Adobe walls preclude corner bass trap frame touching wall

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Adobe walls preclude corner bass trap frame touching wall

Postby wicca » Wed Jul 10, 2013 9:55 pm

My first post so I hope i don't do anything wrong. Not very sophisticated about room acoustics. Have a listening room with brick floor, adobe walls and ceiling/roof of 2"X 6" T&G wood covered with 8" rigid foam insulation. Two small awning windows and two exterior grade french doors. Room is 21.5' X 15.5' X 10.3'. I have built 4 corner bass traps floor to ceiling ( 2' wide 10' tall and 4" thick ) made of outer layer 703 and inner layer of 705 for a 4" thickness with a wood frame. These helped but bass till a little muddy. I am unable to get the wood frames to touch the adobe walls as they are wavy. I made 4 floor to ceiling Superchunks 17" X 17" X 24" out of a variety of 703, 6 lb rock wool and some pink fluffy and placed these in the corners behind the corner bass traps. Bass is tighter but I still have a mid bass boom that is prominent as volume increases. My measurement skills are pretty primitive but with RS meter and Stereophile warble tones I believe that there is problem around 100 hz. The Superchunks make good contact with the corner walls but my Corner Trap being only 2' wide is even further from the walls now than before.Do I need to add more absorber material to the Superchunks? Do I need to rebuild the Corner Bass Traps to make them wider? Is there a relatively pliable material that I could place on the sides of the Corner Traps to better seal them to the corners?
I realize that there are many other variables but I would like to improve my bass absorption in the four corners before I move on to my other problems unless you all believe that I have to do it all at one time.
I appreciate the opportunity to consult with real professionals and I apologize in advance if i am making any mistakes. Joe Fagan
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Re: Adobe walls preclude corner bass trap frame touching wal

Postby bert stoltenborg » Thu Jul 11, 2013 5:43 am

Hey Joe, welcome.

Maybe add some absorption on mirror points?
A drawing of the room or some pics would help.
If you view life with the knowledge that there are no problems, only opportunities, you are a marketing manager.......this is my personal philosophy
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Re: Adobe walls preclude corner bass trap frame touching wal

Postby Bob » Thu Jul 11, 2013 3:44 pm

and what range speakers?
Specifically,
a) stereo, vs
b) 5.1 with a subwoofer you can move into nodes, and front towers with 110hz high pass filter
Last edited by Bob on Thu Jul 11, 2013 3:55 pm, edited 2 times in total.
Regards
Bob Golds
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Re: Adobe walls preclude corner bass trap frame touching wal

Postby Bob » Thu Jul 11, 2013 3:55 pm

The rule of thumb for Superchunks is that they are a great first or second step in an otherwise untreated room, but you have a ceiling covered with 8" of fluff.

I typed your room measurements into
http://www.bobgolds.com/Mode/RoomModes.htm
to look for anything strong around 100hz
- 105.1 hz (4,0,0 Axial)
As an experiment, I might try covering the rear wall with 2" to 8" of rigid insulation,
after finding some way to measure before and after,
both to find out if it had any effect (which might be doable with a sound meter and a frequency generator),
and to find out if it caused a mess (some by ear, and some by a impulse waterfall or some such)
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Re: Adobe walls preclude corner bass trap frame touching wal

Postby wicca » Fri Jul 12, 2013 2:26 am

Thank you all for responding so quickly. I will try to address the issues raised, Speakers are Magnepan 3.7s augmented with Magnepan DWMs ( small woofer panel with range of 40 hz to 200 hz ) They are on the long wall. The main speakers are 6' from the side wall to the center of the panel, they are 5' from the front wall and toed in about 2". Listener primarily for stereo sits about 11 1/2' from each speaker which puts my head about 6" from the back wall. I have rigid fiberglass panels ( 4' X 6' ) on the center of each side wall. Given the response pattern of the Magnepans absorbers in the traditional locations probably don't make much difference. But I do have 4 18" X 48" absorbers on the ceiling where the ribbon tweeter can be seen in a mirror. My description of my ceiling was incomplete and misleading. The ceiling is made up of 2" X 6" T & G boards supported by 10" diameter round logs ( vigas ) with spacing between the logs of about 20". The foam is above the T & G and serves as the roof. Behind each speaker is a 2' X 8' diffuser and in between the speakers is a 6' X 4" diffuser below a video projection screen. Each room corner has floor to ceiling Studiochunks with a 4" thick corner trap in front of the Superchunks. Each side near the ceiling/front wall are RPG Primitive Root Diffusers ( forgot their name ). Floor is about 70% covered with rugs over synthetic hair felt pad over a brick floor over sand. The back wall has two narrow awning windows near the ceiling. About 60 to 70% of the back wall is covered with quilts ( temporary ) I do have a Velodyne DD18 about 4 ft from the sidewall and about 21/2' from the back wall. It has a 40 hz 24db XO. The Magnepans run full range which ranges down to 35 to 40 hz.

My first question has to do with whether or not adding to the thickness of the Superchunks and widening the corner traps would have a useful impact on frequencies 80 to 110 hz which currently are at least 12 db above the midrange frequencies. Similarly as I can not get the wood frame around the corner absorbers to touch the sides of my adobe wall as they are quite irregular I was wondering if there was a flexible material that I could put between the wood frame and the adobe wall.As adobe walls are made of dried dirt blocks covered with plaster using fasteners to the adobe walls doesn'r work very well.

Secondly some of the midrange frequencies are depressed while the highs from 4/5K up are elevated. I previously had wideband absorbers on the back wall behind the listener but they seemed to reduce the midrange frequencies so I removed them and the midrange did increase in level somewhat but not enough. I hung the quilt to the back wall to try to tone the highs down and it helped but didn't solve the problem.

I have used a 1.5 ohm resistor on the ribbon tweeter to reduce its output ( about 3 db ) and I am using a 2 ohm 25 watt resistor on the small woofer panel to also reduce the bass output. I can increase this to 4 ohms. These are options offered by Magnepan to try to match the speaker to the rooms. I can increase the value of each . Also I can use a 1.4 mH iron core inductor to roll of the higher frequencies of the little woofer panel

Any suggestions will be appreciated. Thanks again Joe Fagan

I will try to figure out how to post a drawing of the room as I realize that the description is difficult to follow.
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Re: Adobe walls preclude corner bass trap frame touching wal

Postby wicca » Wed Jul 17, 2013 12:10 pm

Thanks to those who responded. Thus far I have decided to rebuild the corner bass traps to make them wider by so that they will almost touch the corner walls. Where the adobe walls are so irregular that there are significant gaps I plan to use " hair felt" carpet pad to fill in the gaps ( hopefully this will restore the absorption around 100 hz). I have tried reversing the speaker leads on the small Magnepan woofers ( DWM ) and this had a positive effect by cutting the bass output by 4/5 db from 80 to 125hz. Probably the reason that this worked so well is that the Magnepan has an unusual series crossover with considerable overlap between bass and midrange and a mixture of first and second order roll offs or its something else entirely. When I started I had an almost 20 db elevation of some bass frequencies over the midrange. Adding the Superchunks behind the corner bass traps dropped that by about 5/6 db. Reversing the speaker leads on the Magnepan woofer ( now opposite of the hook up on the Magnepan 3.7s ) cut the bass output another 4/5 db. With the bass elevation now only about 10 db above the midrange and quite smooth with no significant peaks I am close to my goal of a 5 db elevation of the bass over the midrange. I will see if widening the Corner Bass Absorbers takes care of this and if it doesn't I will add a larger resistor to the DWMs crossover to further attenuate the bass output.
Joe Fagan
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