Room length twice its width, how big of a problem is this - AVS Forum
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post #1 of 14 Old 09-07-2013, 07:07 PM - Thread Starter
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Should I avoid a room that is twice as long as it is wide by all costs?

I am very happy with my plan for the space I have, but I calculated the room dimensions and the length is almost exactly twice the width 21.2 ft x 10.8 ft. I understand that this is almost a worst case scenario dimension.

Can front corner base traps and a riser base trap correct this or should I change this while I still can? (the walls are framed)
Does anyone have experience listening to a room of these proportions?

Thanks

-Jay
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post #2 of 14 Old 09-07-2013, 07:13 PM - Thread Starter
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Sorry for the bad pic,

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post #3 of 14 Old 09-07-2013, 07:26 PM
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Jay,

Regardless of the dimensions of the room, you will need to treat it. So if it were me I don't think I would let that deter me, especially if I was happy with the space.

If you are really concerned, you could get a pro to create an acoustic plan. Then you would have a specific plan for your room instead of relying on the general acoustic guidelines posted on the forums.

I don't think it is that expensive considering the overall cost of such a project. I am planning on making that investment for my build which will get underway shortly.

Good luck,
Gerry
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post #4 of 14 Old 09-07-2013, 07:52 PM - Thread Starter
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Gerry,

Thanks for the advice, I agree with your view point. But I really have no experience so I look into the cost of obtaining plans.

-Jay
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post #5 of 14 Old 09-08-2013, 02:45 AM
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You don't need to have the dimension differ much from twice to remove the issue. They're already not exactly double... Could be enough to just shorten the room by four inches to be home free. Build a new front wall in bricks or concrete blocks and get a solid wall for your front subwoofers to lean against and put a thick absorber in the rear.


At that width, I would say you need to do quite a job with absorbing the first reflex. I'd probably go for 'gills' myself.

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post #6 of 14 Old 09-08-2013, 05:23 AM - Thread Starter
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Quote:
Originally Posted by Nightlord View Post

You don't need to have the dimension differ much from twice to remove the issue. They're already not exactly double...

Good point, any clue how close the frequencies need to be, do they need to be exact

I worked up the following

Room Dimensions in ft 21.2 x 10.8 x 8.8, Frequency of resonance mode F=1130/2xd
26.6 HZ from length, 52.3 HZ from width, 64.2 from Height

Length Width Height
1x 26.6 52.3 64.2
2x 53.2 104.6 128.4
3x 79.8 156.9 256.8
4x 106.4 209.2
5x 133.0 261.5
6x 159.6 313.8
7x 186.2
8x 212.8
9x 239.4
10x 266.0
11x 292.6
12x 319.2

Seems like I have may have issues at 52-53 HZ, 104-106 HZ, 157-160 HZ. Not sure how I should round these numbers or how close they can be without issue.

I found that a Cube is the worst case scenario, followed by rooms with 2 equall walls, followed by rooms with walls that are all multiples(21x16x8). My room only has two walls that are almost multiples.

Quote:
Could be enough to just shorten the room by four inches to be home free.

if I decrease room length by 4 inches to decrease standing redundant waves.

New dimensions in ft 20.9 x 10.8 x 8.8
26.9 HZ from length, 52.3 HZ from width, 64.2 from Height
Length Width Height
1x 26.9 52.3 64.2
2x 53.8 104.6 128.4
3x 80.7 156.9 256.8
4x 107.6 209.2
5x 134.5 261.5
6x 161.4 313.8
7x 188.3
8x 215.2
9x 242.1
10x 269.0
11x 295.9
12x 322.8

This gives me additional separation of the concerning frequencies.

It turns out this situation may not be as bad as I thought, Does anyone know how much separation I need between frequencies or do they need to exactly match to be a issue.

Quote:
At that width, I would say you need to do quite a job with absorbing the first reflex. I'd probably go for 'gills' myself.

Could you help me out, im not familiar with "gills"

Thanks Nightlord, really helpful feedback
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post #7 of 14 Old 09-08-2013, 05:37 AM
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Angled in walls stuffed with insulation with the openings towards the speakers.

Borrowed image pre stuffing:
CameraPictures%20135.jpg

Speakers to the right in this pic, listening position to the left.

Looks a bit like the gills on a shark, hence the nickname.

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post #8 of 14 Old 09-08-2013, 06:16 AM
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Modal resonances have bandwidth, defined by half-power (-3dB) points. F2-F1=2.2/RT60 The longer the decay time, the narrower the band.

A reverberation time (decay time) of 0.5sec yields a bandwidth of 4.4Hz. 0.8sec yields 2.7Hz. When bandwidths of adjacent modes overlap, energy in one resonance tends to excite the other.

In my personal opinion, refer to Gerry S, in post 3. There are too many other reasons for your room to have its dimensions. Do what you can to make the space as enjoyable as you can - if it needs to be that size to be worthwhile, make the bass work through some other means. Start by planning for at least 3 subs (prefer 4) and strongly consider professional help. smile.gif

Fred
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post #9 of 14 Old 09-08-2013, 08:21 AM - Thread Starter
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Thanks Fred,
Quote:
Originally Posted by HopefulFred View Post

Do what you can to make the space as enjoyable as you can - if it needs to be that size to be worthwhile, make the bass work through some other means. Start by planning for at least 3 subs (prefer 4) and strongly consider professional help. smile.gif

I started planning only for one sub, I am now convinced to set up two. I would like to wire for 4.

