Attic Shaped Studio

[endorka]

Back to speaker soffit design. Delivery charge from the builder's merchant is cheaper with larger orders, so the plan is to design these as well as possible in advance to derive a bill of materials to lump in with the additional plasterboard for room 2. The garden shed roof also needs fixing

The built in bass traps going into the roof eaves at the back of the room have made the room 60cm longer. This and a few other tweaks mean the speakers can be angled at 37° to the theoretical ideal listening position. Not perfect, but I've been working with this angle for some time now and quite like it.

First drawing shows the Genelec 8030 monitor inside a box made with two layers of 18mm OSB. I've sized this to the official flush mount specifications given by Genelec, but the front is extended so the speaker front won't protrude like their design. I will add ventilation holes (not shown) in the top and bottom to allow air in for cooling.

The speaker box is secured to the top of a structure made of 4x2' stud wood that is fixed to the knee wall noggins. This is based on the method recommend by Thomas Barefoot. The speaker has a mic stand thread in the bottom; the isopod has been removed from the speaker, and it is secured to the box and structure by a bolt going through those into the bottom of the speaker. The odd shape of the structure allows a ventilation sleeve or smaller silencer to be placed in front of the air vent.

This knee wall meets the requirements of a good speaker stand. Made from one layer of 18mm OSB and another of 15mm high density plasterboard, it probably weighs at least 20kg. There is green glue between layers, so it is damped. It is very securely fixed. And hanging directly off it on the other side is the HVAC silencer, I estimate at least 70kg. The silencer is only attached to the knee wall, it doesn't touch the ceiling below. So total knee wall / speaker stand weight is about 90kg;

Here's the external frame the soffit baffle will be fixed to. It does not touch the speaker structure or box at any point. It's not clear from the drawing, but the 4x2s at the back of this structure do not touch the knee wall either, there is a 10mm gap there. It also doesn't touch the ceiling as I am paranoid about "shorting out" (?) the load bearing roof rafters by doing that. Instead it will be fastened to the side wall.

The initial baffle design with proportions that fit Stuart's rules for soffits. It is built from one layer of 18mm OSB and a front layer of 18mm MDF. This will not touch the speaker, speaker structure or box. I'll have to route out a hole a bit larger than the front of the speaker in it for this. I bought a Sakertool for this purpose. The soffit will be filled with fluffy insulation.

Would it be valid to extend the top of the baffle to almost meet the sloped ceiling? I say almost as I'll leave a gap of a few millimetres to avoid touching the ceiling.

The baffle here has also been extended to the side, recessed to allow 4' of insulation to be placed in front. The top vent for the speaker box will exit through a hole in this.

Any thoughts much appreciated!

I'm aware that Stuart recommends floating the speaker on sorbothane. I reckon this could be done by mounting the speaker in an "internal" box made from 18mm OSB, which is then floated in a larger "external" box with sorbothane between the two. It sounds complex and expensive though...

Cheers!
Jennifer

[gullfo]

the sorbothane mounts aren't expensive and you don't necessarily need a box in a box approach although with curved speaker surfaces it might be easier to do it that way so the inner box is better fitted to the speaker and have attachment for the baffle plate bezel needed to finished the speaker opening. removing the speaker then is simply taking off the bezel and sliding it out. if the speakers are powered, you need venting. and since your room vent is underneath, you need a path for the venting there. my approach is using ductboard for both as it forms a nice path through all the insulation you'll have in there.

if you want, share the SU model you have and i'll insert a soffit unit in and you can then tweak from there.

[endorka]

Thank you Glenn. I must say the idea of sorbothane mounts is growing on me. While it makes the speaker part more complex, not having to build a decoupled inner structure for the speakers will simplify the soffit framing and also allow greater freedom for the room vent path.

I haven't used ductboard before, I'll check that out thank you!

