# Corner bass traps and sub locations



## dguarnaccia

Hi all, 

I've got a home theater contruction build going here and I had specific question on acoustics. The room is 34'x15.9'x7.4' I'd like to create 4 superchunk bass traps for each corner. The question is this: What's the best location for the subs? Is it best to build a shelf and put the sub tight in the corner (assume I'm going to eventually have 1 sub in every corner) or should I have the sub diagonal across the face of the superchunk trap and have the trap extend floor to ceiling?

The subs are 40" tall, CHT 18.2 subs. I ruled out midwall placement because I'm trying to hide the subs behind curtainwalls. 

Would love some recommendations on how best to place the subs with corner bass traps.


----------



## bpape

Corners give the most sheer output but also normally maximally excite your room modes. Center of walls is actually best if multiple subs. Another option is deliberately non-symmetric to not excite the room reflections in the same way or at the same frequencies. Iterative trial and error is the best way to go about this. Get one dialed in to your mains. Then bring the other into the equation and try to compensate for any anomalies from the first sub by creating offsetting response issues from the other.

Bryan


----------



## dguarnaccia

So you'd advise giving up the corner loading? I'm fairly limited in placement. I have 4.5 feet behind the front screen that I can place the sub/subs and 3.5 feet behind the back cloth wall that I can put subs so I guess the best bet will be to do centers front and back wall and start measuring. I was hoping to try to get a bit of extra umph out of the corners (they are backed by concrete) to help me bump up the super low fequencies (under 10hz). How much would I lose there going centerwall do ya think?


----------



## Prof.

I would suggest that you build your bass traps floor to ceiling (at least for the front wall) and initially place your main sub right next to the trap..That way you still get a bit of reinforcement from near corner location..
If it's too boomy there, then you can gradually slide it towards the centre location and take measurements as you go to find the best position..


----------



## dguarnaccia

Thanks all. I'll start with the centers and test out the best placement. Will start building out the corner traps tommorow. I've read that the less dense pink stuff is better for large corner traps so I'm using r38 in a 2'x2' triangle.


----------



## dguarnaccia

Another question about the sidewalls. I've seen a bit of debate about how to treat the sidewalls. If I wanted to use Linacoustic or something similar up to 42-44" from the floor, do you mount that with the lining facing out to reflect the higher frequencies or insulation facing out to absorb everything?


----------



## bpape

The lining of the linacoustic is pretty thin. It will still absorb mostly all the way up even with the facing out. If you want it full all the way up, then put the lining against the wall.

Personally, I don't care for the linacoustic. It's just too thin to do much of anything. I don't like to use anything less than 2".

Bryan


----------



## dguarnaccia

Thanks for the feedback Bryan. Instead of Linacoustic what do you trypically recommend. I'm trying to avoid a bunch of large panels on the sidewalls for asctetics if it can be helped. Is there other material out there that is better or is it purely a function of depth? How much performance do you lose by using 1" material vs 2"?


----------



## bpape

1" vs 2" is purely a matter of how low in frequency it will be effective. You can still hide 2" 703 or equivalent behind cloth walls just like the linacoustic. You just need to fir it out farther.

Bryan


----------



## dguarnaccia

Thanks Bryan. I'm gonna have to figure out how to deal with a 2 inch bump out. I was planning on leaving the top half of the wall painted sheetrock. 

Can I ask a question about diffusion? I've read a ton of theads about diffusion and have seen some of the debates about how close is too close to sit to a diffusor. I've been looking at the GIK QRD diffusors, and wondered what the minimum seating distances would be. My back row would be about 3 1/2 feet away, and the middle row would be 10 feet away. Is that too close to the diffusors along a rear wall? Also, do you recommend putting diffusors along the side walls towards the back of the room?


----------



## bpape

How far you sit from a diffuser is a function of the wavelength of the lowest frequency a diffuser will work on. 3.5' is a bit close. We usually recommend 5' minimum but technically it's more like 4.5'

On the rear portion of the side walls, that might work OK. Usually the back wall of a home theater is better off with thick bass absorption than diffusion.

Bryan


----------



## ALMFamily

Sorry this is not an acoustical response, but I was wondering about the desire to leave the top portion painted sheetrock. I was thinking that when sidewalls are treated, the closer you are to the speaker, the higher up the wall the treatment goes. As you go toward the back wall, the treatment diminishes to next to nothing.

