Under Garage Home Theater Build

[Rick Lee]

Ah, I remember your build threads. Love how you did the Table Tubas. Time flies....
Okay, the picture is coming into focus: you built the stud walls and ceiling not necessarily for isolation, just as space to run electrical and acoustic insulation. The penetrations are the door and an 8" hole. I'm assuming the hole is down low for plumbing (drainage) and electrical. I would love to see a picture of it.

Is it a two story house (main floor and concrete basement)? The garage is on the main floor on top of the HT room?

If it is two story is the plumbing connected to the upper living space at all? Is it just for drainage and the upper living space sewage a completely different system? If it is separate I would go to the little extra trouble to make it a real room within a room environment.

HVAC sounds good except for one detail: You have a movie marathon on a cold winter's day and spend many hours in the room with 3 or 4 people. The room is completely sealed. Have you thought about fresh air ventilation? Humidity? It's something to seriously think about.

Without digging into HVAC and just going with what you have, first, I would buy Rod Gervais' book: https://www.amazon.com/Home-Recording-S ... 143545717X Well worth the $30. Written in laymen's terms and covers all the basic details to give you a better handle on things.

No blown in densely packed celluose insulation. It settles and doesn't have any acoustic advantage.
No foam. People are finding it may have some health issues and it doesn't do anything for sound. It's good for thermal but not acoustic.

Without going overboard on cost, for isolation here's what I would do: (this incorporates some of what Tom said earlier)

Seal the 8" hole. Without seeing it I'm not sure what to tell you except that Mass is your friend. Don't use foam because it won't stop the sound very effectively (low mass). If you can inject concrete sealant like this: https://www.lowes.com/pd/Sika-10-1-oz-L ... /999977050 or trowel in concrete etc. that might be best. Yes, sound will travel through the pipe but if it's not connected to the living space it's not such a big deal. Does it penetrate the stud wall? If so that's a real killer of isolation. I'll assume it doesn't.

If you can lift the stud assembly a little at a time and put thin rubber strips underneath and caulk with the real stretchy caulk (Sashco Big Stretch, etc.) to completely seal the perimeter that would help a little. Not a huge deal with the concrete floor but it would help some.

Then I would consider the wiring (and equipment if it's noisy fans, etc.). One penetration is best if possible. I'm assuming you have lights on a circuit and then floor outlets. You already have the light boxes in I see but I'm telling you what I would do Take the lights down for now. Have one penetration through your stud wall and run surface wiring on the inside of the room.

The step after taking care of the hole and wiring and HVAC is to put pink insulation in the studs, completely filling all spaces in the walls and ceiling. For this step pink insulation is just as effective as mineral wool or celluose. Then put a layer of 5/8 drywall up on all the walls AND ceiling. Keep the drywall 1/4" off the floor all around. Put stretchy caulk/sealant on the studs before hanging the drywall. Caulk all the edges of the drywall with sealant. Use backer rod where needed. Other than the door opening it needs to be completely sealed. The penetration for electrical should be as tight as possible. If you have multiple penetrations with outlet boxes caulk all of them. (I would run surface wiring and hang a cloud with lighting built in so I could only have one electrical penetration through the wall)

Then put up a second layer of 1/2" drywall on all the walls and ceiling. Before placing the drywall I would liberally trowel carpet adhesive on all surfaces as I go: https://www.homedepot.com/p/Roberts-4-G ... /202261817 This adhesive stays flexible and helps decouple the layers (lowering the resonance). Studio builds with big budgets use dedicated "green glue" but the Roberts has very similar properties and is much cheaper. The second layer should alternate to the first layer- for instance if the first sheets were laid horizontally hang the second layer vertically. Overlap the inside edges. Caulk with sealant all the joints.

At this point you shouldn't have spent too much. Just an extra layer of drywall and caulk should be the biggest expense.

If you get Rod's book it goes into excellent detail on the door. Again, it doesn't have to be expensive but it has to have a lot of mass and be acoustically separated from the inner wall. If you don't get the book I can help walk you through it.

Okay, a lot so far. I need to get some work done. The basic idea is that the inner room needs to be completely separated from the outer room. The floor of course won't be but it's not a big deal. ~ 10hz and down.

