New idea for Bass mode treatment

banpuku

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Apr 24, 2010
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Please correct me if I am wrong, but my understanding is that bass modes occur when 2 parallel walls reflect bass energy to/from to create nulls and peaks for a specific frequency depending upon the distance between the walls. Historically, absorbers have been the most common method to tackle the bass anomalies. BUT, what if we took a different approach, much like what is done in auditoriums and symphony halls. Instead of absorbtion, we change the underlying cause (parallel walls). And, instead of just skewing one wall, we create a very diverse surface that scatters the bass sound waves much like a diffusor.

So, I stumble across a product call Woodskin. It comes in several sizes that can be turned / twisted to create the diverse surface that would deflect bass sound waves in multiple direcdtions that would minimize room bass modes. I was thinking of placing a large woodskin surface on the front wall including 2 corners. See the attached photos that give you an idea of how flexible this product can be used.

Imagine that the front wall has Woodskin covering floor to ceiling in one corner. At the floor level, the woodskin in the corner would be close to the front wall surface. Then, as the woodskin traverses to the ceiling, the woodskin would undulate away from the front wall. The woodskin would also move toward the other corner, so we get about 50% coverage on the front wall.

Thinking out loud, there might be unfavorable scattering that would be hard to control. But, would this still be better than room modes.

Please let us know your thoughts. this stuff isn't cheap. It runs $75 - $125 / SF. I would put rock wool in the space between the woodskin and the front wall.
 

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Mike Lavigne

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or build an oval room with lots of diffusion built in and a ceiling with chambers. which better uses space in a room of a particular size.

to use a product like what you show, a room would need to start out somewhat over-sized. but if you have a room with lots of extra width but flat parallel walls then that looks like a good solution.

and i would expect that there are plenty of real world domestic rooms close to square where this approach might be optimal. you would have to do the acoustical math (of which i have no clue) on it before making the considerable investment. unlikely one can 'demo' and return this.
 
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Kal Rubinson

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All reflections, not just from parallel surfaces, can create modes; axial, tangential and oblique modes are significant. Using non-parallel walls distributes the resulting nodes/peaks over a wider range of frequencies. However, although those distributed modes will, individually, be of lower magnitude, they may be harder to deal with due to their increased number.
 

banpuku

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Kal, yes I have been thinking the same as well. Today I spent quality time rearranging my listening position and the bass traps that I have (2 Realtraps, 4 corner Mondo RealTraps, 2 Mondo RealTraps). It was very interesting to hear the limited impact of the traps. Across the spectrum, they impacted at most 2-4db (nothing to sneeze at), but considering that I have a 15db null, the 2-4db just isn't enough. The RealTraps are good products, but I don't think any of the absorption products are a panacea. I am looking into the artnovion sub traps which are tunable. My only fear is that I will need an army of these (or any other products) to make the difference i am seeking. 15db nulls are tough to solve. Any thoughts on attacking these nulls at 70 and 110hz?

Thanks, Pat
 

Cohnaudio

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Dec 24, 2016
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Given the size of bass waves, particularly those below 150hz, i have real doubts that those will be effective addressing bass nodes. They may be effective for mids and highs, but are not real diffusion. In my experience, to address anything below 100hz, good diaphragmatic bass traps are the only things that really help. The real traps and the like, even many of them, help only above 80-100hz
 

Kal Rubinson

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I have a pair of MSR Acoustics Dimension4 SpringTraps (https://www.stereophile.com/content/music-round-56-page-2) which, in principle, are similar to what you have and they do make a significant difference but you/I need more multiples to really kill a null. I suspect I would need 4-6 more but the budget for that would have to include a divorce. DSP to the rescue.
 

sbnx

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Not to discourage creative thought, but ...

It is a common misconception that making the walls non-parallel will eliminate room modes. All containers, aka rooms, have (eigen)modes. It is a fundamental property of the partial differential equations that govern fluid flow. The shape of the room will change the way the modes are distributed in the room as well as the frequencies (eigenvalues) which the modes occur. For example if your room was a cylinder the modes will be Bessel functions. If your room is a Sphere then they will be spherical harmonics. etc.

Kal said it best..."there is no free lunch". You need a LOT of bass traps to get smooth frequency response. Here is a formula for success. Start by finding the position in the room that has the flattest bass response. This will be your listening position. Then find suitable speaker positions. Add bass traps to the corners as for a rectangular room this is where all of the modes intersect. Hang bass traps from the ceiling. Put traps behind your listening position and to the sides.

Hope this helps.
 

banpuku

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Thanks to all who have responded. After further research and your input, I am going to pursue adding some bass traps, in particular the springtraps or Artnovion sub bass traps.
 

Folsom

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Given the size of bass waves, particularly those below 150hz, i have real doubts that those will be effective addressing bass nodes. They may be effective for mids and highs, but are not real diffusion. In my experience, to address anything below 100hz, good diaphragmatic bass traps are the only things that really help. The real traps and the like, even many of them, help only above 80-100hz

Ya... the problem is the bass frequencies are too large. While this could work, you'd need the panel pieces to be way too large to work in the room.
 
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microstrip

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Besides dimensions we need to the consider the wall materials and type of building to properly analyze bass in a room. Most simple calculators assume 100% reflection of the bass energy at walls, something that only happens in extremely rigid and solid walls.

Perhaps this old thread is of interest to you - these tuned bass traps improved a lot my room, both the measurements and the sound quality . https://www.whatsbestforum.com/threads/diaphragmatic-versus-membrane-bass-traps.7710/

I should have added a few more at the back wall to deal with the 50 Hz, but as it involved a lot of moving it has been systematically postponed ...
 

