Bass management -- the trick of putting absorbers at a front null


Very interesting discussion with Anthony Grimani. He advocates both traps and multiple subs and goes through the basics and tactics of bass management.

What's of interest to me is a "trick" he mentions -- putting an absorber panel at null between the listening position and the front wall. This, he says, can help even out the bass and take the place of the brute (and impractical) physics of trying to absorb the standing wave with absurdly thick absorbers.

He mentions it very soon after this point (which provides some context): https://youtu.be/QYpAbv7gKrs?t=1853

Has anyone tried this? Any details or outcomes you can share will be welcome.

P.S. He mentions Todd Welti, who did a Ph.D. thesis on using multiple subs. He's now with Harman. There is a paper by Welti, here: "Low-Frequency Optimization Using Multiple Subwoofers" https://www.semanticscholar.org/paper/Low-Frequency-Optimization-Using-Multiple-Welti-Devantier/00da...

This looks interesting, too: "How Many Subwoofers are Enough"
https://www.semanticscholar.org/paper/How-Many-Subwoofers-are-Enough-Welti/96b772af4ea937d8028c2f845...
8700e65e 845e 4b1b 91cc df27687f9454hilde45
I'm not sure what he calls absurdly thick absorbers.  I use soffit traps in the corners behind the speakers and find them very effective below 100 Hz.  In my current case I'm pretty sure I can't put big bass traps anywhere near the null locations.
I find that adding this is enough for a single sub with DSP correction. See my blog post about it here:
https://speakermakersjourney.blogspot.com/2021/03/how-to-equalize-subwoofer.html
He explains what he means in the clip or perhaps shortly before. Thanks for the link to your post. I'll take a look. 

So I got to watch about half of that video.
While the idea of putting traps at high velocity/low pressure points makes sense to me, I think he underestimates the benefits of corner placed traps if well constructed and exaggerates the required size.

It is true that your average 4" thick fiberglass panel is completely ineffective < 100 Hz in the corners, but that doesn't mean you need 5' of fiberglass. :)  As I mentioned before, I've had very good luck with GIK Soffit traps in the corners for this.  See my blog post for an example of what I'm calling good results.

Fortunately, his theory is easy to test for yourself with the right tools.  As his graphs point out, if you correctly attack the problem the bass will immediately and obviously even out.  If it doesn't work, you can always use the panels elsewhere. :)

Best,
Erik
Thanks, Erik. Since I last posted about traps, etc. I was able to find someone locally who was literally giving away their traps to any local audiophile who was interested. I scored 14 traps in all, including stuff from GIK. I'm just not quite making a dent yet in some peaks and nulls, so I'm interested in additional tricks. 
Hi hilde45
So, it is absolutely true that your traditional 2-3’ panels have a very hard time with this. If you look in the GIK site you can find specs which show how effective the panels are at different frequencies. Their absolute best bass traps are the Soffits.


Also, it IS true that without EQ you need massive traps, so the recipe is not just to rely on bass traps, but to rely on a combination of traps, placement of sub and listener AND EQ. Without the traps, the EQ wont work down low.
Thanks, Erik -- good to have that reality check -- "a combination of traps, placement of sub and listener AND EQ."

I've figured out placement of listener and speakers, and am trying to dial the room in now with traps and, to a degree, a single sub. I have a second sub in the house, but it was too complicated initially (because I was figuring out the other things). I hope to have a couple more good traps to work with, soon, and will try to work those in as best I can and then see how the addition of subs can help. Then, I'll move to EQ. I hope that process sounds logical.
I sure like the spring traps. I would pay the 900.00 for that trap. Heck I’d pay for 4 of them. That is a bass trap. It will handle 3 different frequencies. that is my room. 40-50, 75-85 and 110-120.

A problem might arise with what ever the surround material was made of too. Need 20-30 years of life too. Treated BR or silicone

A single axis accelerometer, shouldn’t be any more expensive than a tonearm and phono stage, PLUS there is a phono stage between you and the readings.. That part didn’t make sense to me.. The tuning part with a tone arm. I’m not sure it’s accurate, but a test after will surely tell..

Doesn’t a phono stage EQ stuff? :-)

I also have suckout at 250 -6db and 350 -8db.

I like classical guitar. Start messing around in those ranges it really messes with what I like.

Who would make "Spring Traps" NOW? Me I guess. My hands are already hurtin’. :-(

50 years ago. Carpeting, acoustic ceiling, book shelfs and curtains.. Tamed a room pretty good.
Actually there wasn’t a lot of bass back then either come to think of it.. Wasn’t much of a need to tame it :-)

Some rooms just start out right...
The idea is that a frictional absorber necessarily is placed away from a wall ... not always very practical.
Angled corner traps work a bit differently (angle of incidence ... ?) with angled (design dependent) half filled corner type trap.
Something to remember is that the illustration (flat 2D) references only left / right / up / down and no depth ...
@hilde45, if all those traps are piling up and not being used I can take one or two.
Thanks, Rego. I will message you if I am buried under traps! When you say, "The idea is that a frictional absorber necessarily is placed away from a wall ... not always very practical."