You may be thinking, well, what about the acoustics of the room? Don't worry, you've already addressed some of the acoustics, because the walls and ceiling of the inner room has their own resonances at slightly higher frequencies (still in the bass range). They will flex both fundamentally and in harmonics when the bass is booming and absorb (convert) some of that energy. Some of the slightly higher frequencies will penetrate the drywall, be "slowed down" some by the insulation, bounce off the concrete walls, go back through the insulation and double layer of drywall and dribble back into the room. All the while being converted from audio energy to heat. Yes, there's bass traps and upper frequencies to deal with but not too bad.

I'll be glad to give more detail as needed. All kinds of fun stuff. If you really want to dig in buy a cheap sound level meter, a measurement mic and appropriate software and make it an adventure. Oh, maybe go with Dayton Audio DSP-408 for your front L/R/C/subwoofer setup. It's a ton of fun.

Alright, I'll shut up. Gotta finish some work on the roof of my studio....

[Bill Fitzmaurice]

Still seems like massive overkill. Who's on the other side of that concrete to aggravate? Worms?

[Rick Lee]

Ooo, more comments and pictures!! Yes, seal the hole with concrete and flexible caulk. That's a huge weak area sound wise. Looks like the pipe doesn't penetrate the inner wall. Yay! I've got lots of thoughts on your door but it looks like you're headed in the right direction. The main thing with the door is taking care of flanking sound which means sealing it correctly while keeping it decoupled.

Yes, just having a comfortable basement with a slab floor is nice. Mine isn't so there's not much I can do to keep the sound from traveling outside the media room. And to many people going to the trouble of room within a room is overkill but really it's not expensive at this point for you. Just labor and time mostly. Bill's less tedious ideas will get you 98% of the way there.

Hah! Yes, massive overkill but he's already built the stud wall. Why not have some fun? IF he's worried about sound traveling to the living areas it may be worth it. If not, I agree with you. Just finish out the wall simply and do some sound treatment.

[ecut1]

I'm actually not too worried about sound traveling to the home. This room is under the garage at the furthest end of the house. I currently have the table tubas set up in the other larger room under the larger garage. When they're cranked up you can barely hear it from the upstairs. Anything will be an improvement over the last house. Things would fall off shelves upstairs when we watched movies.

My main goal is to make it sound awesome inside the room and if some sound escapes I'm not too concerned.

The plumbing line is just for one drain from the garage floor. It travels into the other room under the garage and never actually enters the basement of the house.

[Rick Lee]

Well, you have it easy then. Don't worry about sound leakage but I would still drywall all of it (with caulking and fiberglass on the backside) because the low resonance of each complete unit will help dissipate peaks and smooth out the low end. However, I wouldn't use the same thickness of drywall on all the walls. Use 1/2" on one wall, 5/8" on another, etc. There's pretty easy formulas on the internet to figure the fundamental resonance of each panel if you wanted to go to the trouble. This is a good one to figure room resonances: https://amcoustics.com/tools/amroc?l=21 ... ue&r60=0.6 Several forums use that resource to figure the rooms acoustics. If it gets overwhelming just try to vary materials so resonances overlap. If that's too much trouble just drywall like normal.

After the walls were finished, if I were going "seat of my pants" without any acoustical measuring, I would first do a subwoofer crawl to get an idea of resonances and where to best place the subs. Within the room I would build a fake 2x4 wall at the front (screen end), about 3 feet in front of the concrete wall. I would frame in L,R,C speakers and the screen in the fake wall at desired height. Behind the screen I would hang 24" wide mineral wool on edge from the ceiling to the floor(like hanging clothes in a closet) with several inches of space between each (~6"). Ideally the long THT's could fit behind the fake wall, maybe even on top of each other with the mouths facing opposite directions, or facing up, etc. They probably won't work in each corner because it may affect ~80hz too much. If they end up stacked behind the screen I would put "closets" on either side behind the L/R speakers instead of behind the screen. Maybe under the speakers or center speaker I would build my equipment rack(s). If the subs end up right behind the screen I would build a shallow sound absorption panel the size of the screen, tightly surrounding the center channel TLAH. Use 2" rigid fiberglass backed by thin plywood like Luan. If the "closet" works behind the screen (subs somewhere else) don't do the sound panel and I would strongly consider soffit mounting the L/R speakers to alleviate sound bouncing off the wall behind them. Yes, that would affect the transmission line of the TLAH but it may be worth it.

