Full rear wall bass trap, balancing absorption and diffusion

[jaminjames]

As it stands now the space is 19 ft long and 17ft wide. The ceilings are vaulted, 8 feet at the walls, 13ft at the apex. It is a two car garage. Construction will begin sometime in the beginning of next month. The only thing tracked in the space will be vocals and piano. It will not be a room focused on mixing, though I do rough mixes on most everything I work on. I mostly do film scoring and commercial music and predominately work with keyboards and synths, sample libraries, drum machines, etc. I will be running lines to a separate space for recording amps. I record bass direct 99% of the time. I prefer to not work at loud volumes, though I'll reference fairly loud several times a day for a minute or two. I have never measured, but most of the time I could easily carry on a conversation with someone sitting next to me during playback. My desire is not to have a perfect room. I don't think I like perfect rooms anyway. I want a functional room that sounds pleasing to listen in. If there are some bumps or dips here and there, so be it. I don't mix the material I work on and the mixes I get back sound great... and the space I'm in right now is FAR from perfect. My budget is $15,000 but can be pushed to $20,000 max. This has to include the cost of HVAC, decorating, permits, etc. I'll be doing a lot of the work myself. The contractor will be doing the framing though.

The exterior is composed of wood shingle siding and 3/4" solid wood sheathing on 24" oc stud wall. The exterior wall stud cavities will be insulated with 2" Roxul comfort bats for thermal insulation (after they are caulked for air tightness).

A second wall and ceiling will be framed out with a 3" air gap, 2" of Roxul Safe N Sound, and then 2 layers of 5/8 dry wall.

The rear wall will be sightly different. There are existing garage doors that I want to keep for exterior appearance reasons. Those will be sealed air tight and insulated, then a second wall will be built with an air gap and filled with Roxul. Beyond that will be 24" of R30, and then a fabric barrier.

The front corners of the room will have large floor to ceiling super chunk traps.

There will be 6ft by 2ft Roxul absorption panels 2" thick spaced 2" from the wall, placed in the needed spaces around the room for reflection points and overall absorption. I'm guessing around 8, based on rough calculations.

A cloud will be constructed over the desk, 4" thick, made of Roxul. As of now I don't have any ceiling absorption plans, I want to hear the room first before I do anything to ceilings. The floors are stained and polished concrete with area rugs. The windows will be built over.

After build out, the room dimensions will roughly be 16.5ft long (2ft of that consisting of bass trap) and 16ft feed wide. I'm not exactly sure on the ceiling dimensions because I'm having an architect restructure the truss system so the vaulted ceiling are completely open. As of now they have a farm house style truss system. I will be losing about 6 to 8 inches I'm guessing.

Which brings me to my question. The back wall will be a bass trap, 16ft wide and 8ft high (floor to ceiling line, the gable will not, at least at this point, we'll see though). I'm afraid leaving the entire surface fabric will suck too much high end out of the room. I was thinking 50/50. My plan was to construct the fabric wall and then an inch or two out from that build wooden 2 inch slats spaced 2 inches apart at varying angles. This would give me around 50/50 absorption/diffusion. Well, I understand it's not true diffusion, more reflection. I'm looking in to finding some kind of formula for this kind of thing for true diffusion.

Outside of my question about the bass traps, are there any glaring problems with my design ideas? I have a friend who is way in to room design and taking measurements. He's going to measure once the walls are up just to make sure there aren't any HUGE problems, and then we'll work from there.

[Gregwor]

I'm looking in to finding some kind of formula for this kind of thing for true diffusion.

Here is the closest thing to a formula to create what you're describing:
http://www.johnlsayers.com/phpBB2/viewt ... c&start=15

are there any glaring problems with my design ideas?

You haven't REALLY designed anything yet. You've just stated some ideas about what you think MIGHT work. Until you draw everything up in Sketchup (each stud, each piece of insulation, each piece of MDF) and do every single calculation, you haven't designed your space. Once you start drawing it, you'll find issues and then you can research more or ask questions.

Greg

[Soundman2020]

Hi there " jaminjames", and Welcome!

Construction will begin sometime in the beginning of next month.

Probably not a good idea! Way too many people want to rush blindly into building their studios, with great enthusiasm but little to no preparation. Enthusiasm is fine, and a good thing, but lack of preparation is a terrible, terrible idea for building a studio.

As Greg pointed out, you don't really have much of a design yet! Just a few thoughts jotted down on paper, and a very basic SketchUp model that shows only the outline of the building. A studio design needs to show every single part of the entire studio, including the existing parts and the new parts that will be added, in detail. That's the only way to catch mistakes, problems, errors, and unexpected situations that you didn't see coming initially.

