[E-Zee]

I settled on 12" concrete tube forms to make in-attic back-boxes or cabinets for some ceiling speakers. Speakers are 8" ceiling speakers with approx 10" opening diameter for install.

My criteria was simply being light-weight, easy to install, sealed to maintain some insulation value and keep insulation out of the speaker and the opening.

I have the skills and tools to have made boxes out of more traditional materials such as mdf boxes, but i wanted to keep the weight low and wanted something that could be dropped into any of my selected locations without customization or securing heavier boxes to the framing.

I won't go into too many details, but here's some photos.
Materials were: 12" concrete tubes, cheap 4mm or 5mm plywood for base and lid, aluminum foil tape, liquid nails (a premium version, can't remember which)

12" tubes are not all same diameter.  Manufacturer makes them in slightly different diameters so that a few different sizes can all ship inside of each other. 

I selected a couple of the largest diameter tubes and a couple of the 2nd largest diameter, so that the smaller diameter tube could be glued inside of the larger diameter, allowing a somewhat dual-wall for additional strength from flexing (thick liquid nails type adhesive is necessary for this as it is thick enough to bridge the somewhat significant gaps between the different diameters.

As you see in the photos, i cut each of the larger outside diameter tubes in two pieces. A slightly smaller diameter piece was glued inside of the larger piece in a way that it stood taller, creating an offset so that the second larger diameter piece can fit on the base like a lid.

A slightly larger rectangle piece of the plywood was used for the base, while round plywood panels were cut for the lid portion.  I wanted a wider platform at the base for weight distribution, as well as more sealing surface with the flat back side of the ceiling sheetrock. I used a 1/4" router bit in a circle jig to cut a thin recess into each plywood panel. That small recess gave the concrete tubes a little something to register against during glueup.

Once in the attic, I used the foil tape to secure the bases in place temporarily. The clamping force of installing the ceiling speakers later secured them very stable. (The speakers install with a screw driver adjusted clamping lever that naturally clamps the plywood sheet against the sheetrock).
While in the attic, the speaker wire was passed through a snugly drilled hole in the side of the concrete tubes. The cylinder lid was sealed to the cylinder base with the foil tape.

The primary benefit of the two piece tube was to be able to perfectly align the opening in the base with the cut opening in the sheetrock. If I had a helper, I could have built one piece tubes, set them roughly in place from the attic, and the second person could have aligned the holes from underneath, before being temporarily taped in place, in the attic.

[E-Zee]

Photos are loaded here

























All together, I'm pleased with having the space sealed off and maintaining some type of insulation factor while also protecting the speakers from being buried in endless insulation material. 

I looked at the backer boxes available for retail or even commercial purchase and they were much smaller in air volume than what i wanted. They also seemed rather expensive for what they were. I didn't see any options that appealed to me, even if they had been much cheaper.

[JLM]

A friend did an infinite baffle in his ceiling with 12 inch woofers.  Supposed to be the best bass performance, couldn't hear it from outdoors, so no complaints from neighbors.  But barely made an acoustical difference in his room.

[richidoo]

Genius!

Cylinder is terrible box shape for diffraction but as back box on infinite baffle it is excellent. Round shape like pressure vessel resists vibration, not that you'd hear it anyway.

Did you select the box volume to achieve a certain system Qts?  Looks big for 8" driver so the Qts will be lower than  max flat Qts .7.  My speaker designer friend likes larger box  Qts .57 for his DIY woofer boxes.   

[E-Zee]

I didn't chase or reference a specific QTS. I'm aware there are some strong opinions out there on the topic for in-wall/in-ceiling box construction, but I'm content with compromises. Internal volume for these is about 1.1 cu ft (in hind-sight that's potentially 2x the ideal size). I did deliberately trend towards larger. They won't be driven hard so I'm not worried about over excursion. Sealed was important to me from an insulation standpoint as my location can have some long hot summers. Additionally, the 6 ceiling speakers that I installed are surround sound only and for those positions I wasn't concerned with absolute precision or critical listening. These aren't used for music. These speakers are playing rain-drops, helicopter passes, doors slamming, and whatever other sound effects make it to the peripheral channels.  My priorities after sealed were light-weight, low build effort, easy drop-in.

Once the boxes were complete, I did have two different speakers to test between, Yamaha and JBL. Having a clean sealed enclosure did make it nice for super clean swaps between the two. I could certainly tell the difference between the two speakers. Despite differences, I couldn't pick which was "better".  I ended up choosing the Yamaha based purely on the fact that they measured higher Ohms, were a little bit more efficient, and would be a lighter load on the amplifier.

Admittedly, for ideal performance I should shorten the boxes to reduce the internal volume. Maybe at some point I will, but not a priority for now. 

[richidoo]

If you are happy with the low frequency output then a lower Q is better imo.
Thanks for sharing your process and build!  Very impressive!

[Pez]

This is very clever! Thanks for sharing.

[WGH]

Nice work! An elegant solution to the vexing problem of installing Atmos ceiling speakers.

Usually Atmos and surround speakers are limited to frequencies above 80 Hz - 100 Hz by the surround processor, low frequencies are handled by a subwoofer. A receiver would have to have a huge power supply to send low frequencies to all the speakers in a 7.1.4 setup like mine.

I had to install my ceiling speakers from below, the attic space was too tight to crawl into and like yours, filled with 16" of blown fiberglass insulation and hot. I was concerned with the ceiling speakers spot lighting the sound, Atmos works best with 9' and higher ceilings so the effects have space to diffuse, my ceiling is a little under 8'. There are not many factory made bipole speakers in a sealed box that install from below.

I ended up using Triad In-Wall, On-Wall Bronze/4 Surrounds for Atmos and surround speakers. Triad is a high end home theater company that most people haven't heard about, their speakers are usually sold by professional installers with just a few retail outlets. I bought mine on eBay, otherwise they are very expensive.

And because Triad caters to the high end market they can be only concerned about the speakers design, not the efficiency. The Bronze Atmos and surround speakers are only 84 dB efficient, too low for most receivers to handle. I have 4 stereo and multi-channel amps (from my closet or bought used) for a total of 1740 class AB high current watts, dynamics and volume are not a problem.

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