Attic Shaped Studio

#391

a few thoughts on this:

mounting the speaker on an isolated pad within the speaker box - use some duct board to create the "back" and the vents top and bottom - from just above your air vent and to the edge with it meets the ceiling and baffle plate. this provides maximum venting.

use a bezel plate for matching the curved speaker face(s) to the baffle plate so you're within 15mm or less of flat to the plate. then if you need to service the speaker, you remove the bezel and then have access to the speaker. you don't really want it jutting out very far from the face as is can become resonant to mid/mid-high frequencies getting diffracted...

depending on electrical code - e.g. an "extension" cord into an enclosed space, vs a proper electrical box located on/in the speaker box should be checked. you'll want to be able to switch them on and off.

#392

Thanks Glenn, much appreciated. I reckon I can get a close fit of the bezel to the speaker with the help of my router and a profile matching tool I bought. A very good idea to make the bezel removeable thank you!

There's already a couple of proper electrical sockets that will end up contained within the soffit. I'll check code on this. I was planning on plugging a multi way extension cord into this and mounting it on the side of the soffit to get easy access to on/off switches.

The vent is for air exhaust, and runs into a silencer then outside. There's no backdraft shutter so it acts as passive as well as active ventilation. It has a duct running to it from the back of the room. Do you reckon it would be ok to add a plenum onto this instead of the current OSB plate. This would allow me to run the existing duct (going to increase this to 200mm diameter) and also a smaller one to feed the bottom of the box round the speaker? I'd have to make sure not too much exhaust air was coming back into the room from the top of the speaker box, of course. The smaller cross section duct to the speaker would give offer higher airflow resistance to help achieve this.

The left side of the room has a similar arrangement with the supply duct, so this could be done for the left soffit too.

Look at the state of the speakers in those corners! It's amazing they perform as well as they do :lol:

Cheers!
Jennifer

#393

yes, extending it to the room and tapping off it should work fine.
the speakers are designed to be on stands, not soffit mounted :-)

so they are going to work nicely in the way, and you can readily adjust the rolloff etc to accommodate most positions for them.

#394

Thanks Glenn.

I've built one speaker box from 18mm OSB. Later on I'll beef this up by adding two layers of 5kg/m2 MLV to the outside.

Drilling holes for the mounting bolts in the correct locations was quite challenging. The bottom bolt is 3/8" UNC, like a mic stand. The two bolts on the back are M6. Very important to drill the holes accurately to prevent the speaker being skewed in the box! The bottom bolthole location was reasonably easy to determine from the Genelec drawings and use of a ruler. The two on the rear were trickier though, as the only dimension specified by Genelec was the distance between them, 3"

I did it by packing the speaker in the correct location with offcuts of OSB and thin trim. Before this I'd screwed the stub of an M6 bolt into one of the speaker boltholes. Once in place I marked the location on the speaker box by drawing round the stub with the world's smallest pencil

The speaker will be held correctly in place with spacers / washers on the bolts as required.

I'm aware that accessing the rear bolts to remove the speaker for servicing etc. could be tricky once the box is embedded in the soffit. Therefore I'll built the structure to allow some speaker box rotation once the lower bolt has been loosed to give easy access when the soffit bezel is removed.

Cheers,
Jennifer

#395

Looks good, Jennifer!
I'd suggest a couple of things.

As Glenn mentioned, it really is best to use resilient isolation pads around the speakers. I always use Sorbothane for that. Pads of their softest material, cut to the right size, positioned all around the speaker (under, over, both sides). Note: "Cut to the right sizes!". You create a tuned system by isolating your speakers on pads (which is a very, very good thing), so it's important to get the resonant frequency down low enough to get real isolation. The dimensions of the pads tune the system. It looks like the gaps you have there would be nearly right for Sorbothane pads. Sorbothane has a calculator on their website that you can use to figure that out. They also publish the equations, so you can figure it out in even more detail.

Also, you are going to need to cut large holes in the rear panel of your boxes, as well as slots in the top and bottom panel. Two reasons for that: #1 is ventilation: You do need to ventilate around the speaker, especially if you plan to run them at high levels for extended periods (and this IS a control room, so that goes without saying...). #2 is for resonance. There will "stuff" going on in there, between the speaker and the box. Cutting large holes and using abundant insulation around the speaker can help deal with that. I suggest very low density insulation partly filling the gap between speaker and box, such as the type of filter material used in kitchen hoods, and more typical insulation all around the outside of that box, within the soffit.

I could give you some more tips, but it would have to be off-line, not in public, as I've spent years developing the method that I use for soffit-mounting speakers.... Including the egg-shaped Genelecs!

- Stuart -

#396

one strategy for this is to build a turnbuckle system to compress the speaker onto the isolation pads - this ensureS the speaker is properly compressing the pads for best resonance setting and the top pads in combination form the strong friction fit to keep it stable. the turnbuckles in addition to setting the pressure, also enable fairly easy ability to remove the speaker and re-set it.

if you build the assembly as shown, you can then create the soffit box it fits into with the supports and guides to enable easy in/out for servicing.

#397

Thank you Stuart and Glenn, your posts are very helpful and have opened up some significant avenues of thought. I'd like to explore some questions if I may. Please don't think I'm being dismissive, it's more about expanding my understanding of the acoustic properties of this structure.

With this in mind, here's a recap of my existing plan. It's not entirely clear from the drawing below, but the shelf the speaker box sits on is not directly connected to the outer frame that the soffit face will be attached to.

- Soffit baffle structure is directly connected only to the right wall and floor.

- Speaker shelf is only directly connected to the knee wall at the front. The knee wall is 18mm OSB, green glue, then 15mm high density plasterboard. I've beefed up the studwork on it.