If this is the case, I would think that would create a "ladder" effect on your painted walls that might not look like you want. Perhaps Bryan can set me straight on this one.......


----------



## bpape

Actually, we would do more higher up in the front portion of the room and then drop it down outside the reflection zone on the sides to leave the surround field a bit more lively.

Bryan


----------



## dguarnaccia

I had planned to ladder up the absorption to match the riser height. Is this not a good idea? For the rear, I have some 242 traps, are they good enough or so I need monster traps?


----------



## bpape

Just make sure you cover the reflection level the entire way. In the rear, Monsters would be far preferable to 242's.

Bryan


----------



## dguarnaccia

ok cool, I'll do that. I was originally going to use a fabric mountain system like Wallmate, but they only support 1 inch panels. Anyone know of a fabric mountain system that supports 2 inchs?


----------



## bpape

Just fir out the 1" channel a bit and you can use the same stuff.

Bryan


----------



## NBPk402

dguarnaccia said:


> Hi all,
> 
> I've got a home theater contruction build going here and I had specific question on acoustics. The room is 34'x15.9'x7.4' I'd like to create 4 superchunk bass traps for each corner. The question is this: What's the best location for the subs? Is it best to build a shelf and put the sub tight in the corner (assume I'm going to eventually have 1 sub in every corner) or should I have the sub diagonal across the face of the superchunk trap and have the trap extend floor to ceiling?
> 
> The subs are 40" tall, CHT 18.2 subs. I ruled out midwall placement because I'm trying to hide the subs behind curtainwalls.
> 
> Would love some recommendations on how best to place the subs with corner bass traps.


Do you have a sub floor or basement? I am thinking of installing my next subs under the floor and having floor grills for the bass to travel through. By going this route it is out of site... which works for me.


----------



## dguarnaccia

No, this is actually in the basement with concrete flooring. I'm building 2 false walls covered in fabric in front for my screen wall and in back to hide my rear subs, back surrounds, equipment rack and any rear wall absorption. Rear rear false wall also gave me the ability to keet the seating I the 60-80% zone and off te back wall.


----------



## dguarnaccia

Yet another acoustics question: 

I've been looking for the best, most absorbative carpet underlay and I've come to the conclusion that Jute fiber is the best for sound absorbtion. I realize it's nowhere near as good as what I'll get from fiberglass or rockwool, but this 40 ouch jute underlay looks promising: 

http://www.rugpadcorner.com/superior-felt-jute-rug-pads.aspx?page=2 

Bryan and Prof, would this stuff be worth the money to help tame some of the reflection from the floor? I plan to do some panels on the ceiling as well ar the first reflection point. My speakers are the Abbeys from Earl Geddes, and while they do a decent job controlling the horizonal directivity, their vertical CD based on the polar plots isn't quite as good so I'd love to be able to tame the reflections vertically as much as possible. 

So the question is, it a pad like this worth the extra money?


----------



## Prof.

I'm not familiar with that particular underlay, so I can't say how affective it would be..
It doesn't mention anything about acoustic properties on the web site, so it might not be suitable..

There are several brands of acoustic underlay available, specifically designed to dampen reflections..
Damtec and Silent Step are two that come to mind, but if you do a search for "acoustic carpet underlay", you will probably find others that are available in your area..


----------



## dguarnaccia

Thanks Prof,

I couldn't find Damtech or Silent step from vendors in the US. I think I'll have to visit a local carpet store to see if they can source it, everything I find are either UK or AU companies. The Jute fiber I had read about was from the acoustics master thread over on AVS. I think one of the guys was saying it had the highest STC rating, but I'll ask the vendor if the have any specs on that.


----------



## dguarnaccia

Quick question. On my front wall, I'm mounting rockwool mounted on 2x3's to create a 2 inch air gap between wall and Rockwool. Now, for my corner basstraps I'm doing super-chunks. Is it ok to leave that same 2 inch air gap between the wall and the superchunk or does it need to be tight in the corner?


----------



## bpape

You can though it's not really necessary. The chunk style works just fine tucked tight into the corner. 

Bryan


----------



## dguarnaccia

Thanks Bryan,

I was going to stand it out to be flush with the Rockwool so it was one continuous surface for the fabric but didn't want to reduce the effectiveness of the superchunk if the gap was less than ideal.


