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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...
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Showing 4 responses by erik_squires

erik_squires
16,471 posts
 
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.
erik_squires
16,471 posts
 
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.
erik_squires
16,471 posts
 
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
erik_squires
16,471 posts
 
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
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