The silencer for ventilation exhaust is cantilevered on the other side of it. The silencer does not contact anything else. If the combined mass of the silencer and wall was 100kg I'd not be surprised. It's a heavy, very sturdy and well damped structure. The OSB at the top of the silencer was temporary, behind it the silencer is attached to another noggin.

- My line of thinking was that this wall, in combination with the speaker shelf, would make a heavy, sturdy and well damped speaker stand with no direct connection to the soffit baffle structure. Therefore structure borne sound transmission from speaker to soffit baffle face can only happen indirectly e.g. down the shelf prop, into the front knee wall, onto the floor, then into the soffit structure, then into the soffit baffle face.

My assumption - and I may be in error - is that this indirect connection will transmit less structure borne sound than a direct connection between speaker and soffit baffle face.

A potential downside of this arrangement is that structure borne sound will go from the speaker into the house structure. At the moment I assume this is already happening with the speakers on metal stands. How significant this is for sound transmission into adjacent rooms (or rooms adjacent to those) in comparison to airborne sound from the speakers passing through walls and floors, I have no idea.

First question: are those reasonable assumptions or complete nonsense? :lol:

Cheers!
Jennifer

#398

nope - reasonable. the idea is to have the baffle plate mass and damping sufficient that it only has minimal resonance and thus vibration. as the speaker itself is disconnected, then you're primarily dealing with the air borne acoustic energy as the primary source into the structure.

the baffle plate can be decoupled if desired - typically by decoupling the frame from the rest of the structure so it's "floating". as noted, mass + damping will be fine and the speaker being decoupled is the primary objective.

#399

Thanks Glenn. With that in mind I'll proceed with the current plan and attempt to test it to make sure there are is no significant structure borne transmission. Do you know if there's a reliable way to test for this? I imagine it might be crudely done by running a slow sine sweep through the speaker while resting one's hand (or an accelerometer?) on the surface that should not resonate. If it resonates at any frequencies, there might be a problem worth investigating further. I'll leave enough space in the speaker box and structure to float the speaker with sorbothane should it be required.

Thank you for the advice about ventilation and speaker box resonances Stuart. I've already designed for this and taken delivery of the tools and materials to make it happen

Related to this, the speakers have a small panel of dip switches at the rear containing settings for low frequency attenuation, potentially useful for flush mounting if speaker DSP is not being used. Would it be acceptable to make another small hole on the back of the speaker boxes to provide easy access to this without having to remove the entire speaker from the box? If not, I could always make a removable plug to cover the hole.

Cheers!
Jennifer

#400

endorka wrote:Source of the post Would it be acceptable to make another small hole on the back of the speaker boxes to provide easy access to this without having to remove the entire speaker from the box?

Definitely! In fact, don't make the hole small: make it BIG. That rear panel can end up looking like Frankenstein's Swiss cheese: Large holes. As long as there's enough wood left for good rigidity in the enclosure box, that's fine. I usually cut a couple of large rectangular holes in the top half of the rear panel, and a larger one in the bottom half. You have to get the cables in somehow, and you very likely will need access to those switches, so leave yourself plenty of room, and an access panel in the side of the soffit itself.

- Stuart -

#401

for structure - get a mechanics stethoscope. usually not expensive and allows you to pinpoint sound via it's contact tip.

#402

Thanks Glenn, good plan, I already have one of those.

Thanks for the swiss cheese tip Stuart, that makes things much simpler. I had assumed one would keep the holes as few and as small as possible on the speaker box to prevent sound "leaking" out as much as possible, but I suppose the insulation in the soffit will help with that anyway?

How does this look for ventilation, cable and access holes? From top to bottom on speaker box (top removed for better viewing):

Ventilation 80mm diameter circle
DIP switch access
Cable holes (separate XLR and power)
Ventilation 80mm diameter equivalent ellipse, likely bigger than illustrated.

I looked into using duct board but it's far too twisty turny in these to make it happen, so I've bought 80mm diameter acoustic flex duct to connect the ventilation holes to outside the soffit structure. Because of the sloped diagonal ceiling There's not enough room for a vent hole on top of the speaker box, so it's in the rear instead, to be routed to a hole at the same height on the side of the soffit.

The small hole (80mm diameter) on top of the the ventilation grille plenum gets routed to the bottom of the speaker box. The large hole (150mm or 200mm diameter) on the front of the ventilation grille plenum goes to the room ventilation ducts.

#403

#404

The build has begun, thought I'd share something I've been pondering. The wood coloured beams in this picture represent the existing house structure. The red stud represents the verticals in the soffit structure.

My design has the red beam stopping 6mm or so short of the sloped ceiling. It doesn't connect to it at all. Why? Because if I connect it, I am concerned about short circuiting (?) the existing load bearing truss structure, perhaps causing roof flexing loads to transmit through the red stud and into the floor below.

It might be ok to leave the soffit structure unconnected to the sloped ceiling if the connection to the wall and floor is secure enough. If not, perhaps a telescopic fixing would allow the red stud to connect to the ceiling without risk of transmitting loads from roof to floor, while preventing back/forwards and sideways movement of the red stud.

Are such fixings available off the shelf? I imagine this is a fairly common requirement in building.

The existing stud built partition walls within this structure connect to the house structure with nails. There's a gap between them, so I wonder if this is deliberate, i.e. the nails allow more "telescopic" movement than screw fixings would? I'd rather avoid nails in the soffit structure for this purpose though in case they creak

Any input welcome.

Cheers!
Jennifer

#405

isolation sway brace to stabilise the red stud. something like this: https://kineticsnoise.com/unibrace/isolation-bracket

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