Basically the front end I would make as dead as possible from the mid 40hz on up. The "closets" will help smooth out the resonances down into the 40hz range.

The rest of the room would be fairly simple. If I weren't able to do the "closests" at the screen end I would do one at the opposite end or two in the corners, maybe incorporating it with storage cabinets, popcorn maker, refrigerator, etc. Over the seating area I would consider a "sound cloud". Basically a suspended sound panel off the ceiling that would help with the audio illusion of height. You could also incorporate indirect lighting and help absorb some of the sound of your projector, etc.

On the side walls I would use the mirror test. That is, slide a mirror along the wall. From the seating area every where you see one of the front speakers is where you hang a DIY sound panel. I wouldn't worry about sound diffraction too much because the room is acoustically small. If you are concerned about diffraction and you wanted a rustic Idaho look you could build floor to ceiling sound panels with wood slats loosely spaced over them for a little Helmholtz action.

Doing those things would have you well within the park of good sound. There will be a lot of bass, which is good but doing those things should help smooth the response a little bit. Because it is an acoustically small room and it's set up solely for HT I wouldn't worry about making it too dead. RT60 should be pretty short, like 1/2 a second. You may find that you want "bass traps" in all the corners, including wall/ceiling intersections. I'd try it without them first though. Mainly I would vary the absortion as a general rule so no one frequency would be damped too severely.

And I would still think about fresh air supply because the room is going to be pretty tight.

[ecut1]

I took a look at the link you send. I think I'll have to dedicate a few hours to research before even really understanding what's happening on that site, LOL. Using different thicknesses of sheet rock on different walls is a pretty easy step. Another idea is maybe I go around a few days before hanging the sheetrock and put beads of stretchy silicone on all the studs. Let that dry, then hang the sheetrock. That would pretty much separate the sheetrock from the wall.

I was planning on building a false wall for the screen. I don't really want to lose 3 feet of the room though. I was thinking about 12 inches. Basically just enough to get the speakers behind the screen. With the right access I could still hang mineral wool insulation inside the cavity. I just have the two TT's right now and might add a THT later. I'm planning to set all the audio components in a stand on a wall behind the seating position. Not super fancy and maybe a bit annoying to look behind you to see the amp but it keeps all the lights out of sight. The basement has a kitchen just about 20 feet or so away so we won't be doing a wet bar or anything in there.

I like the idea of the bass traps and the mirror test for the sound panels. I think I could easily build those using a wood frame with with mineral wool insulation wrapped in speaker grille fabric. I also really like the idea of good old pink fiberglass insulation. It's much cheaper and DIY friendly!

Another topic: I'm also certainly torn on building a raised floor in the back of the room vs leaving the floor level throughout the room. I like the idea of a second elevated row but it seems to me I'd be adding a whole mess of logistics: like, subs placed on the wood platform vs concrete floor, etc. Or what if you want to slide the floor level couch back 12 inches, etc. It seems like a raised platform for rear seating is the go-to for home theater but I'm not sure if it's a good idea.

Here's a question off the topic, sort of... we're also finishing a bedroom in the basement that is under a main floor bedroom/bathroom. To minimize sound I was going to have cellulose blown into the 12 inch floor joists. Do you think 4 inches of mineral wool will give me similar results? Again, DIY friendly and less expensive.

Thanks for all the suggestions!

[Rick Lee]

If the sub(s) modeled well in the back of the room you could build a raised platform with the subs built into them. Otherwise..., eh.

Sounds like you're not really understanding absorbing sound and isolation. I only understand it coming at it as a musician and not from an engineering angle so I'm certainly not preaching as a know it all...
To isolate sound you need mass. That is, weight. Everything resonates at some frequency(s). Even a thick concrete slab sitting on mother earth resonates. However, that massive, heavy concrete slab is only going to resonate at ULF frequencies, below let's say 10hz and is going to need some pretty good energy to get it resonating. As things get lighter for the same general size it's going to resonate at higher and higher frequencies and is going to need less energy to start resonating. The higher the frequencies get in the audio range the less (perceived) isolation you have. Another way to think about it is think about how a 4x8 sheet of foam sounds "higher" than a sheet of plywood, which sounds higher than a sheet of drywall....Some big generalizations here but hopefully you get the idea.

