Acoustics help needed on new HT construction

[Vanhoecke]

I’m a newbie. I’ve tried to do a lot of research ahead of this posting, but I’m sure I’ve probably missed something –information overload. I'm building a new HT as part of a basement build-out and need some advice. I have reserved a rectangular space in the basement that is 19'(l) x 13.5'(w) x 9'(h) - unfinished. The walls on two sides of the space (the front screen wall and a side wall) will sit in front of cement foundation walls. About 9' of the 13.5' back wall sit in front of another concrete foundation wall and the balance separates the HT from the mechanical room. The remaining side wall (19’) separates the HT from the remaining basement, which when finished will be a rec. room with bar and pool table. This space is directly under the kitchen, which has wood flooring.

I want to make sure that I'm planning the sound isolation and acoustics properly. I’d rather spend the money up front then spend it on a divorce lawyer later. I’ve already been down that road once. For sound isolation, I'm planning on the following:

1) The walls the sit in front of foundation walls will be 2x4 construction with RC, fiberglass insulation and double dry wall (1/2 and 5/8) screwed and green glued. They will sit approx 1 inch off the foundation wall, which is wrapped with insulation. There are two small (1'x2') basement windows along the top of the side wall which I plan to wall off. I plan on keeping a small gap (to be chalked with sound chalk) where the wall meets the ceiling and the floor in order to isolate it more.

2) The side wall adjoining the living space will be double 2x4 construction (1" gap), RC on the wall inside the HT, both walls insulated and double dry wall on both sides (different thicknesses, gap at top and bottom, same as above). This wall will have an exterior grade metal door with a weather seal in it. (Unfortunately, it will be located in the front half of the HT, just ahead of the first row of chairs.)

3) The ceiling will be heavily insulated (6"+), 1#/ft mass loaded vinyl, RC double dry wall (similar to walls). I plan on building a 1’ soffit around the outer 3” perimeter so that the recessed lights will not penetrate the ceiling.

4) For the floor, I’m considering either using 2#/ft mass loaded vinyl under the carpet and mat or a floating floor using the pads and two layers of sub-flooring. (I’m strongly leaning toward the mass loaded vinyl route.)

If I’m careful to plug all the cracks, I’m hoping to get a STC over 60. Do you think this will be adequate to keep the noise from reaching the kitchen and/or rec. room? I’m planning on running a Denon AVR-4306 with 7.1 Axiom Epic 80/600 speaker system. I like a true cinema experience (i.e., I tend to crank it up). What am I missing in terms of isolation?

For acoustics, I saw a HT where the four side walls were lined floor to ceiling with 2” mineral wool fiber covered with fabric (for absorption). I probably need to put a panel on the door to stop any first reflections that will occur at that point.

The front of the HT will be a 6” high stage, 3’ deep in front of a 100” screen. I would like to find some way to hide the front speakers and sub. Or alternatively find a set of in-walls that will perform the same as the Axioms (without spending significantly more money). I have seen HTs where the LR front speakers are placed behind black fabric walls that are build at a 30-45 degree angle both sides of the screen. As long as I frame the wall so that the 2x4s are well clear of the speaker fronts (i.e., put by cross support 2x4 at least two feet above the top of my LR speakers) does this create a problem? I would have the speakers in direct contact with the back of the fabric and could even remove the speaker grills. I would also put base traps in the front corners of the HT behind the speakers and fabric walls (any suggestions?). For the center and the sub, I was planning to build a framed-in cabinet (open across the front along the back of the stage just below the screen. It would be covered with black fabric like the fake side walls. Again, I would keep the framing material away from the speaker fronts. I know most hard core audiophiles cringe at the thought of anything being between he speaker and the listener. But would this set-up appreciably reduce the sound quality by any noticeable amount?

Will the mineral wool on all four walls be too much? Should I do something different for the back half of the HT (for diffusion) vs. the front (for absorption)? If so, what would work and keep with my general design?

Thanks for all your comments. Happy Turkey Day!

Bob

 

[Glenn Kuras]

>For acoustics, I saw a HT where the four side walls were lined floor to ceiling with 2” mineral wool fiber covered with fabric (for absorption). I probably need to put a panel on the door to stop any first reflections that will occur at that point.<

That is going to be way to much thin treatment within your room. You are going to end up absorbing all the high end but the low end is still going to be bouncing around the room. I would focus on more bass trapping in the room and use 2" mineral wool for the first reflections only on the left and right wall. Also you will want to treat the back wall (4" panels) with a few panels to help with standing waves coming off the back wall.

Glenn

 

[Vanhoecke]

Thanks for the input Glenn.

