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Building a small house for mixing in
Old 14th April 2019
Here for the gear

Building a small house for mixing in

I know there are some threads about similar situations, but I have som ideas i think some of you will be interested in talking about so I'm starting a new one.

I plan to build a small building only for mixing in.
I live in a remote area. no traffic. no neighbours. I will record in the existing house that has an acoustically treated room will 4m ceiling 35m2 space.

The new building will have the outer dimension: H 3m W 4m L 5.5m

I will treat the room with bass-traps and broadband absorbers in the right places.

I will build using a reinforced concrete structure 9 pillars, slab floor slab roof.

My questions:

1. Material for walls. Choosing between solid concrete blocks filled with concrete, or perforated clay bricks. Since it is small and stand alone in the woods I could let some bass escape/ leak instead of trapping. This could be a good idea since its a small space and modes will be tricky to treat if all the bass is kept inside. Could this be done by replacing some of the concrete blocks with holes and covering from the outside with slate, or even plaster. Or I could build with perforated clay bricks with the openings facing into the room. I would then plaster on the outside giving me a outer wall effectively only consisting of plaster ( which will let the bass out... ) the inner wall will consist of hundreds and hundreds of 20cm deep wells ( the openings of the perforated bricks ) would this give a useful diffusion effect? I would cover most of the walls with mineral wool and fabric in this case, but I imagine ai would need to fill less of the space with treatment material since the bass will mostly leak out and the wells formed by the bricks would act as space behind the absorbers... (?) in this senario I could invest most of the mineral wool in turning the entire ceiling into one big absorber making it invisible acoustically.

So the main choice is between building with solid concrete and leaving openings (covered on the outside with slate rock or plaster ) along the modal resonance areas, or building with perforated clay bricks.

Thanks in advance for your input to the project!
Old 15th April 2019
Here for the gear

I'm in a terribly remote area... so no acoustic experts around here... help is appreciated... best way to vent the bass?
Old 16th April 2019
Lives for gear

This could be a good idea since its a small space and modes will be tricky to treat if all the bass is kept inside.
Modes are caused by the dimensions of the room, and they won't disappear if you just cut holes in your walls: they will still be there. The only way to get rid of the modes in your room, is with a bulldozer.... knock down the walls! Besides, "getting rid of modes" is a bad idea anyway: the entire problem with modal response in small rooms is not that there are too many modes in the low end: the problem is that there are not enough modes in the low end! If only you could get MORE modes into the low end, that would considerably improve the overall acoustic response of the room. But getting more modes into the room is also impossible, because modes are related to the dimensions of the room. The only way to get MORE modes in there, is to move the walls further apart and raise the ceiling.

So you don't have a lot of choice here: you can't add more modes, and you can't take any away. All you can do is to damp the modes, such that they don't ring and resonate too long, and don't cause excessively extreme peaks in your frequency response. There are several ways of doing that, and one of them is, indeed, to use the walls as absorbers, tuned to the resonant frequency of the lowest axial mode in each direction. This only works for one mode each way, of course, so you need to chose carefully which one you want to treat. Usually, it will be the first axial each way, but not always. It depends on how the room actually turns out, and unfortunately, the only way to be sure of that, is to build it! At which point it is too late to tune your walls, because you already built them...

You can tune them a bit more broadly, and hope that you manage to hit the right frequency. It is possible, but it takes careful planning. And careful construction. Basically, you just build your walls as tuned panel traps (membrane traps), choosing your materials and dimensions carefully so that each wall is tuned to the lowest axial mode that you expect will affect it.

But even then, you will still need bass trapping on those walls, to deal with the other modes (higher order axials, all the tangentials, an the obliques), as well as the other low frequency issues that always affect small rooms: SBIR, reflections, phase cancellations, etc.

- Stuart -
Old 16th April 2019
Here for the gear

Hi Stuart, Thank you very much for your knowledgable reply, I really appreciate it!

So this talk about buildings with thin walls that let much of the bass go through the walls having less issues with frequency respons becoming terribly uneven in the lower register isn't true? It is my empirical impression that you do get much more issues with bass in a basement than in a tent, just to mention two extremes

Just to see if I got your answer right: its mostly the shape that creates modes regardless of the material? And using thinner material ( more transparent to bass frequencies ) in areas where modes are known to build up is not going to change that?