To absorb sound is basically using material that will convert acoustic energy into heat. Fiberglass, mineral wool, etc. Absorption is the other side of the isolation coin because resonance is also converting energy into heat. I'm taking a lot of liberties with this explanation but that's the basic idea.

Anyway, what you want to minimize sound for the ceiling/floor of the bedrooms is Mass. Basically, double or triple layers of drywall (mass) in between the joists screwed to the subfloor. Then a layer of fiberglass, then a layer of drywall for the ceiling. Or, use resilient sound isolation clips if you don't want to use that much drywall. Or, use that much drywall and the risc clips also.

Since the bedroom below is concrete I don't think you need to worry too much about flanking sound. If you REALLY want isolation and absorption do the drywall and on the upstairs floor lay 1" or 2" rigid fiberglass down, then a layer of 3/4" plywood, then a layer of mass loaded vinyl or green glue or carpet adhesive, another layer of 3/4" plywood and then floor finish of your choice. How's that for overkill!

Oh, since you're not really building an airtight isolating double wall assembly in the HT don't worry about putting the caulk on the studs before hand. It's still a good idea to caulk, but I wouldn't worry about letting it cure first.

[Bruce Weldy]

I don't think it's been mentioned yet, but the first rule for getting rid of unwanted reflections in a new build is to NOT have parallel walls. That allows the reflections to find a corner to dissipate and not keep bouncing back and forth.

Do a little research on building a studio. It doesn't have to be stupid elaborate, but making a few adjustments in wall placement will help reflections.

[nivlek]

Just my $.02 ...

I know this is a little bit like comparing oranges to tangerines, but, the very best room acoustics
I ever encountered was in a club that I was playing in. I asked the owner about the walls...he replied
that the burlap material on the walls was covering the fiberglass insulation, which was turned around
so the fuzzy side was against the burlap and the backing was against the exterior walls.

The room had 2' x 4' acoustic ceiling tiles, but a tile dance floor and parallel walls. It was not
absolutely dead, but it was a pleasure to play there and there seemed to be no such thing as playing
too loud. You just needed to stay in balance.

Just a side note: They even had BEAR WRESTLING at this club for the mighty rednecks!!! Off the
chain crazy I tell you!

Regards to all.

[Bill Fitzmaurice]

Normally that wall treatment would be way too dead, but it's offset by the tile floor.

[ecut1]

I'm fully wired and spent some time gluing and bolting the sill plate to the concrete floor yesterday. One item I'm wondering about before moving on to insulation. The 2x4 walls sit about an inch or so off the concrete wall. I have the chance now to connect them to the concrete wall by simply shimming them and gluing the shims in place with liquid nails. I'm thinking about half way up so there's only 4 feet of 2x4 flex instead of 8 feet. Would this be worth the trouble to keep the walls from flexing? Maybe overkill?...

[Bill Fitzmaurice]

Since you have the ability to fasten the studs to the concrete wall so easily you might as well. May not help, can't hurt.

[ecut1]

Sheet rocked, painted, carpet installed, screen installed... just a bunch of wiring and speaker/projector mounting to go... Getting close!!!

[ecut1]

I began hanging speakers and placing the table tubas last night. This room is more difficult that I thought it would be. We tried multiple positions for the subs and the best so far is firing forward into the false wall at the front of the room. (I haven't tried the rear of the room yet.) I found the phase HAS to match, otherwise in position 2 (see pic) the bass sounds like a blown woofer - almost no bass at all. In this scenario (opposite phase) if you step into position 1 or 3 it sounds amazing. Once they are in the same phase position 2 sounds pretty good. Still a bit better in position 1 and 3 though. One interesting thing I found is the distance from the right and left walls seems critical. At first I had them about 6 inches away from the wall. Moving them within an inch from the wall made the most difference. (I left them about 6 inches away from the front wall.) I'm thinking the furniture will help and I plan on adding some bass traps, sound panels, etc. Just not sure where to start with those.

[Bill Fitzmaurice]

Most of the problem is the listening position. You're getting a cancellation null from the reflection off the rear wall meeting the original wave out of phase. That null will occur where the distance from the LP to the rear wall is 1/4 wavelength. I'd try placing the subs right behind the couch. Rear of room won't help, then you'll be getting the cancellation from the reflection off the front wall.