Fixing my riser: Broadband bass trap and from 5 ft 6 inch to 7 ft deep - AVS Forum
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post #1 of 14 Old 01-02-2009, 01:09 PM - Thread Starter
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Well;
Like many here I have built "stuff" w/o proper knowledge, then sometimes decided to try and fix it afterward.
DIRTFT, yes I know, if we all knew then what we are aware of now....

Take my 2nd row seat riser, I built that 3 years ago, way before I even heard of HT acoustics/bass traps etc.
Not that I know much technical analysis - I have read the book "Master Handbook of Acoustics" twice, so I'm aware of the need to address it for HT usage for proper sound quality (RT60/delay time) and reducing bass "boominess". But I don't live/walk it like the experts.

I'm trying to retrofit that riser into a Broadband bass trap and also improve the walking ergonomics of it by making is 7' deep instead of 5'6" deep.
7' deep will allow the rear seats some more space from the rear wall, and people can walk in front of the seats reclined w/o tripping over them while still on the riser. (yes, that happened already, so before carpeting I need to fix that)

I've read these threads:
pls review riser bass trap
Riser as Bass Trap- Still in or out?
Riser as Broad Band Bass Trap

And saw this article Dennis Erskine wrote and someone posted links too:


The marked up picts is my plan:
The riser is currently filled with pink insulation, this morning I removed the top.
Upon all this being done I will put 30lb roof felt under the complete unit / floor and also add a 2nd sheet of 3/4" OSB on top of the 1st sheet, separated by 30lb roof felt.
Yes, my 2 x 8's run the width of the room, not the length of the room - not correct to the article Dennis wrote, so now I have to cut slots in them for the bass to breathe and be absorbed by the insulation, or just the rear chamber is "active".
The volume of my chamber will then be not individual like the riser in Dennis's article, rather much more. I sure there is mathematical relationship to volume and frequency, just like 1/4 wavelength is a "magic" length when you want to address some frequency response. (It's been 25 years since my MSME and vibration analysis).
I'm open to comments/suggestions, and appreciate any/all.
I have speaker grille cloth to put under the vents so the pink is not visible/getting into living space.


 

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post #2 of 14 Old 01-03-2009, 02:49 AM - Thread Starter
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After sleeping, I've decided slight change, below is modify #1.
I'm cutting the slots now, after some coffee, it's 5:30am here in Michigan.

Break up the riser into 3 separate zones of volume, to give different peaks to the bass broadband absorption.

I've got REW and laptop/Behringer 8000 Mic, I'll try taking some nearfield speaker and seat charts prior to cutting this to document changes.

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post #3 of 14 Old 01-03-2009, 03:42 AM
 
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With REW you should be able to find good locations for the vents. I might suggest trying to keep slightly to the sides of the middle of the room then working your way inward.
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post #4 of 14 Old 01-03-2009, 07:49 AM - Thread Starter
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Covered equipment to keep out sawdust, and I thought the sawdust stage was past for me

My wife thinks I've gone overboard with this one, she's been quite supportive so far with the whole HT.

I'm telling here "but honey, we want flat response and the correct delay time for the 'real' HT experience", she just shakes her head and accepts what I'm doing.
The kids want to get involved but too much powertools.

In process pict, I have more slots to cut and the front 18" to build then.
Nice Sat day here in SE Mich, so my wood is outside for now.
This should all be done by 8pm tonight.

I'll put the tops on w/o cutting and then put all the seats in their intended place, then use REW to measure current state.
Then, cut vent slots in corners and 3 along back wall, one RH side wall and 2 LH side wall, take REW measurement to see difference, and see if more vent slots should be cut.
Trial and error approach to bass acoustics.
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post #5 of 14 Old 01-04-2009, 09:39 PM - Thread Starter
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Sunday night status:


a) Slots all cut, roofing felt installed under riser:


b) Pink insulation re-installed, 1st and 2nd row 3/4" OSB installed, 1st row screw/glued to riser box 2" x 8"'s and 2" x 4" blocking , 2nd row screw/roofing felt. 2" lip all around:


Revision to vent plan:
Since my 3 bass chambers are different volume, for this "experiment"/broadband bass trap I've decided to at first just cut 2 vent holes per/chamber as some control to the analysis/effects.
(2) 4" x 14" vents will be in each corner, and 2 more 4" x 14" on the LH wall 3rd chamber. 4 cut, 2 more to go - those are Tuesday as tomorrow is my wife's b-day.



edit: I'm asking Alan to move this thread to audio theory, setup, chat forum
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post #6 of 14 Old 01-23-2009, 09:52 AM - Thread Starter
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Just follow-up:
Riser trap vents final image, I installed (14) 4" x 14" vents.
(thx Ethan Winer for advise)
(1) on RH side, (4) on LH side, and (9) along back wall.
I'll post graphs of the vents shut and open, hopefully all my effort showed some broadband absorption.

