Quick Slot Absorber Questions?

[seamus]

Hello.
I have been working on our 20.4'L x 13'W x 10.75'H control room for a couple of weeks and I just have a quick question or three about slot resonators.

I want to build a 4'H x 8'W x 9"D slot absorber on the middle-bottom portion of the back wall (behind the couch) to get in the low 70Hz range, which is the 3/4 wavelength of my listening position, where I have a measured null. There will be cloth-covered R38 above it extending to the ceiling.

1. Is the surface area and/or placement of the proposed slot absorber enough to make a difference?
2. Are 2x4's adequate for slats? If spaced 1/16" apart, that should theoretically hit the low 70Hz range.
3. Is fluffy pink insulation ok for behind the slats, or should I use OC703?
4. Am I insane?

Thanks,
Seamus

[jbassino]

Hello there
Have you looked at membrane resonators? They are easier to build plus you can get even deeper absorption using another design, and you'll save an expensive resonator with 2x4

[seamus]

Thanks for the reply!

My first thought was a membrane absorber, actually.
It just so happens that I have a bunch of extra 2x4's, though.

However, if a membrane absorber would be more effective, I would go that route, instead.

I did some more measurements last night.
After some speaker repositioning I got the nulls smoothed out a little.
They (Quested S8r) are 1.5' from the front wall and 27" from the side walls.
The high pass filter is on, which helped a great deal.
As it is now, I have a 62Hz null (-16dB), as well as some various nulls between 95Hz and 150Hz (@ -12dB).
There is also a -16dB null at around 350Hz, which is weird.

I should probably use some sine waves and an spl meter to try and find out where they are happening first before I bother building anything.

Just for the sake of giving more information, the only bass treatment in the room at the moment is that the entire front wall is covered with 12" FRK faced R38. The whole wall except for the 24" hollow core door in the center, that is.
This helped out a lot with some huge problems that we used to have.
I was planning on Covering the entire back wall with 18" of pink fluffy (2 layers R30), as well. Right now, there is only an 8' x 8' section of 4" OC703, which is not doing a whole lot, I suspect.
I am, also, planning on doing a 2'W x 8'H section of R38 on the side walls right next to the speakers.
The side walls are plaster and lathe on brick.
The front wall is just one layer of 5/8 sheet rock.
By one layer, I mean that I built an inside-out wall with the 5/8 rock on the outside of the room with the 12" R38 facing the room.

Thanks, again!
Seamus

[jbassino]

Some of your problems might be solved with "standard" treatment, saying, corner traps (a bunch of insulation stuffed or thick panels straddling the corners), also place some absorption at your first reflection points (you know, you should do the homework too), and then measure again. If you still have a deep null then consider treating it accordly
Thats my advice

[seamus]

Thanks, again.
I do have first reflection point treatment on the side walls and the ceiling.

I've been ruthlessly studying acoustics for about a year, now.
I've just finally had time to put it into a tangible form.

Seamus

[jbassino]

Make some superchunks then (insulation triangles stuffed at the cornesr, ideally all 4 of them, covered with nice cloth), and then measure again

[seamus]

Yeah, I decided not to go with the superchunks quite a while ago for money and time reasons.
I'm going to stick with the low density fiberglass, for now.
It's cheap, quick and easy to install and seems to have excellent specs.
It, obviously, takes up a lot of room, but done strategically, isn't a big deal.

If I still have the nulls, I'll look into the tuned traps more.
I'll be able to just remove sections of the fiberglass and replace them with whatever trap will work out best.

[jbassino]

You know a easy and cheap way to find out? Go buy some insulation packages, and just stuff them in your corners, with the plastic cover and everything. And measure

[xSpace]

Yeah, I decided not to go with the superchunks quite a while ago for money and time reasons.

"Right now, there is only an 8' x 8' section of 4" OC703, which is not doing a whole lot, I suspect."


There's your material<looks up> for your super chunks.

[seamus]

Ha!
Yeah, that's true.
There is still the time and the "where do I cut all that fiberglass?" part, though.
Also, I had kind of ear-marked those panels for the live room.

I'm getting the feeling that you guys think that the superchunks would be worlds better than low density fiberglass soffits (roughly 2' x 2') in the corners...

[seamus]

You know a easy and cheap way to find out? Go buy some insulation packages, and just stuff them in your corners, with the plastic cover and everything. And measure

That's true, as well.
However, those bales are compressed quite a bit in the plastic.
I fear that it wouldn't give me an accurate idea of what's going on.

[jbassino]

Im sure it will. give it a try

[seamus]

Yeah, ok. I'll try it.
Although, I only have two bales of R30, so it'll only be 4' high.
I'll put them against the wall (next to the speakers) and measure.
Then I'll do the R30 in 8'H x 2'W x 18"D frames on the sides of the speakers, and measure.

I probably will end up doing the superchunks, if I need to.
I do have the OC703, as it was helpfully pointed out.
The live room can wait.
There are some structural limitations to the back wall that would prohibit me from doing all of the corners, but there is some room here and there.
I'll probably try a tuned trap as a last resort.

Thanks so much for the advice and attention.
Seamus

[seamus]

So, a little update.
I built front wall/side wall corner soffits that are 2' wide, 8' high and 18" deep.
After repositioning the speakers, the problems between 95Hz and 150Hz improved quite a bit, but that 62Hz is a killer.

I also built a back wall absorber that is 6' wide, 8' high and 18" deep.
It didn't change the frequency response at all.

This really frustrated me.
I hadn't covered the side corner soffits with fabric, yet, so I pulled the insulation out and measured again.
There was nearly no change.
It was the speaker repositioning that had improved the 95Hz - 150Hz, apparently.
There may have been a slight change in the lower 100Hz area, but not enough to justify eating up all that space.
The decay time in the room (by ear) increased, however.

Anyway, I'm left with a back wall absorber that doesn't do anything for the response and no corner treatment.

I'm going to do the superchunk idea in the front corners and use the pink fluffy that I have left to extend the back wall trap up to the ceiling.
We'll see if those two things can touch the 62Hz.

Thanks,
Seamus

[Soundman2020]

I think you might be working under false assumptions: Absorption cannot eliminate a room mode. It will always be there. You can damp it somewhat with absorption, but it won't go away. To eliminate it entirely, you have to change the geometry of the room somehow. You can break up standing waves with angled surfaces, which is one reason to not have parallel walls in your room. You can also suck some of the energy out of it with resonators of one type or another, but absorption alone can't do the job. Maybe that membrane resonator might be a good idea, after all?

I'm also not sure where that 62Hz issue is even coming from! Based on your dimensions, you do not HAVE any modes near there! You have one at 59.4, and the next one is at 68.2 (both tangential), but nothing at all near 62. Are you sure your dimensions are accurate? Are you sure your frequency measurements are accurate?

The decay time in the room (by ear) increased, however.

I guess I must be misunderstanding what you said: The RT60 time INCREASED after you put a whole bunch of absorption all over the room? That doesn't make sense!