Framing Went Up Yesterday - I Think I Made A Big Mistake - AVS Forum
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post #1 of 20 Old 11-08-2007, 06:19 AM - Thread Starter
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Framing went up yesterday in my basement for my new HT...

Everything looks great except for a last minute adjustment that was made. In order to get a small equipment room under my basement stairs, the rear wall of my future HT was moved in to accommodate. The contractor moved it in further than I anticipated and the result is a room with dimensions of 15-9 1/2 x 15-2, dangerously square according to everything I've read here. The room will be treated with linacoustic later, but I don't want to be fighting an uphill battle, and I'm not overly knowledgeable on acoustics.

A lot of work (as in several days, at least for me) would be involved, but I could probably ditch the closet and move back the rear wall and gain a foot back. As I said this would be a lot of work for me as the soffits and doorways are all up as well. But, this would make the room 16-9 x 15-2. Do I need to do this? I could also bring the side walls in some (easier), but I hate to give up the space.



I appreciate any thoughts or suggestions.

Chris
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post #2 of 20 Old 11-08-2007, 07:42 AM
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I am by no means an expert on accoustics, but if it were me I would just leave it as it. I dont think 1 foot is going to make or break it especially if you are going to treat the room properly. If you were adjusting the size in order to accomidate larger seating or a screen than I would say that would be worth it (and I did do that 16" to get larger seats and better alignment with the screen), but you seem to have a pretty good width to your room already.

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post #3 of 20 Old 11-08-2007, 07:43 AM
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You didn't state you ceiling height, but I through it into a mode calculator at 8ft and it isn't too pretty. You may want to play with one (I am guessing there are some spreadsheets around the forums), or chat with someone here who does design.

Curt


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post #4 of 20 Old 11-08-2007, 07:53 AM - Thread Starter
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Thanks for the replies.

Yes, the room height is close to 8' - probably around 7' 10" once drywall is up. Screen size I'm hoping for ~ 104-106".

Anyone have a link to a mode calculator?
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post #5 of 20 Old 11-08-2007, 08:10 AM
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Rather than just moving in the walls, could you add some built-in storage for movies,pillows/throws, maybe even a little built-in cooler?
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post #6 of 20 Old 11-08-2007, 08:11 AM
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One of the charts used to look at performance in small room is a bonello chart. Here are some quick numbers for your room. I simply took an inch away from your dimensions for finishing, since I didn't know your actual plan.

All rooms have modes, and the point is to keep them as smooth as possible, so the idea of the chart is a smooth graph going up.

As you can see, they are both a bit rough, but the 16' 9" (finished) gets you a bit better. Also, while squares are generally "bad", there is more to it and you can't forget about the ceiling as well. My space ended up much more square than originally planned, but still turned out acoustically pretty good, 23' 5"x25'6" and the chart is also below.

Again, it is all a big can of worms and it comes down to what performace you demand and what you want to spend.

Best of luck,





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post #7 of 20 Old 11-08-2007, 09:20 AM
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Which utility did you use to get those charts? Thanks.

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post #8 of 20 Old 11-08-2007, 10:41 AM
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One thing your might want to consider are your French doors. They probably won't stop the lower frequencies. So, at lower frequencies the lobby may have to be considered part of the room.

Human perception is not a direct consequence of reality, but rather an act of imagination. - Michael Faraday
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post #9 of 20 Old 11-08-2007, 11:36 AM - Thread Starter
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Tradewinds, I found a similar utility at this link:

bobgolds.com/Mode/RoomModes.htm

Just type in the numbers, and the chart appears on the bottom right.

Using that, I find that just a couple inches either way makes a noticeable difference in the smoothness on the chart.

Mdputman, you think the frequencies will still get thru if it's a solid-core door? That was the plan... the doors will be 42" (~21 each) total width.
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post #10 of 20 Old 11-08-2007, 11:49 AM - Thread Starter
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By the way, does everyone agree this chart looks better than the initial? It's basically 15-10 x 15-0 x 7-10 or 7-10.5 . I should be able to get those dimensions.

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post #11 of 20 Old 11-08-2007, 12:11 PM
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Thanks....nice calculator. My chart came out decent. I think.


LL

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post #12 of 20 Old 11-08-2007, 12:25 PM - Thread Starter
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That does look good - especially next to my choppy chart What are your dimensions?

Strange thing is everything I type in between 17 feet and 15-9 (depth) comes back choppy. I'm thinking it's the ceiling - if there was any way to get it above 8'2 or so, the charts look much smoother.
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post #13 of 20 Old 11-08-2007, 12:28 PM
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Focus on your seating array requirements and viewing distance screen size.

It is a lot worse theater to cut it down to be rectangle from a square - but you are asking if you should build out to go from a square to a rectangle - my answer is yes - but not because of modes - but because of needing the HT space. My answer would even be the same if building it out meant going from a rectangle to a square!

You are not building a reverb room with perfect modal distribution above 125Hz. You are building an HT with horrible modal distribution below 125Hz - regardless of your dimensions. Homes simply are not big enough to be able to shift the chart so there are lots of modes in the subwoofer region.

That is what the charts are telling you - that there are not enough modes in the subwoofer region. Ocassionally you get one or two in a third octave - then the next third octave has none. Once you realize you can deal with that with other methods - like seat(s)/sub(s) positions and pEQ - then you will realize it is not worth chopping a room up.

