helmholz in corners; schroeders in front of corner traps

[unqlenol]

Hi all,
I have been visiting this forum on and off for a couple of years while I trickle along with my studio acoustic DIY mission.
This is my first time posting and I hope that my questions are valid and not overly common.
I have done a fair bit of searching but can not seem to find any concrete answers to a couple of questions which I have.
I thank anyone in advance for any advice that can be offered and if anyone feels that these answers are already readily available I would appreciate any tipoffs (ie: URL's) that could help me ( I have scoured Ethan's forum too).

My two questions are essentially:
1. Is it reasonable for me to build rectangular MDF helmholtz slat absorbers to straddle across the room corners?
2. If I were to build simple schroeder diffusers and place them in front of my current chunky broadband bass traps, would the traps effectiveness be reduced?

I would like to flesh out my scenario in case this information is helpful:

My room could be called a control room or mix studio, but ultimately a bit of everything happens in the room.
Dimensions are 4.2m(w) x 6m (L) x 2.4m (h)....sorry for the metric.
Each wall corner is currently 'closed off' from floor to ceiling with towers of super chunky broadband absorbers.
The absorbers are each 200mm thick(200 x 600 x 1200), high density Rockwool.
The rest of the room has 100mm thick broadband absorption at 1st reflection points.
The ceiling is about to become one giant broadband cloud. Not much can be done with the ceiling over and above this (it's a long story).

Things are sounding decent enough but I want improvements.
After some testing with REW it seems apparent that I have strong modal troughs at about 32hz and at about 85hz.
The problem persists when changing listening or speaker positions.
This is one problem.

The other issue is that the room has no diffusion plan implemented yet.

I was hoping that I could convert the lowest trap at each corner (ie: the traps that actually sit on the floor across each wall corner) into helmholtz slat resonators. The idea is to build the boxes with the existing 'superchunks' inside them; Then I would straddle the boxes across the wall corners and put the other existing super chunks on top of the slat absorbers (to recreate my big 'towers') and then, finally, to place schroeder diffusers in front of the superchunks. My concern about the helmholtz plan is that when I look at the *corrected* helmholtz calculator, the diagram shows the 'depth from wall' as a parameter. I am wondering if I should, in this case, consider the back of the proposed MDF box as 'the wall' since the idea is not to build these things directly onto the wall but rather to make standalone boxes.

My concern with placing schroeder diffusors in front of the rest of the superchunks is that perhaps the diffusors will reduce the absorption efficiency of those absorbers in the low-mid range.

I hope this makes sense. I can draw up a picture if it doesn't, please let me know if this is necessary. I would also appreciate any other fundamental criticism in the case of the whole idea just being completely misguided.
My aim is to have a flat and reasonably accurate room via a combination of broadband absorption, tuned absorption, first reflection point absorption as well as a healthy dose of decent diffusion....sort of a compromise between Reflection Free Zone and Early Sound Scattering.

Can anyone advise as to whether I am wise or foolish?

Again thanks in advance for any help.

Sincerely,
Nolan

[unqlenol]

Hi, it's me again

I would like to add to my above questions:

Another confusion that I have relates to what I see as an information gap with regard to helmholtz resonators:
On this forum, and on many others, there is always a reference to the helmholtz formula as well as to the respective available calculators.

The calculators have a single variable for depth (ie:calculator assumes a constant cavity depth) as well as a single variable for slot width and slat width.

What puzzles me is that I also see references to triangular 'corner' helmholtz slat resonators (which obviously do not have a constant depth since the cavity is triangular) .
I also see references to mixing different slat widths and slot widths.
I am puzzled because none of the calculators I have seen seem to account for this, they all work on constant values per resonator. Strange to me is that on some of said calculators there is the recommendation to combine different slot widths despite the calculator not catering for these extra parametres.

So, to sum up, and to add to my above questions, I humbly ask: ' What gives?'

As always, thanks for any enlightenment.
Nolan

[Soundman2020]

Hi Nolan,

First, please read the stickies!

Regarding your question: If you have a slot resonator placed at an angle away from the wall behind it, then it is a broadband device. So calculate the frequency for the smallest depth, calculate the frequency for the largest depth, and your device will affect all of the frequencies in between.

For several slots on a slot wall (such as John's slot wall designs) where you have several different slot/slat dimensions, you do them one at a time. For example, start with the slot at the top of the wall and calculate the frequency for that one. Write it down. Now go down to the next slot, and figure out the frequency for that one. Etc. Repeat for the entire wall. If the slot wall is also angled, then of course you will need to do as above, and calculate for the lowest and highest frequency that each slot will affect, then write down the range rather than the individual frequency. When you are done with the entire wall, then you will have a list or frequencies that the wall deals with, or for an angled wall, you will have a list of the ranges that it deals with.


- Stuart -

[unqlenol]

Steve, thank you very much for this answer. I have indeed read the stickies but somehow I have not found my specific issues addressed. Perhap I was not vigilant enough

The varying depth concept I almost guessed as it seems logical, I just had to be sure.
I still am not entirely sure I understand the part about implementing various slot widths. I think I understand that I'd have to have separate sealed enclosures for the different slot width choices. Eg: one box with 5mm slots, another box with 10mm slots. If I'm wrong on this, ie: if one can incorporate different slot widths between slats on one cavity, then I still don't get how that would be calculated since the helmholz calculator only provides for a single slot width parameter s'far as I can tell.