Hipper

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Jun 12, 2011
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I had a 15dB null around 70Hz after using copious amounts of bass traps and careful positioning of chair and speakers. I also checked all the gear to make sure I'd done nothing silly.

I considered using tuned traps to deal with it but first I had to find where to put them. I measured with a suitable test tone and SPL meter high and low all around the room but could find nothing conclusive.

In the end I tried EQ to solve the problem, and it did. I've no idea why but there we go!

The other apparent solution which I've never tried is group of four small subwoofers carefully placed.
 

heihei

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Thanks to all who have responded. After further research and your input, I am going to pursue adding some bass traps, in particular the springtraps or Artnovion sub bass traps.
I'd be interested to hear how you get on with these.
 

Krelle

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Aug 5, 2013
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Please correct me if I am wrong, but my understanding is that bass modes occur when 2 parallel walls reflect bass energy to/from to create nulls and peaks for a specific frequency depending upon the distance between the walls. Historically, absorbers have been the most common method to tackle the bass anomalies. BUT, what if we took a different approach, much like what is done in auditoriums and symphony halls. Instead of absorbtion, we change the underlying cause (parallel walls). And, instead of just skewing one wall, we create a very diverse surface that scatters the bass sound waves much like a diffusor.

So, I stumble across a product call Woodskin. It comes in several sizes that can be turned / twisted to create the diverse surface that would deflect bass sound waves in multiple direcdtions that would minimize room bass modes. I was thinking of placing a large woodskin surface on the front wall including 2 corners. See the attached photos that give you an idea of how flexible this product can be used.

Imagine that the front wall has Woodskin covering floor to ceiling in one corner. At the floor level, the woodskin in the corner would be close to the front wall surface. Then, as the woodskin traverses to the ceiling, the woodskin would undulate away from the front wall. The woodskin would also move toward the other corner, so we get about 50% coverage on the front wall.

Thinking out loud, there might be unfavorable scattering that would be hard to control. But, would this still be better than room modes.

Please let us know your thoughts. this stuff isn't cheap. It runs $75 - $125 / SF. I would put rock wool in the space between the woodskin and the front wall.

A link to a similar design (SMT) can be seen from Klangloft FB below
I asked SMT (I liked the design for my new listening room ) and they claim that the modules also work as broadband bass absorbers
Notice also the transparent wings infront of the skyline modules /Krelle
https://www.facebook.com/KlangLoft/photos/pcb.2400847340035394/2400847033368758/?type=3&theater
 

banpuku

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Just an update for those interested. My wife and I have decided to build a new house. This means a new stereo room. Exciting and yet fatiguing. The room will be 18L x 15W x 9H. The walls will have 3/4" plywood on the studs covered with drywall. I plan on using some sort of bass traps on the front wall corners, probably with a helmoltz resonator. Also, the front wall will likely be undulated to break-up the standing waves. More to come! Any recommendations are welcomed.
 
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Duke LeJeune

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The other apparent solution which I've never tried is group of four small subwoofers carefully placed.

Reports I've gotten from many customers who have tried bass trapping and four small subs intelligently distributed is that the latter makes a bigger difference, even when the bass trapping was extensive enough that the cost was similar.

Disclaimer: If it's bass traps vs subs, I've got a dog in the fight... well, more like four small ones...
 

Kal Rubinson

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Reports I've gotten from many customers who have tried bass trapping and four small subs intelligently distributed is that the latter makes a bigger difference, even when the bass trapping was extensive enough that the cost was similar.
That makes sense but one needs to consider that adding subs for this purpose to a system that, otherwise, would not have subs means that the user needs to integrate them into the system.

That is one reason why I have great hopes for DiracLive's new bass management which actively manages all the bass sources in the system, rather than simply crossing over the main speakers to the subs at a fixed frequency. Seems like an advance on Harman's SFM.
 
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Duke LeJeune

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That makes sense but one needs to consider that adding subs for this purpose to a system that, otherwise, would not have subs means that the user needs to integrate them into the system.

Agreed, multiple subs are not a one-size-fits-all solution.

That is one reason why I have great hopes for DiracLive's new bass management which actively manages all the bass sources in the system, rather than simply crossing over the main speakers to the subs at a fixed frequency. Seems like an advance on Harman's SFM.

Sounds like a powerful system. Earl Geddes was doing something similar for his subwoofer customers, wherein they would make multiple in-room measurements of each sub in a distributed three-sub system, and he had a proprietary program which would then generate optimized DSP correction for each sub.

My guess is that the best results may include a combination of acoustic solutions (bass trapping, multiple bass sources) and signal processing solutions.

Ime it can be perceptually beneficial to introduce a significant phase difference (typically 90 degrees) between the bass sources on the left-hand side of the room and those on the right-hand side of the room. This synthesizes a low-frequency environment similar to what one might experience in a large hall. Credit to David Griesinger for this idea.
 

Kal Rubinson

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Sounds like a powerful system. Earl Geddes was doing something similar for his subwoofer customers, wherein they would make multiple in-room measurements of each sub in a distributed three-sub system, and he had a proprietary program which would then generate optimized DSP correction for each sub.
I do not have enough information at hand to say how and what exactly the Dirac system does although I have heard a demonstration of its performance. I would hope that it would not merely "make multiple in-room measurements of each sub " but also make measurements of subs and other LF sources in groups.
 

Duke LeJeune

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I would hope that it would not merely "make multiple in-room measurements of each sub " but also make measurements of subs and other LF sources in groups.

I assume the summed response could be predicted given enough frequency and phase response measurements taken over a sufficiently large area for each bass source, but I could be wrong.
 
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