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post #1 of 15 Old 10-01-2018, 06:28 PM - Thread Starter
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Acoustic Experts: Recommended Length For This Room?

Hi guys, I'm converting part of a long storage room into a media room. I'm loosely familiar with the idea of desired ratios as they pertain to acoustics and am wondering what you guys would recommend. I'm attaching a rough diagram of the room (drawing is obviously not to scale). It has vaulted ceilings, so I'm assuming I should consider the average height and width with regards to the desired ratios? Not exactly sure how to calculate the average, maybe 75" H and 135" W? Though my grade school diagram looks slightly lopsided, the actual room is symmetrical, the 96" H and 80"W ceiling portion is centered and the vaulted portions are both the same lengths. The height and width are what they are, but the length can be adjusted to anything within reason (though preferably somewhere between ~14' and 19'). Thanks for any and all help.

Diagram: https://imgur.com/a/LfRWQCI



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post #2 of 15 Old 10-02-2018, 08:43 AM
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Theater dimensions are mostly concerned with distance between parallel surfaces. The parallel walls create resonant frequncies. You want to avoid having the room length, width and height from being equal or multiples of each other. That can create a stacking of standing sound waves at the same frequency resulting in very uneven room frequency response.

Long answer why not to not use the average room height or width, measure between parallel surfaces.
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post #3 of 15 Old 10-02-2018, 09:38 AM - Thread Starter
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Quote:
Originally Posted by BIGmouthinDC View Post
Theater dimensions are mostly concerned with distance between parallel surfaces. The parallel walls create resonat frequncies. You want to avoid having the room length, width and height from being equal or multiples of each other. That can create a stacking of standing sound waves at the same frequency resulting in very uneven room frequency response.

Long answer why not to not use the average room height or width, measure between parallel surfaces.

Thanks for the response, so you're saying I should consider the room to be 8' H and 15' 5" W as those are the parallel surfaces? Is there any particular length you would recommend?
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post #4 of 15 Old 10-02-2018, 12:00 PM
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What do you need to fit?
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post #5 of 15 Old 10-02-2018, 01:41 PM
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Quote:
Originally Posted by Orta View Post
The height and width are what they are, but the length can be adjusted to anything within reason (though preferably somewhere between ~14' and 19').
The closer you can get the room length to 19', the better. Your 15'5" room width will resonate (peaks & nulls) at certain problem frequencies: 37Hz, 73Hz, 110Hz, etc. The 19' length will resonate at 30Hz, 59Hz, 89Hz, etc. Those numbers aren't close, so the problem frequencies for peaks & nulls in your room will be well separated (easier to detect, easier to EQ).

Also, don't bother calculating average height. Instead, try to keep the listeners' ears roughly within the 80" wide flat section of the ceiling. BTW, how many seats were you planning on having?

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post #6 of 15 Old 10-02-2018, 03:28 PM - Thread Starter
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Quote:
Originally Posted by BIGmouthinDC View Post
What do you need to fit?
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Originally Posted by sdurani View Post
The closer you can get the room length to 19', the better. Your 15'5" room width will resonate (peaks & nulls) at certain problem frequencies: 37Hz, 73Hz, 110Hz, etc. The 19' length will resonate at 30Hz, 59Hz, 89Hz, etc. Those numbers aren't close, so the problem frequencies for peaks & nulls in your room will be well separated (easier to detect, easier to EQ).

Also, don't bother calculating average height. Instead, try to keep the listeners' ears roughly within the 80" wide flat section of the ceiling. BTW, how many seats were you planning on having?

Thanks for the help guys, don't have the budget for anything elaborate unfortunately, just a 3 seat row of those home theater recliners centered, an ~70" wide TV stand, subwoofer(s), and speakers/stands (2 towers, 4 books, 4 ceiling atmos). Room will be carpeted, and I do intend to do some basic GIK treatments. If you guys could point me to treatment pieces that would make biggest bang for the littlest buck (the 80/20 rule in other words) for this room, I'd greatly appreciate it.
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post #7 of 15 Old 10-02-2018, 04:00 PM
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Since your subwoofers are going to be handling the low frequencies, no need for towers. Instead, consider getting bookshelf speakers all the way around (7 of the same speakers would help with consistency as sounds pan from the front soundstage to the surround field). Simple start for treatments is broadband absorption across the front wall, especially between the L/C/R speakers. This will prevent reflections from bouncing off the front wall and muddying the critical front soundstage. Same with the middle of the back wall (directly opposite the Centre speaker). You can add more treatment to other locations as you go. No need to do it all at once. How big a screen were you planning?

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post #8 of 15 Old 10-02-2018, 07:27 PM - Thread Starter
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Quote:
Originally Posted by sdurani View Post
Since your subwoofers are going to be handling the low frequencies, no need for towers. Instead, consider getting bookshelf speakers all the way around (7 of the same speakers would help with consistency as sounds pan from the front soundstage to the surround field). Simple start for treatments is broadband absorption across the front wall, especially between the L/C/R speakers. This will prevent reflections from bouncing off the front wall and muddying the critical front soundstage. Same with the middle of the back wall (directly opposite the Centre speaker). You can add more treatment to other locations as you go. No need to do it all at once. How big a screen were you planning?