The design process starts with a number: how many decibels of isolation you need. That's the most basic parameter. So get out your trusty hand-held sound level meter (or buy one if you don't have one yet: it's a key tool in your studio build and tuning), and do some measurements to figure out what your number is.

Based on that number, you can start looking at building materials and construction techniques that will get you that number. There are equations for calculating that, or if you don't like math then there are tables and charts and books with images of numerous isolation designs, showing the level that they will attain, and the details of the materials. Right now you are guessing: You have no idea at all if "framed out with a 3" air gap, 2" of Roxul Safe N Sound, and then 2 layers of 5/8 dry wall" is going to provide the isolation you need: Is it enough? Too much? Insufficient? There is no way of knowing that, since you don't know what your goal is! So first define your goal, then choose the path that will get you there.

I mostly do film scoring and commercial music and predominately work with keyboards and synths, sample libraries, drum machines, etc.

Fine. But at what level do you track and mix?

I prefer to not work at loud volumes,

Define "not loud"? What you consider "not loud", your neighbor might consider "deafening", and a session drummer might consider "way too quiet". "Not loud" is a subjective opinion, but there's no place for plugging "opinions" into the equations for calculating wall mass or air gap depth. You can only plug in real numbers, like "50 dB isolation" or "30 dB" or "70 dB", or whatever your number turns out to be.

I have never measured,

Well, then, the time has come for that! Measure with "C" weighting and "Slow" response set on your meter.

My desire is not to have a perfect room.

That's rather strange.... So you don't mind if the room lies to you? You don't mind if you are not actually hearing what the true sound of each instrument is? You don't mind of the room colors the sound, adding some tones and reverb that weren't in the original, while sucking out other tones? You don't mind if the directionality is all screwed up, so your stereo balance is off? I get it that you don't need to do precision final mixes for big-name blockbuster albums, but you still do need to hear what the speakers are actually telling you!

I don't think I like perfect rooms anyway.

That's even stranger... I've never met a tracking or mixing engineer who didn't want his mixes to sound good. Very unusual...

I want a functional room that sounds pleasing to listen in.

You might want to take a look at the document ITU BS.1116-3 (google it) to understand why a room that "sounds pleasing" is a terrible room to mix in... If it "sounds pleasing", then it is distorting the true sound coming out of your speakers. Control rooms should not sound pleasing: they should have no sound at all of their own. It beats me how musicians, producers, or engineers would want to work in an acoustic environment that mangles the true sound. Sorry, but I just don't get that concept at all.

I don't mix the material I work on and the mixes I get back sound great...

So you want the mastering engineer to fix your mistakes? You want to send him sub-par material that was poorly tracked, then get him to try to improve it in some way? I suppose some people do work that way, but it's not what I would recommend. I don't see too many of my clients asking me to design their studios like that. Mostly people want to turn out the best work they can, then hand it off to the final mix engineer or mastering engineer to put the "cherry on top", so to speak. A cherry on top of a pile of garbage doesn't make it into a cake: it just makes it look prettier. But a cherry on top of an amazing cake, makes it even more amazing.

The exterior is composed of wood shingle siding and 3/4" solid wood sheathing on 24" oc stud wall. The exterior wall stud cavities will be insulated with 2" Roxul comfort bats for thermal insulation (after they are caulked for air tightness).

Will that provide enough mass for the MSM resonance and isolation that your wall needs? Did you check that, or are you guessing?

A second wall and ceiling will be framed out with a 3" air gap, 2" of Roxul Safe N Sound, and then 2 layers of 5/8 dry wall.

Ditto. Same question: Will that provide the actual isolation that you need, or did you just guess? What happens if it turns out that you really should have used a 5" gap, or a 2" gap, or that only 1 layer of 5/8 was needed... or three layers? How can you be sure that your plan is right, if you don't really have a plan?

There are existing garage doors that I want to keep for exterior appearance reasons. Those will be sealed air tight and insulated, then a second wall will be built with an air gap and filled with Roxul. Beyond that will be 24" of R30, and then a fabric barrier.

You didn't mention what those doors are made of, but assuming the typical thin sheet metal or light wood, your plan is not going to provide much isolation. That will be a serious weak point in the isolation of your studio. It would be better to deal with those doors in the usual manner.

There will be 6ft by 2ft Roxul absorption panels 2" thick spaced 2" from the wall, placed in the needed spaces around the room for reflection points and overall absorption. I'm guessing around 8, based on rough calculations.

I'm not sure what equation you used to calculate that, but it is WAAAY off. By a factor of about 200%. You will need around 500 sabins in that room, and you have less than 230, from what I see.

In addition, 2" is not enough for first reflection points, and having all the absorption mounted identically implies that it will all be concentrating on the exat same frequency ranges, so you will have be over-doing that range while "under-doing" other ranges. How did you determine that 2" of Roxul over a 2" air gap is the right frequency range for your room? And what type of "Roxul" are you talking about? They make quite a few different products....