----------



## dguarnaccia

Corner traps and front wall finished. Thanks for all the advice so far guys. Really appreciate it. Here is the finished front wall before mounting the fabric covering:


----------



## ALMFamily

dguarnaccia said:


> Corner traps and front wall finished. Thanks for all the advice so far guys. Really appreciate it. Here is the finished front wall before mounting the fabric covering:


Sorry, probably being a bit dense here, but how are you mounting your screen?


----------



## ALMFamily

As long as I am asking questions, :bigsmile: I have a question about corner traps. Did you find info that suggested building the traps directly on the concrete would be best?

I had planned on building mine on top of the carpet once it was installed, but seeing how yours is done has me questioning if that is the best choice.....


----------



## dguarnaccia

I'll be building a screen wall about 4 foot in front of the actual front wall. Planning on doing an acoustically transparent screen. Most likely the Acousticpro4k from Elite. 

I wasn't planning on carpeting behind the screen which is why I went on the concrete. Not sure it maters either way though.


----------



## ALMFamily

dguarnaccia said:


> I'll be building a screen wall about 4 foot in front of the actual front wall. Planning on doing an acoustically transparent screen. Most likely the Acousticpro4k from Elite.


Ah - I did not realize you were doing an AT screen - I wish I had the space to do that. Your screen wall will be so clean!



dguarnaccia said:


> I wasn't planning on carpeting behind the screen which is why I went on the concrete. Not sure it maters either way though.


OK, thanks!


----------



## dguarnaccia

It was a really long room at 34 feet long so I decided to just create a nice large space in the front and back of the room to hide equipment, speakers, subs, etc and hide the treatments as well. I'll put a AT wall in the back as well so I can do some cool stuff with acoustic panels or diffussors (if I wind up building a QRD) or something like that.


----------



## dguarnaccia

Sound proofing questions for everyone. I've trying to reduce the sound transmision through 2 doors in the theater, and so far, I've sealed the doors, and put on a special bottom seal system that seals the bottom gap when the door closes but I'm still getting a ton of noise through the door. I'm using a solid core wood door, and I'm looking for other things I can do. 

I was looking at this material http://www.supersoundproofingsales....ick-by-48-wide-per-foot/productinfo/09-42760/ and wondered if adding 2 inches of that heavy foam material would be worth it across the entire door?

The other option of course is to do the airlock system, but I'd really rather not have to have a second door on each side of the room, as the doors open out now to avoid disrupting movie watching when someone comes in, but if I have to go there I will as a last resort. 

Any thoughts on that material or other suggestions of what I can do with my door to increase the sound blocking?


----------



## bpape

Unfortunately that foam isn't going to do much of anything for isolation- regardless of what they advertise. It might damp the door a tad from resonance but won't block sound. 

If you want to stay with a single door, the best option is to add MDF to increase the mass and lower the resonance. Green Glue between it and the door will also help some. You may have to beef up the hinges though to support the additional weight.

Bryan


----------



## dguarnaccia

Well fiddlesticks! I was hoping that material would help block the sound but it sounds like the only thing that will really do that is mass. Looks like I'll have to do the double door airlock thing. The wall is a full double wall about 12" thick so I have the room, I just didn't want to have to tear apart the doorway again but I guess I will. Anyone know what the STC rating would be around a double door system like that?


----------



## bpape

What level is getting through now and what frequency range?

Bryan


----------



## dguarnaccia

I haven't measured it, but I would guess it's only reducing the volume by 20-30% subjectively. My goal is to be able to watch a movie at reference and hear only a murmur outside the room. The ceiling and walls seem to be performing well and they shoud be at STC 56 (sound Iso clips, double sheeetrock with green glue) so it I can get close to that I'll be set.


----------



## bpape

If you want that kind of performance from the door, most likely the double door is the best solution.

Bryan


----------



## dguarnaccia

Ok, thanks Bryan, I think I'll have to go that route then...


----------



## dguarnaccia

Hi Bryan,

I finally got a decent measurement system for xmas, and did some measuring. It looks like I'm getting some ringing between 5k and 15k. I also did a bunch of other measurements, and can't quite tell if this is good or bad. Could you look at these and let me know if there's something you guys make that could tame the stuff and let me know if there is anything else I should be dealing with? I have the Omnimic, so if there is any other measurements you'd like me to take let me know:


This is the toneburst sweep:












I originally thought the ringing between 5-15k were my sconces, but I caulked all them last night, so it's not them rattling...wonder what that could be...



This is the total spectral decay. Not a clue what that really is:














This the wavelet spectogram. Again, no clue how to read that...














