Advice on porous absorbers in a near ideal sized room

[mike zietsman]

Good Day!

(Apologies in advance for the long post and thanks in advance for reading it. Some questions are inline but everything is repeated at the bottom nice and neatly)

I am about to move into a large, near ideal sized control room where I will be mixing, mastering and composing. The room was purpose built for recording and abandoned before acoustics could be installed. The walls are double layer brick and sound leakage is not a problem or concern. This post is all about making the room behave properly for the purposes of mixing and mastering.


I am based in South Africa.

The room dimensions are:
6.3M (L)
4.0M (W)
2.88M (H)

Attached is an image indicating the layout of the room and it’s doors.
I will be placing the speakers against the dimension at the bottom of the image, which I will consider to be the “front wall”. The door on the left of the image is the entry to the room. The door at the top right goes to a machine room and the door at the bottom right leads to a vocal booth.

My limitations are these:

1: I need to be up and running by mid-january.

2: My budget is constrained post covid. I’m obviously aiming for bang for buck. Max 3000usd-ish

3: I would ideally be able to get all of the traps I build in and out of the room. I have been given permission to affix them to the walls for basic stability. This means that no two dimensions can be bigger than the door to the room (2mx 0.9m) and no single dimension can be bigger than the width of the room (4000cm).

The speakers I am using are ATC SCM50ASL


My Advantages are these:

1:I have access to some very decent helping hands in the form of a studio partner, a carpenter friend and some other very lovely people who have offered help.
2:I have suffered through shitty rooms and through lots tweaking and getting things wrong, I know that acoustics always takes “more work than you think” to get right. And also that it is very often, completely counterintuitive. Which is why I am bouncing these ideas off of you lot before I embark.
3: I don’t need all of the space that is available in the room currently and so I can use a fair bit of the real estate available.




The Current Plan:
Whilst I know that the accepted standard at the moment is usually a combination of Porous absorbers, waveguides, limp mass traps, Helmholtz resonators and diffusion.
I also know that the the trend seems to be something like a non-environment room with some careful managing of late reflections for location cues in the space.

Given my limitations my current thinking is something like this:

A Soft flush mount, basic wood frames, and a WHOOOOOLE lot of porous absorbers.

A depth of 400mm on the front wall, side walls and the entire ceiling, with a 600mm depth for the back wall. After that, the plan is to buy up second hand diffusors, or build in some wood stripping to the back wall and above the engineer's head so as not to make the place a complete audio vacuum.

My local options for insulation are pretty vast.

The obvious options are:
1: “Think Pink” which is the local equivalent of pink fluffy.
It comes in 135mm thick rolls of 5 x 1.2m and has a density of around 11kg/m3 and, by all accounts, a GFR between 3000-5000

2: Knauf Acoustic Batt: Comes in 100mm sheets.
From what I can tell it has a GFR somewhere between 5000 and 9000 and density of either 11kg/m3 or 14kg/m3
(https://www.knaufinsulation.co.za/produ ... ustic-batt Is the product page, if anyone has any solid numbers for this stuff it would be a blessing.)

Using a lot of this product seriously helped my previous room.

3:IM475, which is the local equivalent of OC703. It is semi rigid yellow stuff with a density of 47,5kg/m3. From what I understand it has a GFR of around 15000

4:IM24, Same as above but the density is 24kg/m3. I am not sure the GFR


First: The soft flush mount.

Doing a softflush in my previous (radically undersized) space was one of the best things I have ever done. I managed to kill most of the sbir issues as well as the vertical mode in the room.
While I dream of a hard flush, I am concerned about the cost and complexity, as well as the time limits I have before I need to be working again.
So I have decided, given the lack of available budget to speak to a real professional, to go with what has worked historically.
Though, given the good advice available here, I am hoping to improve on what was previously a mostly intuitive tweaking process.

I will be remote rack mounting the amps of the ATC’s, then stacking the 40kg speakers onto solid 40kg MDF monolith stands of the same width and depth as the speakers so that they form a sort of, floor standing monolith. I will then stacking that onto four 86x86x50mm pads of blue sylomer. I haven’t done the maths, but the local sylomer provider (who I trust) has informed me that this will isolate resonance down to 20hz very effectively (with the actual resonance of the speaker stack being around 8hz!) and still be very stable.