I also have still not found an answer to my concern about placing schroeder diffusion in front of my superchunks, so if anyone can chime in with an opinion I'd be much appreciative. For the record my superchunks are fabric covered, hence pretty darn broadband. I am happy if schroeders in front of them would serve to reflect and diffuse higher freqs, but I don't want the chunks to lose LF absorption efficiency. The schroeders would not be touching the superchunks...I'd leave a gap....

Thanks again for the help and for all the forum in general!

[Soundman2020]

Perhap I was not vigilant enough

It looks that way! This is the one you seem to have missed...

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=3231

I think I understand that I'd have to have separate sealed enclosures for the different slot width choices. Eg: one box with 5mm slots, another box with 10mm slots.

No, you don't need to divide the space behind the slots into cavities. It's not intuitive, but basically a sound wave only "sees" the air space directly behind the slot that it "gets through" (Not really like that, but a good way of thinking of it). It does not matter if the air space behind is divided or not.

if one can incorporate different slot widths between slats on one cavity,

Well, no, normally you would not do it like that. You would not vary the width of a single slot. Each individual slot normally has only one width. The usual way to do it is have several slats over one cavity, and the gap between each PAIR of slats is fixed. So for example the gap between slat #1 and slat #2 might be 4mm, then the between slat #2 and slat #3 might be 12mm, followed by the gap between slat #3 and slat #4 being, maybe 7mm, etc. So the entire length of each "slot" (the gap between a single pair of slats) is fixed, but the gap on the next slot might well be different.

still don't get how that would be calculated since the helmholz calculator only provides for a single slot width parameter s'far as I can tell.

That's right: You calculate one slot at a time. So if there are nine slots in a slot wall, you'll end up with a list of nine different frequencies (or ranges), one for each slot. It's not that hte entire wall acts as one slot: each slot acts individual on its own frequency (or range of frequencies, if the wall is angled).

I also have still not found an answer to my concern about placing schroeder diffusion in front of my superchunks,

I don't think I'd do that. If you have a problem with your superchunks absorbing too much in the way of highs, then normal recommendation here is to put plastic sheeting in front of the insulation (or wrap it completely). The plastic supposedly reflects the highs while not touching the lows.

In any event, if your room is small, then a diffuser is probably not a good idea anyway (including QRDs), since there isn't enough space for the lobing effects to dissipate (smooth out) before reaching your ears. I forget the exact distance, but I seem to recall that you need something like six meters between the diffuser and your ears before it is reasonable. Based on your dimensions, you simply don't have enough room for that to work.

If you have very specific frequency issues, then perhaps a tuned Helmholtz resonator would be the way to go, or a slot wall with several slots tuned to the problematic frequencies, or maybe a panel resonator, or some such, targeted at your specific issue. So your plan to do that on part of your wall makes sense, but I'm thinking that doing it in the tricorners is probably not the best location. Those are the best for bass trapping all on their own, so maybe the side walls would be a good place for slot walls. Take a look at some of John's designs to see how he does it.

The absorbers are each 200mm thick(200 x 600 x 1200), high density Rockwool.

High density???? How high??? You don't want it too dense! In fact, it seems that less dense is better for low frequencies. How dense is your wool?

I can draw up a picture ...

Use SketchUp to create an accurate model of your room, and post it here. Photos would also help!

- Stuart -

[unqlenol]

Hi Stuart. Sorry for delayed response, been crazy mixing for the last week.
I've finally filled in my location I'm guessing that that was how I was naughty since I had read that sticky and did not find my answers there. Again, humblest apologies.

I totally get what you are say now about the slot absorbers. I'll do the different calculations to come up with a unified design. Excellent.

Regarding schroeder diffusion: I has the idea since Ethan Winer had suggest that it might be effective if I was at least 4 feet from the back wall/ diffuser. You say possily 6 metres is more the distance. I am roughly 3.5 metres away, that's around eleven feet. At the rate things are going it looks like I will focus on suitable absorption. So an RFZ, broadband 'trap/absorbers' in most corners and slots on available walls where suitable. Your advice has already helped me find more resolve. The diffusion thing seemed like it could be a bonus in my scenario if it could work. The room already has a decent enough wet/dry balance though it will dry up more when I install my huge ceiling cloud. For the record: my cloud is specifically large to help another situation that I have which is that the ceiling slopes across the width of room and compromises the stereo image. The large flat cloud/absorber will absorb AND flatten the 'overhead reflective area'.

I downloaded Sketch up. A learning curve in it's own right. I will try get some pics to this thread when things calm down for me. No promises for sketch up unless the video tuts sort me out quickly. I will endeavour to at least post some photos in the next day or two, even if it's just for the reference of other who may read this thread.

The absorbers are each 200mm thick(200 x 600 x 1200), high density Rockwool.
High density???? How high??? You don't want it too dense! In fact, it seems that less dense is better for low frequencies. How dense is your wool?

My wall corner broadband supechunks are rockwool of a 80kg per cubic metre density. Do you feel I went to far? I made this choice based on co-efficient charts and based on homework I did which indicated the this sort of density would absorb to a lower frequency. The primary objective of my corner absorbers is to provide broadband LF absorption.

Thanks as always for the invaluable input. Please hang in for some pics.
Nolan