I have a couple more questions.

If I start two subs, should I

1. get a 7.2 channel receiver to set up the initial two front subs
2. split the line out from a 7.1 or
3. daisey chain the two subs from a 7.1

When I move to 4 subs (if needed), Should I then
1. split the two outputs from 7.2 to feed two front subs and two back subs
2. from 7.2 run one to front and one to back and daisey chain second subs
3. Split the line out from a 7.1 to front and back, then daisey chain the other sub on that wall


Is there not really a difference in the wiring, I am just trying to decide how to prewire for 4 subs and I dont think that running all RG6 from the AV closet to speaker location is the answer.

Should I start 7.1 or 7.2? Should i have 7.2 to run 4 subs?
For two subs I should plan for 2 subs on front wall (1/4 length of room from corners)?
For four subs should I mirror this on the back wall?
I know the extra subs are for acoustic balancing and cancelation not shear volume, but is there anyway 4 subs could be too much for a room 21 x 11, I am currently looking at the BIC PL-200's

-Jay
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post #10 of 14 Old 09-08-2013, 09:01 AM
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All four subs up front.

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post #11 of 14 Old 09-08-2013, 09:56 AM
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How you prewire depends on the philosophy you have for multi-sub optimization. You and I have similar spaces, especially in that they are width constrained. Also, we both have a door near the midpoint of a wall. So, let me lay out my approach, and hopefully that works as a bit of a case study. Of course others will disagree, and since my room is not set up and calibrated yet the jury is still out, so to speak. Clearly, Nightlord would do it differently from what I'm planning.

My personal philosophy is still in flux - which is to say that I haven't tried anything and come up against failure or success - but I'm trying to minimize seat-to-seat variation and provide myself with enough flexibility and EQ options to overcome whatever may develop. I like the ideas that you can read about here articulated well by Amir, via Madrona Digital, as well as here, from Dr. Earl Geddes. I expect that with symmetrical placement, as found to be optimal through Todd Welti's work at Harmon (see first link), a global EQ setting may be adequate, while Dr. Geddes' approach relies on specialized EQ for each channel. My approach to prewiring allows for both approaches in optimization.

The best reasonable solution Welti found was four subs at wall midpoints. You and I would have trouble with that since we have doors at wall midpoints. The next best is four corners. Some pro calibrators (most?) would discourage corner placement, since it excites all the room modes. Still, it minimizes seat-to-seat variation, so EQ can be brought in to get everyone great bass. My plan then, is to deliver four equalized sub signals - one to each corner. My personal choice is speaker wire (large gauge) to those locations and rack-mounted amplification. I also will be routing XLR cables to those locations for line level signals, should I decide in the future to add powered subs. I'll give myself enough slack to move any of the subs at least three or four feet to get them out toward 1/4 wall points. For EQ, I'll get a miniDSP (four outputs, only need one input) and put it between the subwoofer output on my AVR at the amps.

So how you wire it is less important that making sure you have the EQ options you (may) need. I would not rely on an AVR to calibrate my subs, though I understand that Audyssey XT32 with subEQ is pretty good. No matter the quality, no AVR I know of can deliver more than 2 EQed sub signals, so I figure bypass the AVR's EQ and go outboard entirely. I'm not familiar with the EQ options available within the amp of a PL-200, but it looks like it has pass-through, which is a plus. I have a strong feeling that good LF performance is crucial for deeply satisfying HT, so the extra costs of the wires is minimal. I've already forgotten how much it costs. smile.gif
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post #12 of 14 Old 09-08-2013, 10:10 AM
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Quote:
Originally Posted by HopefulFred View Post

Clearly, Nightlord would do it differently from what I'm planning.

Yes, as I'm fully convinced we can tell direction of first arriving wave front and that the best sound in the sub range is when we get as little of something later as possible. Thus, hard wall up front and massive dampening in the back. If one does not believe in this, then optimizations will get different.

If you put subs further back, then the need of dampening the front increases, otherwise we get a stronger late reflex there, and then we damage the originating front subwoofer signal in the process...

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post #13 of 14 Old 09-08-2013, 11:34 AM - Thread Starter
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Thanks for the opinions guys,

Looks like I have a lot of reading to do. But I have come a long way in determining my options. I will have some time before I am ready to close the walls up. I could wire for the 4 corners and that would cover, 4 on the front wall if I connected the extra two with daisey chain. I plan on running way too much wire and conduit, so I hope I will have all of the wiring options I need down the road.

How would I get a Line out for the buttkickers, how many times can I split the line out? At this point should I consider1 amp for subs and 1 amp for buttkickers each with its own lineout from my receiver?

Nightlord,

Would you daisey chain the 4 front subs?

-Jay
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post #14 of 14 Old 09-08-2013, 01:33 PM
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Quote:
Originally Posted by jasonsmithpharmd View Post

Nightlord,

Would you daisey chain the 4 front subs?

-Jay

If they're active it's only extra cable hassle. If they are passive, then it would be up to their impedance what to do. Myself I run stereo subs, so with four it would be two channels with modules in parallell and then an amp that would be ok with 4 ohms assuming they were 8 ohms each to begin with. My 8 subs in the livingroom are run two and two in parallel using 2 stereo amps. They're actually 6 ohms, but the amps are ok with 3 so I'm ok there.

For my theater I will be running two channels with three bass modules in parallell, but they are 9 ohms each, so it will be ok for that amp too.

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