I will be placing these stacks firmly against the front wall and building the faux front (flush) wall around them with 38x38mm pine framing which will be stuffed with insulation and wrapped in fabric.
The front frame will be 2.4m high, approximately 400mm deep and the cloud system will be mounting to the top of that. The outer walls of the front will be angled to match the angle of the speakers.

Here is where my actual questions start:

I know, when looking to absorb deep bass using porous absorbers, that the rule is to look for good depth (more than 400mm and as deep as 900mm) and to match the GFR to the depth of the trap being used (which generally means dropping density of insulation as you get deeper). Does this same logic apply when building a soft-flush for your monitors?

I assumed, when I did my first soft flush, that squishing the insulation would be bad. But when I actually installed the faux walls, they worked best (by a lot), when I absolutely stuffed the crevices with as much, densely packed insulation as I could. I have no idea if a denser packing around the speakers and inside the faux wall around them is better in that it gets us somewhere closer to a hard mount, or if that was just situational, just because I was in a smaller room.

Are there any rules or guidelines given here? Has anyone here done this before?

Should the front faux wall be loosely packed with a good, proven, low gfr absorber like pink fluffy, or should I be cramming it in there and making it as dense as I can?

My intuition says “Cram that sh*t in there, make it dense, turn that rock wool into a fu**ing rock wall.” But, historically, my intuition is completely wrong whenever it comes to acoustics. I would really rather avoid having to staple and unstaple a bunch of fabric to find out that I am wrong though. Does anyone have any solid advice on this?

Secondly: The traps themselves.

I done a very crappy sketch of the room and my plans for it. Called “Bad Sketch”. The areas I plan to place trapping are either red or yellow and their dimensions are included. All traps will be 2,4m High.

Not shown here are the ceiling clouds. Of which there will be 5. Their dimensions will be 0,4 x 1,2 x 3,6m each. I plan to stuff those with 400mm of pink fluffy, with a 100mm airgap. The front wall will be stuffed with whatever we decide on here. The sidewalls I plan on doing with Knauff acoustic batt. The back wall will be 600mm of pink fluffy with a probably 100mm air gap.


My trapping will take up 23 cubic meters of volume, in a room of approximately 72,5 cubic meters.

I know that, in terms of space, porous isn’t efficient, but as far as I can tell, this is the most efficient way financially to get this room into tip top shape, quickly. Between myself and my helping hands, we can bang out those frames in a couple of days.

So, All my questions in one place:

1: Am I silly for thinking that I can get away with using this much porous absorption as a way to deal with such low frequencies?

(The bottom axial mode is 27hz, Which I think it is recommended to have about 900mm for effective treatment, between my front and back wall I have about 1000mm. Or does it not work like that?)

2: Counter to number 1: Am I using too much? Is it silly to think that I could use wood stripping and skyline diffusors to make the room not sound like a vacuum?

3: When doing a soft flush, should I be stuffing the surrounding structure as densely as possible or should I stick to something with an appropriate absorptive gfr?

4: How should I orient the insulation in the flushmount? I was thinking to have it always facing the speaker. Ie. I wouldn’t stack the insulation facing outwards from the wall, I would chop it and face it towards the speaker and stack it like that.

5: Can anyone see anything obvious that I’m missing. Something reliable that I can do without much guesswork?

6: Is my speaker placement likely to work? My gut feel is that I should use it as a starting point and tweak a bit before really digging in. As it stands, my head is at 37,5% the length of the room, no woofers are in a null (Nulls occurring at 1m, 2m, and 3m from the side wall in a room that is 4m wide right?). Intuitively it feels a bit far, but I have seen it written that ATC SCM50 need 2m of space to really shine (despite being phase coherent at 500mm). I am also not usually a fan of speakers placed too widely apart.

7: I’m guessing that with a soft flush, I won’t be dealing with any baffle step compensation?

Thank you SO much for your help.
Please let me know if I have forgotten anything or excluded any vital information.

[gullfo]

a couple of thoughts come to mind.

Amplitude Linearity (±2dB): 70Hz-17kHz
Cut-off Frequencies (-6dB, free-standing): 38Hz, 25kHz
Horizontal Dispersion: ±80°, Coherent
Vertical Dispersion: ±10°, Coherent
Max. Continuous SPL (1 metre): 112dB

your speakers only go down to 38hz with any level. so in a room of 72m3, you might find you don't need nearly as much absorption as you may thing.
if you have access to the facility, i strongly recommend using REW to test the space as-is to ensure you know what the modal response actually is before building.
just like the "soft soffit" approach to your baffle assembly, you might find you can achieve a significant amount of "shaping" through angling treatments rather than simply flat or parallel on the walls and ceiling.