For reference info:
Room layout and seating plan view:


Side walls:


Back wall:


Needless to say my kids are asking countless questions about everything I'm doing.
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post #7 of 14 Old 01-23-2009, 09:55 AM - Thread Starter
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Multi plot taken at:
- the 2 near field positions (2" and 32" from bottom/middle speaker cone)
-4 at 2nd row seating/on the riser.
-2 at 1st row seating
Room modes show themselves, I changed the scale 0-200hz since that's all I measured.



Riser broadband bass study, same location, one with the (14) vents open, one closed:
The blue (vent open) is so much smoother than the green (vent closed).



My first waterfalls, was interested if the riser broadband bass helped with delay time or smoother response, looks that way to me.
Vents Open:


Vents closed:



A few more REW graphs from 1st row position 7, showing the riser broadband bass trap is "working".
Similar to 2nd row position 2 open/closed vent graphs.

Red is vents "closed" and purple is vents "open"


Waterfall plots (I can't get these to plot past 90hz for some reason, yet the measured plots go to 200 hz)
Red vents shut:



Purple vents open, shorter decay time in both rows!
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post #8 of 14 Old 01-23-2009, 09:58 AM - Thread Starter
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I've never seen any before/after of riser broadband bass trap analysis, hopefully I'm doing this right.
Feedback welcomed.

It's possible/probable even thought the vents are "closed" some air is getting by them so the results would show even more affect is 100% blocked.

btw, room is bare w/o carpeting.
Carpeting is Monday 1/26,
For me, this will be a good learning experience, going from:
-no absorption (can you say echo echo echo and long delay time...)
-to a broadband bass trap riser
-to carpeting
-to creating and installing side wall panels per reflection points (2" OC 703, many threads on DIY those)
Then,
-additional bass traps if needed via measurements
-possible ceiling absorption
-lastly making a DIY diffusor to go onto the back wall, if "needed" or desired to show my family/friends how far off the deep acoustic end I've dived into....

Since my walls are kinda bland/bare, once the carpet is installed I was going to use colors on the side walls acoustic panels to slightly brighten up the room (visually).
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post #9 of 14 Old 04-16-2011, 04:17 PM
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I am trying to decide wether to use Fiberglass or Roxul Safe N Sound for my bass trap.. any suggestions as towhich I should use?
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post #10 of 14 Old 09-02-2012, 03:00 PM
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Use the thickest pink stuff you can find. That will be good enough.
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post #11 of 14 Old 09-07-2012, 07:23 PM
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Quote:
Originally Posted by pgodden View Post

I am trying to decide wether to use Fiberglass or Roxul Safe N Sound for my bass trap.. any suggestions as towhich I should use?

+
Quote:
Originally Posted by cdika17 View Post

Use the thickest pink stuff you can find. That will be good enough.

I have some ROXUL left from a wall soundproofing project. So, cost is not be the issue, would you RECOMMEND the "pink stuff" to use as a bass trap?

The reason I ask, I know the ROXUL has a higher price point.
I'm afraid because higher price, it has a "bad rap".
After with working with the stuff, I can say first hand... IT is definitely denser than fiberglass.

Also, almost every Home Theater demo room I have been in, they will use ROXUL (or other brands) for their custom bass traps.

Not being confrontational, just wanted to make sure it is not a bias, but TRULY a non-inferiority comparison.
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post #12 of 14 Old 09-07-2012, 10:39 PM
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Quote:
Originally Posted by Johnny14o View Post

I have some ROXUL left from a wall soundproofing project. So, cost is not be the issue, would you RECOMMEND the "pink stuff" to use as a bass trap?
The reason I ask, I know the ROXUL has a higher price point.
I'm afraid because higher price, it has a "bad rap".
After with working with the stuff, I can say first hand... IT is definitely denser than fiberglass.

The difference between the two materials is negligible. Even though they are very different materials, at a depth of around 8-12" they will both perform closely to each other. The Roxul has a small advantage of going a little deeper in frequency but not by a ton. If you only had 6" or less, though, the Roxul would be better. (This is all specifically in reference to Roxul Safe N Sound, btw). So, if you already have the Roxul, go for it.
Quote:
Originally Posted by Johnny14o View Post

Also, almost every Home Theater demo room I have been in, they will use ROXUL (or other brands) for their custom bass traps.

Roxul is still quite fairly priced, and like I said above, works better for smaller depths.

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post #13 of 14 Old 09-26-2012, 06:51 PM
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Do you carpet around the vents? Any "after" pics? Safe to safe that furniture in front of the front vents won't impede the flow of the bass entering the riser?
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post #14 of 14 Old 09-26-2012, 10:10 PM
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Quote:
Originally Posted by aviantheater View Post

Safe to safe that furniture in front of the front vents won't impede the flow of the bass entering the riser?

This would depend on what furniture and how close it is to the vents. Low frequencies wrap around objects quite easily most of the time, so furniture shouldn't impede too much - its when you get to the higher bass (200 Hz or so) that I see this being a possible problem (likely not much though).

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