Help with placement for Bass Traps / Treatment in unusually shaped room - AVS Forum
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Old 09-18-2012, 12:26 PM - Thread Starter
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OK - Now that I've got most of my equipment in place I'd like to do some treatment of the room to minimize reverb, smooth frequency response, etc. I'm looking for general advice on placement for bass traps and wall (and ceiling) treatments. After reading many articles on here (include Ethan's), it seems the first areas to attack are the corners. Trouble is, in my room the front corners are dominated by my subwoofers. The rear of my room is way far behind the listening position as well. In the front, I could ideally place traps from the wall/ceiling area, but not being able to cover from the floor (actually speakers stands in my room) to mid wall could potentially be an issue. In the rear of the room, since it's so far away from the LP, would it be worth treating these areas?

Limitations: I've tried changing placement with the subs, and they only seem to like this area as far as frequency response. As soon as I move them away from the corners the sound seems to diminish. frown.gif

Also, the right side of my room has windows right in the areas of first reflections - any ideas as to how to deal with these?

Viewing area:
13x22x8

Total room size (front to back wall)
13 x 22 and bar area 19x22
Total (average width) dimensions: 16x44x8

Ceiling: Drop ceiling - currently uninsulated, but I'd like to get some R38 (or similar) up there.

I really don't have terrible frequency response, but I'd like to improve as much as possible. I want to get as much as of my room as I can and really don't mind adding as much as I need to. I'll be making it all myself likely using OC 705 or 703.

My question:Where is my money/time best spent? What should I prioritize? I'm thinking the front and left corners from wall to ceiling should be the first priority. Then maybe fill the corner stands/shelves with some insulation under the subs. Then wall treatments when possible at first reflection points. Thoughts?

Now, onto my room:

Front area:
Theaterpictures143.jpg

View of listening position:
Theaterpictures142.jpg

View from back of the room by the bar:
Theaterpictures135.jpg

A closer look at the front corners without the subs/speakers on them:
photo2.jpg

Back hallway that shows the rear of the room behind the bar:
Theaterpictures134.jpg


Thanks in advance for all your help!

Andrew
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Old 09-19-2012, 12:02 PM
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It looks like there's plenty of corner area above your subs you could cover. There are also wall-ceiling and wall-floor corners, and the bottom of the front wall is a good place for traps because it's out of the way of foot traffic. You also need to treat all the reflection points. The rear wall is another source of damaging reflections. So "all of the above" is the best answer. biggrin.gif

--Ethan

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Old 09-19-2012, 01:54 PM - Thread Starter
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Originally Posted by Ethan Winer View Post

It looks like there's plenty of corner area above your subs you could cover. There are also wall-ceiling and wall-floor corners, and the bottom of the front wall is a good place for traps because it's out of the way of foot traffic. You also need to treat all the reflection points. The rear wall is another source of damaging reflections. So "all of the above" is the best answer. biggrin.gif
--Ethan

Ah, just the man I wanted to hear from. biggrin.gif Thanks for the reply!

Just to clarify - when you say the front wall for traps - does this area include under and to the sides of the screen or strictly top corners above the subs?

Thanks again,
Andrew
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Old 09-20-2012, 11:16 AM
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^^^ Above and below, fully from left to right if possible. Rectangle rooms have 12 corners, and all 12 are valid for bass trapping. Some people don't like the look of traps straddling the wall-floor junction, but in the front of the room they're out of the way, and they're just as effective as any other corners.

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Old 09-20-2012, 12:23 PM
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The first step is to define your problem. What does you omnimic tell you about bass response in the prime seating area? In the secondary seating area?

How does music sound to your ears? I'm not talking how the speakers sound, but rather how the sound interacts with the room. Does the program envelope you? Are instruments localized or spread out? Can you point to the speakers if blindfolded, or have they disappeared into a sound field?

The idea is to fit the acoustic devices to your needs, as defined by instrumented and listening tests. The typical corner bass trap is not a bass trap, but rather a broadband absorber designed for extended low frequency absorption. A true bass trap removes bass and little else, and they are a bit more complicated than a sheet of OC703 hung across a corner. Then again, perhaps some additional broadband absorption would be beneficial. How can you tell without assessing the problem?

And here's the Master resource for all this....
http://andrealbino.wikispaces.com/file/view/Master+Handbook+of+Acoustics+-+5th+Edition+-+F.+Alton+Everest,+Ken+C.+Pohlmann.pdf

Have fun,
Frank
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Old 09-21-2012, 06:39 AM - Thread Starter
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Thanks for the help guys. We have a vacation planned for next week but will start planning and building the traps as soon as I return.

As for the 'sound' - honestly, it sounds really good as of now. Dialog is very clear, bass seems tight, and the speakers and subs can't really be placed with my eyes closed. With that said, the FR of the room needs a bit of work. Audyssey helped a LOT, but as you know that's only focusing improvement in the primary LP and that area alone.

Frank - I actually just purchased that book on amazon last week. Didn't realize there was a PDF out there, thank you though. I was planning on plowing through alot of it during our plane ride on Sunday.