65" for now, 77" maybe next year if HDMI 2.1 is fully integrated and they're a little more affordable. I have a 92" Dalite High Power drop down and have pre-wired for a ceiling mount FPJ, but I don't want anymore lamp based projectors and anything that may compete with an OLED is definitely out of budget. So I should lead with a pure absorber, or one of the diffuser/absorber combos?
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post #9 of 15 Old 10-03-2018, 02:05 AM
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Quote:
Originally Posted by Orta View Post
... I do intend to do some basic GIK treatments. If you guys could point me to treatment pieces that would make biggest bang for the littlest buck (the 80/20 rule in other words) for this room, I'd greatly appreciate it.
With a limited budget the best is to make your own acoustic panels and bass traps. You just need some wood, rockwool or fiberglass, and fabric. Much cheaper than buying the panels, and just as effective. And as a bonus you can make them exactly the size you need.
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post #10 of 15 Old 10-03-2018, 01:33 PM
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Originally Posted by Orta View Post
So I should lead with a pure absorber, or one of the diffuser/absorber combos?
I would stick with absorption on the front wall and middle of the back wall. Make sure the absorbers are thick enough (4" to 6") to be broadband. Reflections from those directions don't really add much.

By comparison, lateral (sideways moving) sound is what gives us the sense of spaciousness and envelopment. So the side walls are where you can experiment with combo panels to find out what kind of sound you prefer. But that can take time. Hence the suggestion to start simple by addressing the front & back walls first.

Seating distance should be based on screen size, so that you end up with a viewing angle that is comfortable & immersive. Acoustically, you're better off placing the seating listeners' ears at one of the odd (thirds, fifths) divisions of room length, since resonant frequencies have nulls at even divisions of room length.

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post #11 of 15 Old 10-04-2018, 11:03 PM - Thread Starter
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Originally Posted by Silva741 View Post
With a limited budget the best is to make your own acoustic panels and bass traps. You just need some wood, rockwool or fiberglass, and fabric. Much cheaper than buying the panels, and just as effective. And as a bonus you can make them exactly the size you need.

Thank you for the suggestion, for the absorption panels that is what I was planning, but was probably going to buy the combo panels. Is the Rockwool insulation Home Depot and such sells sufficient, or does it need to be a particular type?



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Originally Posted by sdurani View Post
I would stick with absorption on the front wall and middle of the back wall. Make sure the absorbers are thick enough (4" to 6") to be broadband. Reflections from those directions don't really add much.

By comparison, lateral (sideways moving) sound is what gives us the sense of spaciousness and envelopment. So the side walls are where you can experiment with combo panels to find out what kind of sound you prefer. But that can take time. Hence the suggestion to start simple by addressing the front & back walls first.

Seating distance should be based on screen size, so that you end up with a viewing angle that is comfortable & immersive. Acoustically, you're better off placing the seating listeners' ears at one of the odd (thirds, fifths) divisions of room length, since resonant frequencies have nulls at even divisions of room length.

Yes I was aiming for around 6' viewing distance given the requirements of 4K and now 8K on the immediate horizon. Would the best placement be the 3/5th's location (assuming a 19' room, I think that'd be around 11.4') and pulling the TV and front speakers off the front wall 5.5' or so? They are rear ported speakers. Or would 2/5ths be best (7.6' back), with the TV and speakers 1.5' or so off the front wall and an around 10' empty space behind the listening position?

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post #12 of 15 Old 10-05-2018, 02:50 AM
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Thank you for the suggestion, for the absorption panels that is what I was planning, but was probably going to buy the combo panels. Is the Rockwool insulation Home Depot and such sells sufficient, or does it need to be a particular type?
It needs to be at particular type, yes. Well, ideally at least, to get the maximum performance possible. You can check that comparison table here...
http://www.bobgolds.com/AbsorptionCoefficients.htm

Owens Corning 703 and 705 is a popular choice, but there are several other that perform equally well.
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post #13 of 15 Old 10-05-2018, 03:36 AM
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Why not 2 storage spaces, and keep all the headroom for the room? Some small soffits could push the end walls back
further, at the expense of storage.

One of those walls could simply be an acoustically transparent wall and AT screen. Even be retrofitted easily, down the road, as a
projection upgrade possibility. A 77" HDR tv will have a better picture, but an AT screen and pj with but 3 seats, means that 77"
might be as little as a quarter the viewing real estate. The AT screen also hides speakers/subs/bass trapping, and the soffits could
represent cable runs to a back wall, in-wall av rack.

The extra volume of a larger space is also going to offer smoother audio response. I personally would looking at just how little
storage I can get away with, to better the theater space.
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post #14 of 15 Old 10-05-2018, 08:27 AM
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Quote:
Originally Posted by Orta View Post
Would the best placement be the 3/5th's location (assuming a 19' room, I think that'd be around 11.4') and pulling the TV and front speakers off the front wall 5.5' or so? They are rear ported speakers. Or would 2/5ths be best (7.6' back), with the TV and speakers 1.5' or so off the front wall and an around 10' empty space behind the listening position?
For your current TV size, I would go with 2/5ths location. IF you get a big screen and projector at some point, move the seating to the 3/5ths or 2/3rd location. If you cover the front wall with absorption, you don't need to pull the speakers 1.5' out from the front wall (the port needs a couple inches behind it).

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post #15 of 15 Old 10-05-2018, 11:03 AM
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Since a picture is worth ...well, you know. Here is a graph of the of the length modes for a 19-foot long room:



Each problem frequency (30Hz, 59Hz, 89Hz, 119Hz) is highlighted in a different colour. Any number that doesn't have "Hz" after it is showing the distance from the front of the room.

Notice that all the nulls are at even divisions of room length (half, quarters, sixths, eights). Worst is at the midpoint of room length, where it's all peaks and nulls. Hence the suggestion to place the listeners' ears at one of the odd divisions of room length.
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