be less than twice the cross sectional areas of the exit and entry

That's what I was trying to say: the cross sectional are in each of those "compartments" must be about two (or more) times the cross section of the duct.

Think of it this way: the silencer works on may principles at once. One of those is "acoustic impedance mismatch", meaning that due to the sudden change in cross-section where the duct comes into the box, any sound waves coming down the duct will be partly reflected back up the duct by that impedance change, and only partly passed through. In order to be effective, the change in cross section needs to be about a factor of two. (the same applies at the exit: once again, there's an impedance mismatch). OK, that's the acoustic part, but there's also the plain old air flow issue: Because the cross section changes to twice the area, the speed drops to half what it was in the duct (because the flow rate stays the same throughout: we are not compressing any air here!). So now you want to keep that air flowing at roughly the same rate as it moves through the box: If you force it into a "tighter" section (less cross section area) then it has to speed up again. Air that moves faster makes more self-noise than air that moves slower. So you don't want it going faster and slower, faster and slower, faster and slower as it moves through the box. Fast moving air also generates more turbulence than slow moving air, and turbulence also causes noises. Etc. So the ideal is to keep the cross section roughly constant. If the cross section at the entry is 200% the cross section in the duct, then that's what you want to keep all along. Or if it is 150% (not ideal, but sometimes you have no choice), then keep it at 150% all the way.. Or if you manged to get 300% change at entry and exit, then 300% is what you want all the way through.

Of course, you don't have to be crazily perfect here: there are variations in area as the flow moves around each baffle anyway, so there's no point in trying to refine things down to tiny percentages. But do try to keep in the ball-park.

Yes, all interior areas will be lined with duct liner. The adhesive neoprene liner I have is approx 7mm thick

Then it isn't duct liner!!! Duct liner is a very specific product, that is porous (fiberous) with some type of surface treatment to keep the fibers in place, and it is made specifically for lining the inside of HVAC ducts. It is usually 1" thick (25mm) or 2" thick (50mm). You can sometimes find 1/2" liner (13mm), but it isn't very common, and isn't a lot of use. But 7mm? And neoprene? Never heard of that for duct liner!!!

This is what duct liner is:
https://www.google.cl/search?q=duct+lin ... 40&bih=470

http://www.industrialnoisecontrol.com/m ... -liner.htm

http://www.certainteed.com/products/ins ... ial/317383

http://www.armacell.com/www/armacell/AC ... .12.09.pdf

If that's not what you have, then return it: it isn't what you need for this application. I almost never use anything less than 25mm for silencer boxes, and certainly not just 7mm.

- Stuart -

[badass_mcfunk]

If that's not what you have, then return it: it isn't what you need for this application. I almost never use anything less than 25mm for silencer boxes, and certainly not just 7mm.

- Stuart -

Thanks Stuart - another error from me in ending up with the wrong product. Have found the right thing now and will get some ordered

[Soundman2020]

Thanks Stuart - another error from me in ending up with the wrong product

Aren't you glad you found the form, and are posting regular updates? Imagine where you'd be right now without that! It's actually a GOOD thing when you find mistakes in your studio build: It means your studio will be a much better place when it is done, because all the mistakes won't be there!

- Stuart -

[tomugli]

The framework will be constructed using 4 x 2 untreated timber.

That's just a UK thing. We like to be difficult.

[badass_mcfunk]

Quick question: I'm wondering about filling some voids in the roof with expanding polyurethane foam. Can this stuff legitimately considered to be mass, or is it more akin to air?

[Soundman2020]

Quick question: I'm wondering about filling some voids in the roof with expanding polyurethane foam. Can this stuff legitimately considered to be mass, or is it more akin to air?

Neither!
It has very little mass, and it is also closed-cell foam (meaning air cannot get through), so it's not much use at all, acoustically. It's good thermally, and also for sealing gaps where water might get in, but not much going for it, acoustically.

What "voids" are you planning to fill with it?


- Stuart -

[badass_mcfunk]

What "voids" are you planning to fill with it?


- Stuart -

The void is essentially a gap above the top of the stud wall where the frame meets the outer skin and the roof (OSB). Hopefully you can see from the photo - I put some sections of 2x4 in between the joists for added support, but used scraps rather than measuring the exact length. With hindsight, I perhaps should have placed timber of the correct size there. Anyway, I will get my caulk gun in there and seal up where the board joints are, and thought I'd finish up with filling with foam.

void.JPG

This is where I've got to -

progress.JPG

I still have some sections of OSB to put on the outside tomorrow, then I'll be going crazy with the caulk before the Tyvek goes on; followed by cedar cladding. The plan is to focus on getting the exterior properly watertight and airtight over the next few weeks before I start on the inner frame; and before Autumn in mid-Wales is upon me..

I have been caulking the studs on the inside where they join the OSB - but not the noggins. is this overdoing it? I was focussing on the studs where the join ran along the centre of the stud, so thought it may be helpful - but may be overkill?

studs.JPG

Another question! -

With the inner frame, my plans are still evolving. I wondered if there would be any value in mounting the inner frame timber on anything (like strips of rubber) in an attempt to decouple them from the concrete pad (which the outer frame also sits on?).

When I get to doing the floor, I will (hopefully - depending on picking one up at the right price) be laying an engineered wood floor on top of standard grade underlay - so it wont be floating to any decent level.

Will attempting to isolate the inner frame as above make any real difference to the overall performance?

[badass_mcfunk]

Really struggling to find a product suitable for lining my baffle boxes in the UK - aside from one product which will set me back over £200 for a roll (I need less than half a roll). I'm wondering if using rockwool slab covered with 2 layers of tight weave fabric would be appropriate? Not ideal obviously, but an option?

[Soundman2020]

I'm wondering if using rockwool slab covered with 2 layers of tight weave fabric would be appropriate?

To be honest, I really would not go that route. It might work, it might not. You'd be pretty much on your own in figuring out the acoustic properties, and how it will stand up to air ablation over time, and if it will be grow fungus inside or not...

Maybe you could contact a local HVAC supplier, and ask for their offcuts each week? It might take a while to get enough offcuts together of the right size, but it would be a lot cheaper than buying the roll! Just an idea...

- Stuart -