A cloud will be constructed over the desk, 4" thick, made of Roxul.

Why? How big? Located where? What angle? Hard backed or not? Why 4"? Why Roxul? Which Roxul product?

These are not trick questions: they are merely meant to draw your attention to all the very important details that you don't seem to be taking into account. Control room acoustic response doesn't just happen by chance: it should be designed in from the start, then tuned during the build.

As of now I don't have any ceiling absorption plans, I want to hear the room first before I do anything to ceilings.

And how would you know by "hearing" where to put the treatment, how to tune it, or even what type? Ears are fallible, and do not have the resolution necessary to tune a control room. Your ears may well tell you "something is wrong here in the bass", or "there's flutter echo", or "the mid range is wonky", but they CANNOT tell you what the exact problems are, not how to fix them. Your ears are incapable of informing you that there is an SBIR null at 108 Hz caused by the floor bounce from your speakers, or that the bass build-up at the mix position is a first-order vertical axial modal problem at 70.6 Hz combined with a second-order cross-wise axial mode at the exact same frequency (which you actually will have in your room, due to the poor selection of dimensions....). Acoustic prediction can tell you what to expect, and acoustic analysis can confirm if the predicted problem really is an issue or not. Your ears cannot.

After build out, the room dimensions will roughly be 16.5ft long (2ft of that consisting of bass trap) and 16ft feed wide.

So you plan to build a square room, where all the axial modes in the length direction will fall at the exact same frequencies as the axial modes in the width direction, and also line up with the axial modes in the height direction, assuming you have an 8 foot ceiling (at least partially)?

You really should do some research modal response, dimensions, and room ratios. You are dooming your studio to failure before you even start by making it square.

The back wall will be a bass trap, 16ft wide and 8ft high

Why? Made of what? What density? What Gas Flow Resistivity? Did you check that this is what you need for your room? Does it fully treat the acoustic issues in the low end? Will it also solve your SBIR issues?

I'm afraid leaving the entire surface fabric will suck too much high end out of the room.

Yes it will. Undoubtedly.

I was thinking 50/50.

Why? Did you calculate that, or are you just guessing?

and then an inch or two out from that build wooden 2 inch slats spaced 2 inches apart at varying angles.

So you only want to recover the very high end of the spectrum above about 7 kHz? Why don't you want to recover the rest of the lost high end? Or the lost mid range?

This would give me around 50/50 absorption/diffusion.

Well,not really... it would give you 50/50 above 7 kHz, but progressively less the lower you go. There would not be much reflection at all below about 3 kHz with that plan...

Well, I understand it's not true diffusion, more reflection.

Correct: it is not diffusion at all. It is only partial reflection in the very top end.

I'm looking in to finding some kind of formula for this kind of thing for true diffusion.

What type of diffusion do you want? Skyline? Schroeder? QRD? PRD? BAD? At what frequency range do you want it? There is no simple equation for numeric diffusion: it's quite a bit more complex than that. You best bet is to download a diffusion calculator, plug in your "n" number and the frequency range, then let it tell you what pattern of wells you will need, and how deep each one needs to be.

Outside of my question about the bass traps, are there any glaring problems with my design ideas?

I think I've outlined some of them, but there's going to be many more as your design progresses.

But I'm curious about one thing: You say that you don't really want a well-tuned room, and you don't care if it is acoustically good or not, so why are you planning on all this treatment? Why not just hang stuff at random around the room, without even thinking about it much, or measuring it? After all, if you don't need a properly tuned room, then it doesn't really matter how it turns out!

Yeah, that sounds a little harsh, "bitchy" even, but the point once again is to grab your attention: Your plan is not a plan: it's just some thoughts. To get decent acoustics, you really do need to plan the room properly, starting with the isolation, then the dimensions (modal response), then the layout (orientation, rough positioning of doors, windows, and major furniture, sight lines, traffic flow, etc.), then the geometry (locations of the speakers and the mix position, including exact positions and correct angles), then the initial treatment design (based on predicted modal response and overall room acoustic response, plus the usual "rules of thumb"), then the HVAC (which is a huge subject, all on its own), then go back to the start and double check each part again, to fix conflicts.... THEN start building it. And test it at each relevant point in the process to make sure the treatment is working as expected, then modify the treatment plan if not.

Here's an example of how the room-tuning process normally goes. This one is in progress right now: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21368 And here's an example of a very well treated and tuned control room, to show you what can be achieved with very careful work: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 I'm not saying that you need to tune your room to the same extent as these, but that's the general end goal for a control room, regardless of the genre. Control rooms should never be built to "sound pleasant". They should be built to "sound neutral", neither adding to nor subtracting from the sound sent out by the speakers. In other words, the room should sound "transparent" and "natural", as though it weren't there at all.