Old 16th April 2019
Lives for gear

Modes form between walls. Modes are standing waves that are set up when the wavelength fits in exactly between the walls of the room. The simplest modes are axial modes, that form between the parallel walls on opposite sides of the room in any of the 3 main directions (front-to-back, left-to-right, ceiling-to-floor). But there are also tangential modes that form between any four surfaces (for example, the front and back walls and the ceiling and the floor), as well as oblique modes that involve all six surfaces. A mode is just a path around the room that a sound wave can take, then arrive back where it started, going in the same direction, and with the same phase.

So it is the walls that create the modes, and it is the distances between the walls that sets the frequency. Modes form only at very specific frequencies, set by the dimensions of the room, and they can easily be calculated using simple equations. Or just use one of these two calculators:

amroc - THE Room Mode Calculator

Both of those are very good, and will help you to decide how best to build your room. They give you tons of information that is really useful to help figure out the best dimensions.

So it is the walls themselves that create the modes. The simple fact of having a hard surface with mass, is what does it.

Yes, if you make the wall very light weight, or cut holes in it, then the intensity of the mode could be reduced, but it will still be there.

It is my empirical impression that you do get much more issues with bass in a basement than in a tent, just to mention two extremes
True, but that applies to all frequencies, not just bass. It's just more noticeable with the bass. The reason is very simple: the more massive the wall (more dense material), the more it resists sound getting out. But it doesn't destroy the sound: it just reflects it back into the room: So the sound that could not get out, stays inside, where it continues to bounce around inside the room for some time, before dying away. Thus, better isolation = worse room acoustics. As I said, it's more noticeable in the low end as you use more and more dense materials. Conversely, for very light weight materials (low density) such as tent canvas, pretty much all of the sound spectrum is going through, with almost none of it bouncing back and staying inside. Modes would be very slight in a tent-control room.

Now, the question is: do you want to mix in a tent? Probably not. You'd have no modal trouble, sure, but you'd have no diffuse field either, and it would be unpleasant to mix in there for long sessions, just like it is unpleasant and fatiguing to mix in ANY type of acoustically dead space. Your ears and brain expect a natural, neutral sound, with a well defined low-level semi-diffuse field, or hopefully a true low-level diffuse reverberant field (if the room is large enough): it's part of what makes a good mixing environment. To get that, you need walls.

There are other reasons why you don't want to let ALL the sound out. For example, if you did, you'd need very much more powerful speakers since there would be nothing at all coming back at you... Plus unwanted sounds coming from the outside.

So, you have the problem of how to balance the two: designing your walls to absorb enough bass while at the same time producing a usable diffuse field. Not easy to do.

If you don't believe what I'm saying, then it's fairly easy to prove it to yourself. Set up your speakers and DAW in the middle of a very large field, with nothing around you for dozens of meters on any side (get a very long electrical extension cable.... ), and try to mix like that. It won't be long before you find it unpleasant, and difficult to mix. You still can mix, yes, but you need to concentrate a lot harder, since there's no natural room sound around you to give your brain the clues it expects.

You do need some type of wall around you to create the psycho-acoustic conditions that produce a pleasant, non-fatiguing listening experience. Finding the balance between that need and the attempt to "dump" sound out through the walls, is a tough call.

- Stuart -
Old 4 weeks ago
Here for the gear

Hi Stuart,
Very clarifying! Thanks!
Great links also.
I am calculating now.
no need to go out in the rain to set up my gear in a field since the info you passed has already elucidated the matter.
So basically you see little benefit in using materials with varying density for different walls, or could that help to smooth things out a bit? would it add any useful diffusion to do so?
Thanks again!
Old 3 weeks ago
Here for the gear

So... summing it up... no use to build some openings covered with a thin material into the corners to releive som om the sound pressure in the low frequencies and thus make room treatment easier?

I was considering substituting some of the concrete blocks with pvc tubes filled with rockwool for instance.
Old 3 weeks ago
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bowzin's Avatar
Acoustics is pretty NON-intuitive... what "seems like it should work" usually doesn't. So I would just recommend not getting too fancy or creative, because that's a bit like getting lost in the woods: you'll probably end up making a big circle and be right back where you started. If you're building from scratch... you have an incredible opportunity. To maximize that opportunity, you'd need to hire a professional acoustician to consult, preferably at the earliest possible stages. They don't need to be local, they can do everything remotely these days. Or you can take a stab at it yourself, and realize you won't be maximally efficient or achieve the best results, and still be ok with that. Pick up the book "Home Recording Studio: Build It Like the Pros 2nd Edition" by Rod Gervais, if you haven't already.
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