I know that frictional absorbers are placed away from walls -- to gain the extra absorption from a distance to the wall roughly equal to the width of the traps, if I remember right. 

But Grimani is suggesting something different than this. He is suggesting placing them very very far from the wall in order to capitalize on a null endemic to the room's modes. That is an application utilizing a different phenomenon, no?
By the way, in terms of having multiple subs, I'm thinking less of this as a solution as I've played around with the Room EQ Wizard's room simulator.

Try it out, but it basically lets you set up a rectangular room with speakers and a sub so you can try out different placements.  Based on those simulations, I'm not sure if multiple subs really does all that. I'm not claiming expertise, I'm just saying that the evening out effect is not as good as I thought.
Thanks, Erik. I'll take a look, but the problem is that my room's irregular shape and non-hermetic situation makes it impossible to put into these room simulators. I use REW a lot but it's as much a systematic try-and-see and try-and-listen approach.
Again the idea for placement as in the video diagram at a null position uses the idea of equalization of relative high to low pressure areas with a velocity.

That is an application utilizing a different phenomenon, no?

Yes-but a same phenomenon as any frictional absorber with the variables being frequency and absorption band.
My emphasis would be to acknowledge the 3D environment being helpful with placement.
The example (diagram presented in video) uses 50 Hz as the frequency range and appears to position the panel perpendicular to the wall and parallel to the end wall (assuming a rectangular plan) illustrating the idea that can work to ’level’ an imbalance at a narrow frequency range.
A narrow band (mid-high frequency absorption) frictional panel could be placed parallel to a side wall, perpendicular, and any angle from 0 deg to 180 deg.

How can it be from 0 to 180 degrees if it needs to intersect with null in the room from one wall to the other.. If it is only so wide (the null) and using the width of single panels, what goes by the panels (untreated) will literally come back and bite you in the As$. I see how it works, and in a LONG room it may be practical, IF you can get the break closer to a front or back wall.. BUT in my listening room?

Pretty darn UGLY having a break dead center of the room. To treat a (50hz) problem wave. What about 80 and 120. LOL It would start looking like huge chess board Wouldn’t you agree..

The spring traps address 3 different areas and just needs to be adjusted for volume and the frequencies you want.. I like that kind of thinking...

Honey I fixed the bass problem..

Click Click, I’m peaceful she’s not.. :-) The spring traps I could get away with in the front room but 1.5 meter hurdles, that’s a bit of a stretch. Get Ready, Set, GO!!!!
@oldhvymec If you watch a bit more of the video, you'll see he's not suggesting such a simplistic solution. He's pointing to a fact about acoustics which, combine with a variety of other techniques -- in some rooms, not all -- can help mitigate problems, and not just at 50hz. So, your off the cuff response while amusing kind of turns him into a simpleton. Not really fair, but hey, this is a just a forum so I guess mere joking is kosher!
I didn't mean to make him look simple. I wanted to point out it was difficult to understand and could have been a LOT easier. 

 He wasn't simple enough. AND it was an easy thing to understand. EVEN the host continued to go back and say "SO IF" several times.. There was a lot of talking, that could have been narrowed to 10th, at the MOST.. LIKE, where to place the weirs? He kept referring to theory. When in fact he was being asked HOW to place the traps, from one side of the room to the other, against the walls, in the center of the room, in front of you? The guy continued to talk but NOT directly answer the questions posed..

NO ONE is going to place a BARRIER in the middle of a home listening room. Not me anyway.. DBA and acoustic treatment are actually polar opposites. Kind of like putting up a WALL of sound, IT'S EVERYWHERE.. Including the neighbors pool... I have a FEW different VBDA systems here. Varied BASS Distributed Array. 

I also wondered WHY the best product he offered, he also quit making.

THAT made ZERO sense to me. Excuse please OP, LOL I may type a lot, BUT I get to the point. Most of my conversations with folks are as long as my replies when typing.. Not a lot of "see how smart I am in the mix". I try to show that through, short, thorough questions and short, thorough answers..  YES and NO is just the right answer sometimes..


NOT KEEP TALKING.. Reminds me of my old boss.. SHUT UP... QUIT TALKING... I use to have to tell him that.. QUIT TALKING, START LISTENING.

I'm so glad I fired him....after he fired me for having a broken neck..

See just two extra lines, including this one..

Regards
@oldhvymec  I understand better now, thanks. I found it easy to follow the first time, so I was unclear about what you meant. Apologies and regards.
hide45 it is funny. I'm just a common man, pretty simple indeed..

I did like the information though.. Gave me an idea or two...

I like the spring traps for sure.. DBA, well it been here for 30 years, in one form or another..

Regards..