Bulding LF absorbers/ Do I really need the wooden frames?

[clarkf]

Hi guys.

I am just about to move in to a new room, and I want to treat it so my mixes translate properly.
My first operation will be to build some "bass traps", using very dense Rockwool and breathable fabric (to reflect M and HF).

My question is this:
The wooden frames around my LF absorbers,
are they for sturdiness, or do they contribute to frequency absorption?

[Soundman2020]

My first operation will be to build some "bass traps", using very dense Rockwool

That would not be a good idea: Contrary to popular belief, very dense mineral wool is not good for LOW frequency absorption: it is good for HIGH frequency absorption (provided that it isn't TOO dense, of course, in which case it isn't good for either...)

For low frequency absorption, you need to use LESS dense materials.

In any event, the factor is not really density at all: the factor is gas flow resistivity. That is the characteristic of the insulation that defines how well it absorbs. There is a rough relationship between density and gas flow resistivity, but it isn't entirely linear, and it varies from one material to the next. For mineral wool, you should be using a density of around 30 to 50 kg/m3.

and breathable fabric (to reflect M and HF)

If it is breathable enough, then it wont reflect much of the mids, or the highs. Better to put a layer of plastic across the front of the mineral wool. That will reflect the highs back into the room, while allowing the lows through.

The wooden frames around my LF absorbers, are they for sturdiness, or do they contribute to frequency absorption?

Sturdiness. In fact, they subtract (slightly) form the overall absorption, since they cover the edges of the mineral wool. That's why many people cut large holes or huge slots in the wood. It exposes the sides to the room, while still providing structural strength to hold the device together.

OK, I'm assuming that you are talking about ABSORPTIVE BROADBAND bass traps? If you are talking about some kind of TUNED trap (Helmholtz resonator, panel trap, etc.) then things are entirely different. In that case, the frame MUST be sealed airtight, and the frame must also be sealed to the wall behind it (or have a sealed back on it).

Tuned traps and broadband absorptive traps operate on two different principals, so broadband absorption should expose as much surface as possible to the room, have no back on them, and be spaced away from the wall. Tuned traps must be sealed airtight, and must either have a back on them or be sealed to the wall, and of course must be right up against the interior surface of the room.


- Stuart -

[clarkf]

WOW.. This is awesome!

You are correct, I guess I am talking about absorptive broadband bass traps.
I want a flat and well translative room, and I figured I needed to start with those, after my monitors are placed correctly.
My room is pretty small, and almost cubistic. (Worst case scenario perhaps, but even slight positive changes are still positive.)

So, If I understood you correctly.

I need mineral wool of a specific density (30/50kg pr m3) to get the wanted amount of gas flow resistivity.
To reflect the mids and highs back into the room, its better to use plastic than material which is TOO breathable.
The wooden frames are mostly for sturdiness, so the more insulation exposed to the room, the better.
AND, I should mount the panels some what away from the walls to get better results?

(Were now talking about LF absorbers to remove unwanted peaks in bass frequencies in my small room.)

Thanks, Stuart!

[Soundman2020]

Yep! You seem to have understood me perfectly!

The only thing that I would add is this: you really should measure the response of your room to find out where your issues are before you put any treatment in it, then put in the basic treatment (large "superchunk" style bass traps are good) and measure the response again. Then you can see what has improved and what has not improved, so you can decide on the second round of treatment.


- Stuart -