In your case - a longer room is always better for a theater. That extra foot may make the difference in terms of being able to fit the seats you want in to the HT - without having to force people to sit on the back wall. Which is bad regardless of what aspect the room is.

Certainly you can consider room aspects during the design phase, balancing modal design vs. seating vs. screen design - but not sure it is worth worrying about the modes anymore once the walls are up. Worrying about fitting the seats you already bought into the smaller theater - that is worth ripping out walls over. Then worry about modes again when your HT is finished and you are ready to start tweaking placements/treatments

While TrikinCurts plot looks smoother than yours - do not confuse that plot with a frequency response. It is a modal distribution chart. Your chart says to expect problems in the 40/80Hz third octaves - but say you have wood walls while TrikinCurts walls are a concrete bunker - in reality his 40/80Hz could be worse than yours! The fact is in all these rooms the lowest octave has no modal support at all - causing uneven frequency response once you hit the fundamental mode which will ring and boom in comparison - regardless of how smooth the Bonello chart is.
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post #14 of 20 Old 11-08-2007, 12:35 PM
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Quote:
Originally Posted by Christopher_C View Post

That does look good - especially next to my choppy chart What are your dimensions?

Strange thing is everything I type in between 17 feet and 15-9 (depth) comes back choppy. I'm thinking it's the ceiling - if there was any way to get it above 8'2 or so, the charts look much smoother.

L - 16' 6"
W - 13'
H - 9' 11"

This obviously does not take into account I have 6 colums and a star-ceiling box (8'x12' in the center of the ceiling) as well as treatments on the wall and the bulkiness of the seats and the high pile carpet and riser and stage etc.

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post #15 of 20 Old 11-08-2007, 01:03 PM - Thread Starter
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I'd love to have those 10' ceilings, TW. I plan on having only 1 row of seating, a stage w/ sand, 4 columns, and a 104" diagonal screen. Not sure if a 15-9 room will accommodate all this...

Thanks for the advice krasmuzik. I agree the space is worth knocking a wall down - I love the equipment room, but definitely prefer getting more HT space.

I think I will go ahead and move the wall back a foot. If anyone has any suggestions on how to do this without destroying everything, let me know - a soffit and 2 doorways are part of this wall.

I'm assuming crowbar for the floor 2x4's... is there anything that could get in between the lumber (and floor / ceiling) and cut thru nails? If I could keep it all in tact and just move it back...
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post #16 of 20 Old 11-08-2007, 02:03 PM
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Quote:
Originally Posted by Christopher_C View Post


I'm assuming crowbar for the floor 2x4's... is there anything that could get in between the lumber (and floor / ceiling) and cut thru nails? If I could keep it all in tact and just move it back...

A recipricating/Skil (sp?) saw with a long (12") blade should be able to slide between the floor and the bottom plate.

I had a few goofs when framing as well.

.................
Michael

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post #17 of 20 Old 11-08-2007, 04:45 PM - Thread Starter
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Thanks Cathan. I flipped thru your construction thread a bit, will be following it now - you're a couple stages ahead of me, looks good so far!
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post #18 of 20 Old 11-09-2007, 07:34 AM
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I'm sure you'll pass me soon enough. Just give a holler if you need anything.

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post #19 of 20 Old 11-09-2007, 08:27 PM
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Hey Kras - I went through the same class as you (I was at Triad in the class after yours), I promise my bass response is pretty good. Actually went with the four sub approach, with Eq to fix what is left. And it isn't like I left it as concrete

Anywho - you start with a good room or you a bad one. I through out the chart because it was the easiest to show, but you there were quite a few coincident modes below 80hz, that was what raised the flags to me.

Like most problems, it can be solved with money...

Curt


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post #20 of 20 Old 11-09-2007, 09:17 PM
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Was wondering why this TrikinCurt was ringing familiar! So are you getting the basement built to meet your HAA Level 2 practical test requirements

Of course you mentioned there is more to it in your first post - which is the point I was trying to make.

Had it been one sub in the corner in a concrete vault with the head in the opposing corner - your 40Hz would blow your head off just as bad as his with his more damped walls that will minimize the modal stackup - ringing is more dependent on the boundary conditions than it is on the modal distribution. So even though the chart says it should be worse because of coincident modes - it is not - because of the walls (presumed to be different - last pic of your thread I saw was a concrete underground bunker!)

The 4 subs and pEQ will cancel out that 40Hz mode making it a non problem because it is non-existant - which of course goes against the Bonello chart which says you need more modes not less. Bonello is just one point of data (which the HAA does look at in context - as I recognized you used the HAA calculator) that points out a potential problem possibly requiring solutions and to watch out for them - but people often confuse it to mean a smoothly rising Bonello curve means smooth frequency response - and it certainly does not mean that with subwoofers in HTs. The fact is - you will have subwoofer modes in HT's that require solutions other than room aspect - which is not a solution thanks to modal sparsity - you don't have enough subwoofer modes to distribute in the first place.. Room size is a solution - as you say - bring money as you will need a huge McMansion to fit the HT that has sufficient volume to remove modal sparsity as a problem....4 subs is much much cheaper than a McMansion HT.

Nobody should be ripping down their theater just because they discovered the Bonello curve and room ratios said it was not good. As the guy from Harman International said in his CEDIA seminars - he can make a concrete cube sound good - but that is because he knows what he is doing!
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