Mike's Studio0001.jpg

Mike's Studio0002.jpg

Mike's Studio0003.jpg

[mike zietsman]

@gullfo,
Thank you for your input.

I’ve found in practice that the atc’s measurements are very conservative. In practice I haven’t seen their speakers in room where they haven’t reached down to 32hz at least. Especially when aided by a mode (which is to be expected for the bottom axial mode in this room @27hz)

Regarding the angling of the traps, has that been shown to improve effectivness?

Regarding the amount, I think I am trying to not leave much room for errors here. I would really like proper, ring free, sbir free low end response. I have never seen that in practice without quite a lot of over compensation in the acoustic design. My current fear is that this will be insufficient, or should I use the wrong material, ineffective.
The angling is a good point, what is your experience with this in practice?

I will be installing the speakers and running sweeps towards the end of the week by the way!

[DanDan]

It is refreshing to see someone intent on use a LOT of absorption. For inspiration I recommend you to check out the work of our sadly departed friend Bogic Petrovic. He has White papers published, but you can get the idea easily from his FaceBook and instagram pages. He was also called Boggy.
He made world class EBU compliant studios using cheap readily available materials with simple builds.
He included vast amounts of diffusion to complement the absorption. But there are other, simpler ways to do that. e.g. John Sayers designs have a lot of reflective wood, but note the angles. And note what lies behind them.

[mike zietsman]

DanDan,
I’ve tried to do a thorough stalk of Boggy’s build but can’t find much detail on his fb. It looks like he was using some very clever cotton roll bass traps, lots of insulation(though maybe not as much as I am looking at) and LOTS of diffusion.

I was thinking to use wood strips across the front wall, ala Jh Brandt. My strategy was to get the room almost anechoic and then add strips of wood to the front wall, measuring as I go, until the rt60 is balanced out.

Historically I like a very dry room, so I would certainly do some listening while I did it. I have managed to find some second hand skyline Diffusors for the back wall. Though I haven’t been able to work out in any way how many I need or what specs they should have.

I just know that a completely dead room will be a bit disorientating and gross to work in.

[DanDan]

Boggy was generous. There were many pictures revealing exactly what he was doing. Things are a bit of a mess after his way too early death. I have been in contact with his partner to offer help.
I would like at least to archive his IMO historic work.
Instagram may be better than FB https://www.instagram.com/bogic.petrovic/?hl=en
He also published two White Papers regarding his My Room concept.
His design intended to address the age old conundrum of us needing Anechoic at least in the early field vs real world listeners rooms.
It used readily available materials. Very affordable ones, simple light fibre batts or rolls, and cheap wooden framing.
60cm of absorption on all boundaries. Then he gradually tried to include a diffuse liveness. At first it was very sparse arrangements of wood laths. IMO I doubt if that small amount of relative area would be really audible, but I haven't been in one of his creations. They all do look lovely though. As time passed he increased the area of reflection vs absorption until we end up with the totally immersive My Room designs, Mk1 sonf Mk2. Many other designers use deep absorption fronted by reflective laths or diffusors. But none but Bogic went quite as far and as methodically and shared publicly.
His published results are EBU compliant, professional quality and performance spaces. All done with cheap local wood and attic insulation.
He also designed and made his own loudspeakers. I investigated buying a pair of his three ways. He told me shipping from Serbia was simply not possible. Then he offered me the entire design, from electronics to Wood patterns, gratis. Boggy was literally THE MAN. For inspiration regarding reflective wood fronting do take a look at the Gallery of John's studios here.
Also Rod Gervais work at Bunker Studios https://thebunkerstudio.com
Those two in particular seem to have a great eye for making beauty out of the tech.

[Paulus87]

a couple of thoughts come to mind.

Amplitude Linearity (±2dB): 70Hz-17kHz
Cut-off Frequencies (-6dB, free-standing): 38Hz, 25kHz
Horizontal Dispersion: ±80°, Coherent
Vertical Dispersion: ±10°, Coherent
Max. Continuous SPL (1 metre): 112dB

your speakers only go down to 38hz with any level. so in a room of 72m3, you might find you don't need nearly as much absorption as you may thing.
if you have access to the facility, i strongly recommend using REW to test the space as-is to ensure you know what the modal response actually is before building.
just like the "soft soffit" approach to your baffle assembly, you might find you can achieve a significant amount of "shaping" through angling treatments rather than simply flat or parallel on the walls and ceiling.