Cheers,
Jennifer

[endorka]

On the the soffit on the other side of the room. This is the exhaust vent;

In the eaves on the other side of this wall there is a silencer, then the inline fan.

As Glenn has suggested, it would be possible to run the vent through the speaker soffit structure with ductboard. In terms of fan & duct noise, the current setup meets the criteria for NR10. But I can still hear some rumble, so why not attempt to reduce this a bit by adding a silencer inside the soffit structure? This is built from 2 layers of 18mm OSB, same as the outer silencer. It takes the exhaust port a bit further out of the room tri-corner, so perhaps this will offer some reduction as well. It's a rough concept sketch, I'll of course add duct liner on the inside;

Now here's the problem: I'm going to have to make it a little smaller to fit within the soffit structure.

I don't want it to touch the soffit structure or baffles at all, certainly don't want those amplifying rumble from being coupled to the silencer! If it was just one layer of 18mm OSB it would more or less fit. To build the surface density back up to two layer equivalent and only add a few millimetres, do you think cladding the silencer with lead on the outside would work?

Cheers!
Jennifer

[gullfo]

the duct attenuator (silencer) could be same square area as the existing vents so velocity is the same, then on the inside of the room you could add a plenum box to expand it and also redirect your register into the room. the duct attenuator only needs a single bend (which will reduce its size) as the plenum also adds another level of indirection. i'd make both out of duct board, and for the attenuator - cover with a layer of 5/8in drywall using construction adhesive. again, making it just the same size the the duct in/out, and let the plenum do the volume shifts to change velocity. with the smaller size, you'll then have a path for the speaker venting if needed. remember to remove the vent cover inside the assembly and put a low noise one on the output/input of the plenum.

[endorka]

Thanks Glenn that makes sense. By adding more turns to the attenuator I hoped to reduce the fan rumble as much as possible, but if one bend is enough it's enough. It certainly frees up space in the soffit for speaker ventilation, as you say. The other port of the attenuator in the eaves is on the same axis as the room vent, which is not ideal according to Rod Gervais book. Having the extra bend on the inside of the room will make the entrance and exit ports perpendicular, which should hopefully offer yet more fan noise reduction.

The vent grilles already in place are low noise intumescent types, 20x20cm. I installed them because my reading of the building code here was that any time a fire resistant partition was breached, it had to be replaced with something of equivalent fire rating. So they'll have to stay in place.

I did wonder about this being overkill as it is common practice here to just cut holes in walls for bathroom and kitchen ventilation fans. But since it was a specialised build with unusual features such as the attenuators in the eaves I thought I should do it by the book as much as possible.

They are incredibly quiet. The one on the right is the fresh air intake, and it barely makes any noise at all. Most of the noise in from the exhaust side with the fan, the noise from which is more significant.

Cheers!
Jennifer

[gullfo]

ah, the units are already pretty complete then. so a straight out run from the vent to the face of the soffits should be ok. if you're getting rumble from the fan units - are you sure its not structural vs as propagated in the air flow?

[endorka]

Source of the post ah, the units are already pretty complete then. so a straight out run from the vent to the face of the soffits should be ok. if you're getting rumble from the fan units - are you sure its not structural vs as propagated in the air flow?

Thanks Glenn. The fan is behind the adjacent landing wall and it's barely audible there or in other adjacent rooms so definitely not structural. Those have only standard 12.5mm plasterboard on the walls. In some of these rooms all you hear is the faintest of thrums, and you really have to listen for it. The fan is connected to the silencer with flex hose, and I took care to fix it to the rafter running right up the centre of the landing so it wouldn't have a direct structural connection to any room or wall.

I measured the fan noise in the main room and it meets NR10, so to some extent I'm just being picky. It was some noise in the 100-200Hz area I'd like to reduce a bit, so I reckoned a bit more attenuation might help that.