I have a friend who is way in to room design

This might seem like yet another silly question, but if you already have someone who is "into room design", then why did he not tell you about all these things that are wrong with your plan? I assume that you outlined roughly the same concepts to him, and he OK'd it all? Is that the case? If so, then that would be a red flag for me.... Also, since you have this friend who apparently knows all about room design, why did yo come here to the forum? Is it that you don't trust his advice? (wise move, apparently). Or is it that you want to compare our advice to his, and see which you like more? Or is it that you didn't actually ask him about it at all? I'm not understanding why you would come to us here on the forum, if you already have an experienced room designer working on your project....

He's going to measure once the walls are up just to make sure there aren't any HUGE problems, and then we'll work from there.

Once the walls are up and he makes that first measurement, I would sincerely hope that you DO find a whole bunch of "huge problems"! That's the entire purpose of making the initial baseline acoustic measurement in the room! It shows you exactly where all of your huge problems are, so that you can then treat them. If you did the baseline test and there were NO huge problems, I'd be very worried: I'd be concerned that the tests were not done properly at all.

The entire and complete purpose of EVERY acoustic test is to see what the problems are, identifying them in both frequency and time, as well as 3D space, so that they can be dealt with. If you plan to do tests and expect to see no problems, then you are going to be rather surprised! I would VERY much hope that your initial tests show up huge problems such as you can see in these graphs, from a totally untreated room:

FRCAUS-REW-FR-20..20k-1..48-BASELINE.png

FRCAUS-REW-WF-20..500-1..48-BASELINE.png

FRCAUS-REW-SP-20..500-1..48-BASELINE.png

If your initial tests don't look something like those, then there's a major problem.


Sorry if I've come across as being highly critical and harsh, but to be honest, that's what we are pretty good at , here on the forum! Getting the attention of people who are not going about things the best way, and persuading them to re-think what they are doing, and hopefully decide to do it the right way. I'm sure you can to the forum because you want your room to turn out well, as good as it can be, and that's what I'm trying to show you: the plan you have right now is not going to lead to that outcome. Hopefully, you'll trust the advice you are getting (after all, you DID ask! , and take the time to do it right. I'd love for your place to turn out great, even if you hate me for the comments here!


- Stuart -

[Gregwor]

Stuart,

Quote:
and then an inch or two out from that build wooden 2 inch slats spaced 2 inches apart at varying angles.
So you only want to recover the very high end of the spectrum above about 7 kHz? Why don't you want to recover the rest of the lost high end? Or the lost mid range?

I've searched high and low (maybe in the wrong places or searching using the incorrect terms) and I cannot find ANY equation to determine frequencies affected by differing sizes of slats and slots. Everything I've read from all the big names in town are "percentage is everything", "make it look nice", "try not to have slats at ear level, "place slats up high and/or down low". That's it. It's really driving me crazy.

PLEASE HELP!!!! hahaha

Thanks as usual!

Greg

[Soundman2020]

I've searched high and low (maybe in the wrong places or searching using the incorrect terms) and I cannot find ANY equation to determine frequencies affected by differing sizes of slats and slots

Hint: Wavelength! A general rule is that sound waves are only affected by objects the same size as the wave, or larger. It's not a hard, sudden cut-off though: its more like the center point of a curve, with shorter and shorter waves being reflected to a greater extent, and longer waves to a lesser extent. Think of it this way: if I put a full sheet of plywood in front of you, it's going to reflect pretty much the entire spectrum. If I cut off a strip off that just 2" wide, it's only going to have an effect on the high end, mostly.

But waves can still diffract around the edges of slats, to various degrees. Hence, rounded edges (such as you see on Frank's up-coming "slotted poly") can be useful, if you want better control.

That's for un-tuned devices. Tuned devices (with sealed cavities) can behave a bit different, depending on the percent open area, slot/slat sizes, and cavity depth, relative to wavelength, internal volume, damping, etc.

It's complicated....

(Which is why it needs careful design, and careful planning...)


- Stuart -

[Gregwor]

Thanks Stuart. That makes perfect sense! I'll dig to see if there are any set-in-stone values for cut off frequencies, their order, and the diffraction traits.

Tuned devices make quite a bit of sense after reading a ton on how they behave. The Helmholtz excel file sure is handy. I think the only part that seems to be up in the air with them is the density and thickness of dampening insulation in relation to the bandwidth of frequencies it's tuned to. I'll have to find some time to see if there are any definitive values for that. Playing the guessing game is never fun (mind you, up until a few years ago, understanding hangers and how exactly their placement affected their performance was a guessing game!)

Anyway, thank you for clearing that up man. You're a legend.

Greg