Mike's Studio0001.jpg

Mike's Studio0002.jpg

Mike's Studio0003.jpg

Glenn, really nice mock up. I am interested in your opinion of angled absorbers vs flat against the wall absorbers of the same thickness and materials. Other than aesthetics what are the main benefits of angling them?

The only one I can think of (and it's a big one) is that they will absorb lower down the spectrum than if they were just flat against the wall, since the depth will approach a little closer to the quarter wave length of certain frequencies at certain distances... however, at frequencies where the sound waves are larger than the size of the panels the sound waves will diffract around the panels instead.

Please enlighten me.

Paul

[gullfo]

it's just an example but in general the face of the angled panels will have some surface for reflection control, and the depth etc needs to be defined yet. angled absorption isn't simply for aesthetics - there is some broadband effects to it although once you're beyond 6-8" it's lessen although gfr comes into play for mids.

in many cases i'll build an angled unit which goes all the way the wall, is sub-divided internally, and has some slat pattern for diffusive effect but may also be sealed for broadband resonator effect. it's a light-weight way to get a room set up without being a fully integrated construction project (such as when i design full baffle systems, etc)

also, using a ceiling mounted soffit for absorption and hvac, and ceiling clouds, there will be significant absorption already in the room. clouds will have some level of solid facing and may also be hard-backed to facilitate some tuning based on suspension and angles.

my best guess - limiting the amount of "construction" to alter the room and go with key absorption spaces (corners, ceiling-wall corners), reflection control, and test, analyze, plan, and adjust as needed. easier on the budget as well.

[Paulus87]

it's just an example but in general the face of the angled panels will have some surface for reflection control, and the depth etc needs to be defined yet. angled absorption isn't simply for aesthetics - there is some broadband effects to it although once you're beyond 6-8" it's lessen although gfr comes into play for mids.

in many cases i'll build an angled unit which goes all the way the wall, is sub-divided internally, and has some slat pattern for diffusive effect but may also be sealed for broadband resonator effect. it's a light-weight way to get a room set up without being a fully integrated construction project (such as when i design full baffle systems, etc)

also, using a ceiling mounted soffit for absorption and hvac, and ceiling clouds, there will be significant absorption already in the room. clouds will have some level of solid facing and may also be hard-backed to facilitate some tuning based on suspension and angles.

my best guess - limiting the amount of "construction" to alter the room and go with key absorption spaces (corners, ceiling-wall corners), reflection control, and test, analyze, plan, and adjust as needed. easier on the budget as well.

Thanks for the reply.

The reason I am particularly interested in this is because I came up with a design concept for my control room involving angled absorption fronted with BAD panels, or slats on the side walls. Each panel is a slightly different width, different depth from the wall and different angle. The idea being to deflect reflections to the rear, absorb some low mids and lows, and maintain the operators' self noise cues.

Since you are someone with a wealth of knowledge and experience, I would really appreciate it if you could take a look and provide some feedback:

http://www.johnlsayers.com/phpBB2/viewt ... &start=270

Continued over the next couple of pages.

Sorry to hijack the thread, but it may be helpful

[gullfo]

here's an example. i think the components are dynamic and live.

paulus87 example.jpg

[Paulus87]

here's an example. i think the components are dynamic and live.

paulus87 example.jpg

Thank you for the mock up, it's a very cool model! I commend you for always being so generous with your skills.

One thing I have not been able to find the answer to is whether or not angled diffsorbers would deflect some destructive reflections to the listening position? If it was simply a reflective panel angled sufficiently then I would expect specular reflections to miss the listening position, but diffusive slats/bad panels, even if angled significantly may "spray" the listening position in some destructive reflections?

Thoughts?

Paul

[gullfo]

it's all possible but then again, it's about the general room modal response, significant reflection reduction, and your adaptation to the room, regardless of perfection or lack thereof, in creating mixes which translate. your desk, equipment, furniture, people, etc are all more likely to impact your space than some weak diffusive reflections from the walls or ceiling. worst case reduce the amount of reflective surfaces initially and see. easy enough to stand up some dummy panels and tweak angles and see how the audio reacts (even before doing any significant treatment, it's a reflections proof of concept). can mock up a desk top as well.