Regarding:

That is going to be way to much thin treatment within your room.

I am planning on using 2" mineral for the first reflections. I've seen the 25% rule of thumb discussed on various sites. My question is where do you apply it. I'm wanting a nice symetrical set of covered walls. I wasn't planning on using individual panels. Maybe this is a mistake.

The back of the first row of seats is about 12' off the front wall and the second row is about 18' off the front wall (i.e., 1" off the back wall). Each row of seats is just over 10' wide, so its pretty tight side-to-side (13.5' width). Since the room is not built yet, I'm just estimating the first reflection points assuming that the reflections will bounce off the walls at the same angle they approach. Is this accurate? I have scetched the first reflections for all eight seats. When I do this it appears that my first reflection points will extend along most of the side walls (e.g., the far right seat in the back row has a reflection point for the left front which is behind the first row of seats). My thought was to use 2" mineral fiber sheets along the walls and cover it with fabric. I would have free standing (1/2 depth) columns along the walls, which will hide my seams and provide support of wall sconces. I'm I making this two difficult? I would welcome any suggections.

Regarding:

I would focus on more bass trapping in the room and use 2" mineral wool for the first reflections only on the left and right wall. Also you will want to treat the back wall (4" panels) with a few panels to help with standing waves coming off the back wall.

Both points are well taken. Is it sufficent to place bass traps in each of the four corners (floor to ceiling)? I checked out your site and I'm interesting inthe corner traps. How far do they extend from the corner? It looked like 2' along each wall. Is there anyway to reduce this down. I don't have 2' of clearance betweent the outside seats and the side walls. On the www.rpginc.com site they refer to a soffit bass trap one of their package options, but they don't describe it anywhere on their site. I'm installing a soffit for my recessed lights -- maybe I could do my bass trapping there.

Will the 4" panels on the back wall provide bass management? Should I use 4"material behind the front speakers? Is this too much acoustical treatment for the room? I'm planning on install a 7.1 Axiom Epic 80-600 speaker system. The QS-8 surrounds in the system are quadpole -- they have four drivers, each facing in a different direction. Given that I'm going to have 16 drivers across all 4 surround speakers (all located in the back half of the HT), can I really create a dead room?

Thanks again for your advice.

Bob

 

[bpape]

A few comments on the isolation.

Please use RSIC-1 and hat channel instead of RC. RC is great for standard building type noise isololation but will be very unpredictable in terms of bass isolation. In many cases, the RC can make transfer of many frequencies worse.

Skip the Vinyl. If you use RSIC-1, hat channel, and double drywall on the ceilings, you're getting a good job done. Also, you'll want to use PAC DC-04 clips to isolate the tops of the side walls from the structure above. If you tied the walls directly into the floor above, you're shorting out a lot of your iso work.

For the floor above, if you want to do it right, use a mat product specifically designed for this purpose. If you have the vertical space, use the mat, then a layer of gypcrete. This is the most effective option.

As for treatments inside the room, it's all about balance. Rules of thumb are just that, generalities that will get you close (maybe) but will almost never be the optimal solution for a specific room. There are so many things that need to be considered.

Bryan

 

[Vanhoecke]

I've been researching the isolation clips for the walls. I think I'll include them in the design. For the walls that sit in front of foundation walls I plan on anchoring the clips into the concrete and the attaching them to the stud wall (i.e., the wall will not touch the ceiling rafters). For the double stub wall, I plan on installing the clips on the interior wall but attaching it to the ceiling rafter (not where else to attach it). The exterior of the two stud walls will be nailed into the rafter -- next rafter is too far away (I assume be this point the vibration will have been significantly reduced and will not transfer upstairs). Do I need to then mount the base of the walls on a mat in order to get them off the slab?

Also, when doing a double stub wall with a door. Is it best to install the door in the interior wall? What is the best way to frame out the door jam between the two seperate stud walls in avoid shorting out the isolation between the two walls?

Still looking for any more advice on the acoustic treatment in the HT.
Thanks

Bob

 

[bpape]

General internal treatment strategy:

Broadband bass absorbtion in the front 2 vertical corners floor to ceiling and across the front wall/ceiling intersection.

1" or 2" absorbers (assuming cloth walls to hide it) floor to approx 4' high (5' high where rear seat reflections fall) on the side walls.

Completely kill the front wall with 2" unfaced. Behind the screen is 1" unfaced.

Rear wall killed completely with 2" but faced with FSK.

As for the isolation, do not connect any of the walls directly to the floor above or you'll short out your work. Even the double stud wall should be isolated with DC-04 clips.

Bryan