Here is the ETC graph














and lastly, here is an updated bass decay chart. This seems reasonable, but let me know if I'm missing something.













Thanks Bryan!


----------



## bpape

I'd be much more concerned with the low end. Your high end looks to already be plenty under control being down 30db after only 5ms?!

Bryan


----------



## dguarnaccia

Thanks Bryan,

given that I've done corner trapping already (24"x24"x37") in all corners and a baffle wall in the front, what else would you suggest to tame the low end?


----------



## bpape

Any way of resizing the graphs so we're looking at say 30-300Hz and with a LOG scale across the bottom?

I would suspect that as in many rooms, the wall behind you when seated is a prime place for some thick bass control. 

Bryan


----------



## dguarnaccia

I'll try that. Which of the graphs do you want? Just the Bass Decay graph?


----------



## bpape

And frequency response


----------



## dguarnaccia

I reran the sweeps form 30-300. I didn't see a setting to change them to log scale, but I think they might be already? Let me know if they aren't:

Here is the Bass Decay:










Here is a FR sweep:










Let me know if that is what you were looking for, or if there is something else I need to measure.

Thanks!


----------



## bpape

That's fine but the frequency response looks like it has smoothing turned on which masks exactly where problems are.

Bryan


----------



## dguarnaccia

it's 1/6 octave. What would you prefer?


----------



## bpape

No smoothing at all so we can pin down certain frequencies.

Bryan


----------



## dguarnaccia

I see what you mean. Here's the FR with no smoothing.


----------



## bpape

I'd bet you a buck the lower null is coming off the back wall.

Bryan


----------



## dguarnaccia

Does it matter that I have 1 sub in the center of the front wall and 1 sub in the center of the rear wall?


----------



## ALMFamily

Have you tried moving the subs around the room at all? I was quite surprised to discover that one in the front left corner and one on the side wall was my best configuration.


----------



## NBPk402

I would think that it might work out better to put them in dif areas of the room as they will sound dif. It might even out the sound better. When I ran 5 subs years ago that is the reason I did it and I loved the way it sounded.


----------



## dguarnaccia

I'm limited in where I can place them. I can put them anywhere along the front wall or the back wall, but can't put them along the sidewalls. I've tried them closer to the corners and in the opposing corners, and it got worse. 

I has trying to follow the Harmon model of centering them on 2 opposing walls which I've been told is the best solution for 2 subs. I've been toying with adding 2 more subs and trying all 4 corners, though my superchunk bass traps would be in the way.


----------



## ALMFamily

I think as long as you locate them close to the corner, you should be fine. None of mine are actually located in a corner - they are all at least a foot or so away from the closest corner.


----------



## dguarnaccia

I think I'll try testing them in the corners again to see what I get


----------



## AudiocRaver

dguarnaccia said:


> I think I'll try testing them in the corners again to see what I get


Corner placement results in more "room coupling," higher efficiency at the lowest frequencies, and perhaps more boominess, depending on a lot of variables. Sometimes it can be tamed sufficiently with EQ, sometimes not. Something to consider. Beg pardon if this has already been covered.


----------



## dguarnaccia

AudiocRaver said:


> Corner placement results in more "room coupling," higher efficiency at the lowest frequencies, and perhaps more boominess, depending on a lot of variables. Sometimes it can be tamed sufficiently with EQ, sometimes not. Something to consider. Beg pardon if this has already been covered.


True, that's why I've avoided them so far. But I do have the Minidsp, and can adjust for some of that. My real goal was to get as flat as possible down below 10hz and this could very well skew that but it's worth some additional testing.


----------



## dguarnaccia

Just thought I'd circle back on this. I wound up trying a variety of things, including some older traps I had lying around and also upgrade my sub amp coupled with a miniDSP. 

Here is the final result (no smoothing)










My overall goal was to get +-5db flat to 10hz. I was also trying to get some sort of house curve though I'm not sure I really got one.

One question for Bryan, does the FR from 500hz - 20khz look problematic? Should I be worried about the nulls in the upper frequencies? I treated the back wall as you suggest with some old tube traps that I'll replace witih monster traps (just wanted to see their effect first) but do you think I need more HF trapping in the room to tame the upper frequencies?


----------



## bpape

Overall the 500 and lower level seems high compared to the rest. Not real fond of the broad dips around 500 and 5kHz. I would do some playing with moving the speakers (not the sub) and see what changes so maybe we can pin down what the problem is.

Bryan


----------