Here is my omnimic sweep for the LFE region - post audyssey, averaged over 8 positions:

capsfrontssetassmall20hztune.jpg
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Old 09-21-2012, 07:00 AM
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Hi Gorilla,

 

Without ETC measurements to define specific surgical treatments, I woul offer the following which one would be hard pressed to go wrong with....

 

  • Bass trap any and all corners you can aesthetically accomodate.
  • Broadband absorbers on side wall reflection points, your L and R are so close to the wall those are sure to be early high gain offenders.
  • Broadband absorption behind center channel, close proximity to front wall.
  • Treat to taste(with lack of measurement) all primary reflection points using the mirror method.  Sides(two each wall), Ceiling, and back wall.  If room becomes "too dead" I would remove those in reverse order.
  • Make sure treatments are true broadband absorbers
  • If HF energy is over attenuated you can cover the corner traps with a substrate which reflects some of the HF energy back into the room, normally not exceeding 1/4" hardboard thickness or it begins to perform like a rigid barrier.
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Old 09-21-2012, 07:55 AM
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Originally Posted by NicksHitachi View Post


* Make sure treatments are true broadband absorbers

Some time spent with the porous absorber calculator trying to get reasonably flat absorbtion down to 50 Hz may be a sobering experience:

http://www.whealy.com/acoustics/Porous.html



BTW the calculations for thickness in inches were in error. The correct thickness of each of the 2 layers is more like 15.26 inches.
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Old 09-21-2012, 09:00 AM
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Quote:
Originally Posted by arnyk View Post

Quote:
Originally Posted by NicksHitachi View Post


* Make sure treatments are true broadband absorbers

Some time spent with the porous absorber calculator trying to get reasonably flat absorbtion down to 50 Hz may be a sobering experience:

http://www.whealy.com/acoustics/Porous.html



BTW the calculations for thickness in inches were in error. The correct thickness of each of the 2 layers is more like 15.26 inches.

I consider anything below 100Hz omnidirectional and best treated with bass trapping.  By broadband absorption I was/am referring to wall treatments for specular reflections, no sobering needed.

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Old 09-24-2012, 06:56 AM
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AFAIK, sounds start radiating omnidirectionally when the wavelength is long enough versus the dimensions of the speaker cabinet. I can't recall if it's half a wavelength or what. But 100 hz has an 11 foot wavelength. Omnidirectional radiation is going to occur well above 100 Hz with most normal speakers. Just based on my experiences here, I think the relevant transition is the Schroeder frequency, which is room dependent and relates to when total SPL is not dominated by refelctions but by pressure phenomena, I think. No sense putting a 3 foot deep absorber at the "first reflection point" for bass when it won't be as effective as corner traps becasue of how rooms behave.
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Old 09-24-2012, 08:01 AM
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Originally Posted by JHAz View Post

AFAIK, sounds start radiating omnidirectionally when the wavelength is long enough versus the dimensions of the speaker cabinet. I can't recall if it's half a wavelength or what. But 100 hz has an 11 foot wavelength. Omnidirectional radiation is going to occur well above 100 Hz with most normal speakers. Just based on my experiences here, I think the relevant transition is the Schroeder frequency, which is room dependent and relates to when total SPL is not dominated by refelctions but by pressure phenomena, I think. No sense putting a 3 foot deep absorber at the "first reflection point" for bass when it won't be as effective as corner traps becasue of how rooms behave.

Yes speakers begin to radiate omni above 100Hz.  Omni radiation increases progressively with inverse proportion to frequency and has no set transistion frequency.  Speaker Baffle size, ported/sealed alignment, spacing from boundaries, driver size,  etc. affect this.  My 100Hz comment was a point at which I feel everyone will agree omni radiation dominates in most rooms.  I made reference to this with the previous comment to treat behind the center due to its close proximity to wall to help attenuate the "back wave" or Allison effect.

 

Most residiential rooms have a schroeder freq between 100-200Hz so omni domination should be agreeable at 100Hz, however you may freely interchange 200Hz if you wish.

 

"...No sense putting a 3 foot deep absorber at the "first reflection point" for bass..."

 

Yes, thats my point.

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Old 09-27-2012, 01:04 PM
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Originally Posted by Gorilla83 View Post

...- honestly, it sounds really good as of now. Dialog is very clear, bass seems tight, and the speakers and subs can't really be placed with my eyes closed. ...Here is my omnimic sweep for the LFE region - post audyssey, averaged over 8 positions...

If it sounds good... Everest is an interesting read as long as you can get through all the intro stuff in the front, which you can't skip as it forms the basis for the rest of the book.

+/-5dB below ~68Hz, in-room isn't bad, but this is an average across 8 positions. What do the 8 individual positions look like? That's what you're really trying to opimize, so everyone gets a good-sounding seat. Also please post a "silent" measurement, so we have some idea of the noise floor; much of this response could be your furnace. Infrasonics are not audible, by definition, so you can't trust your ears to tell if it's quiet.

Have fun,
Frank
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