Cheers,
Jennifer

[endorka]

It's time to add an extra layer of plasterboard to the wall that goes between room 1 and the landing. Before adding this I beefed up the stud structure around the hinge side of the door, in case I want to add mass to the door in the future. It's not as much of a beefing as recommended in an earlier post in this thread by Stuart, but hopefully will allow a medium weight door. The current door is an FD30 fire door weighing about 45kg; the most I would anticipate going to is 90kg, and perhaps a bit less than that.

All the panels etc. around the wall have been removed and location of the studs marked.

I removed a rectangle of plasterboard with a new multitool that made the job almost effortless. It was reassuring to see a noggin already in place and good distribution of insulation.

Two noggins added, one above and below.

Insulation cut to fit around new noggins;

Cut out panel screwed back in.

I sealed the gap around the perimeter with caulk and backer rod. This doesn't have to be neat as another layer of plasterboard will go on top.

[endorka]

I used a circular saw to cut away the edge of the top floor wood so the second layer of plasterboard will go alongside it, not above it. The saw depth was set to just cut the top floor, not the subfloor beneath it!

I thoroughly located all the studs using a magnetic stud finder then marked them so I could see where they went after the second plasterboard layer was applied. Masking tape on the floor shows the stud location too. Did a test fit of the plasterboard before applying green glue, and during the test fit marked all the new screw locations.

The existing layer of plasterboard panels are installed vertically. I decided to run this second layer of plasterboard horizontally as it was the only way I could figure out to avoid overlapping seams that didn't create lots of new seams from cut (not-factory) edges.

Just one panel added as I'm doing this between sessions.

[gullfo]

most times, once you find the studs, use a plumb bob to get the vertical alignment and mark either end (tape on the floor and the angled ceiling portion), then use long ruler to mark on new upper piece. add tape on the lower panel and mark the studs. this way you have the visual reference and covering it won't matter as the ends a marked. doing this to all surface you're going to enhance, before starting the drywalling, will speed things up. remove the tape to do your spackling and finish. use water and dampen a scotch-brand-like scrubber pad instead of sandpaper to get a smooth finish and avoid dust. some towels on the floor to catch any drips.

[endorka]

Thanks for the tips Glenn, I have two walls to do so plenty of opportunity to apply them. That bit about the damp scrubber is genius!

Cheers,
Jennifer

[endorka]

On to the other side of the door now. The 3 gang light switch is flush mounted in the plasterboard, not ideal. To move it onto a surface mount pattress box I removed a rectangle of plasterboard. The goal was to replace this with an equivalent rectangle of OSB to provide a solid backing for the to screw the pattress box into. The OSB might also help stiffen the wall a bit around the door frame.

It was reassuring to see a double stud behind the plasterboard on this side of the door frame!

I didn't want to disconnect the wiring so cut a notch in the OSB to put the wire through, then covered it up with a bit of wood behind it.

OSB screwed into place, the seams caulked, and the notch filled with putty. The next layer of plasterboard will cover all this up.

[endorka]

I've applied the rest of the second layer of plasterboard to the room 1 -> landing wall. Working out the angles and offsets from the expansion gaps was doing my head in, so I made a cardboard cut out from the wall as a sanity check before cutting plasterboard.

All in place and sealed around perimeters and gaps. The marks on the bottom left panel are from green glue, a little bit of which scraped off when I was lifting the upper bit of plasterboard into place. It's high density plasterboard and a very awkward shape to lift and fit into place. Fortunately very little green glue was scraped off, it looks a lot worse than it was due to my attempts to wipe it off

With the extra 15mm of plasterboard I also had to extend the door frame by 15mm so the architraves would go back on. I used oak for this to match the existing frame & architrave. I'll varnish this and fill the gap between this and the plasterboard with sealant. These were screwed into place instead of pinned (like the architrave) as I don't want the frame extension to come off if I remove the architrave again. This would break the seal.

Next task is to cut of the bit of plasterboard covering the window made from glass bricks Photo from before...

[gullfo]

that's a good amount of mass you're adding there!