Quick question about bass traps

[Addni]

I'm finally going to treat my room (now that I have some income from recordings in the sight) And I was thinking about 2 bass traps in the corners behind the monitors, and then just some early reflection absorbers.

I've gotten info from a local hardware/building supplies store about some rock wool.

The units I'm checking out are 120x60cm, 120kg/m3.

They have them in three thicknesses, 1" (where thera are 8 plates in each package), 2" (4 plates) 3" (3 plates) and 4" (2 plates).

the 2" is the cheapest one, with the 4" being just a few kronur (few cents) above it.

For the early reflection absorbers I was thinking about 2" panels

Then, for the corners, I had three Ideas. (look at attached photo) pricing is a bit of an issue, so most for the money is prefered.

The first Idea was just a 60x120 2" or 4" thick panel, 45% over the corner, from floor to the top.

The second idea is a 60x120 2" thick panel in the corner, with another 30x120 2" thick panel behind it.

The third is a 30x30/2 Superchunk bass trap. A 60x60/2 superchunk is a bit to expensive. But with a 30x30/2 I'm only using 1/4 of the material I would be for the 60x60/2

So wich of those would give me the best results for the money, or should I really just go with a 60x60/2 trap ?

Need a response as quickly as possible, since I'll be doing this pretty soon (well, I hope at least)

[Soundman2020]

... local hardware/building supplies store about some rock wool ... 120kg/m3.

That's pretty dense! You don't really need anything that dense. Around 50 kg/m3 is plenty already. You might be able to save yourself some money there.

The third is a 30x30/2 Superchunk bass trap.

That's probably not wide enough. Some folks say that 90 cm width of the bass trap front is where it really starts to be effective, and 60 cm is the minimum where it does anything useful. So 30x30 isn't going to e lot of help. I'd aim to get the largest diagonal coverage you can, at least 60. So I reckon #2 would be the way to go, but make them both 4", or at least the one of them 4" and the other 2". For bass trapping, you need lots of it. More surface area is better. Thicker is better.

- Stuart -

[Addni]

thanks alot for the reply, I'll check into some less dence (is there a word in english for "less dense"? ) wool tomorrow.

Maybe I should go with drawing B, but turn the panels sideways, thereby making the outer plate 120cm and the inner 60cm. both 4" thick

I checked the catalog and they have 30kg/m3 and 80kg/m3, i think I'd rather go for the 80kg since it's much easier to handle I thin. I will get the prices tomorrow

[Ro]

Stu's right. a density of 120kg/m3 is way too high. Tho for acoustic values, density doens't really matter. To 48kg/m3 has proven to be the best average. I can imagen the denser type will cost you more. Go with "regular" stuff. 30kg/m3 will do fine.

For a first reflection panels the 2"will do. Remember thicker panels will absorb better in the lower frq regions. so 2 panels in a corner works better than a single sheet. Unless the single sheet is as thick as 2 seperate sheets (for example 2x2" or 1x4")
Just add sheets together to get more volume. (don't use any paper/foil-backed wool)

[Addni]

Ro:

So I'd be just as well of making a single 4" panel, than two 2" panels ?

Also, about the angle of the corner traps.. Do different angles make difference ? For example should i just keep the panels at a 45° panels. or maybe like 30°/60°.

I'm thinking maybe angling the traps so that they are turning towards me.

If i'm down to 30kg/m3 I might be able to make a superchunk that would be 120x60cm/2 without paying to much for it.

I will also need to make the trap somehow so I don't have to drill any holes in the wall (since I'm not sure how long I will be there, and I was asked not do drill holes until at least this autumn), and being that my room is 320cm high, I thought I would probably need to split it in half (so it will be easier to move around)

So a 120x60/2 trap that is 160cm high, making four of them that i could stack on top of each other.

I'll need to build some frame that i cover with cloth and stuff it with wool.

wouldn't that be my best choice ?

I was thinking about the second package on this page (Þilull)

http://www.steinull.is/Files/Skra_0010235.pdf

[Ro]

So I'd be just as well of making a single 4" panel, than two 2" panels ?

Yup.

Also, about the angle of the corner traps.. Do different angles make difference ? For example should i just keep the panels at a 45° panels. or maybe like 30°/60°.

I'm thinking maybe angling the traps so that they are turning towards me.

corner bass-control-devices don't capture reflections, so the angle, in that respect, doesn't really matter. 45degr would just cover the whole corner and the device will work at highest efficiency. (else it just covers more "wall" instead of "corner") Low frq presure is most active IN the corner.

If i'm down to 30kg/m3 I might be able to make a superchunk that would be 120x60cm/2 without paying to much for it.

Good plan, more wool volume is better low frq trapping. Wool ain't THAT expensive.

I will also need to make the trap somehow so I don't have to drill any holes in the wall (since I'm not sure how long I will be there, and I was asked not do drill holes until at least this autumn), and being that my room is 320cm high, I thought I would probably need to split it in half (so it will be easier to move around)

So a 120x60/2 trap that is 160cm high, making four of them that i could stack on top of each other.

I'll need to build some frame that i cover with cloth and stuff it with wool.

wouldn't that be my best choice ?

Sure, just make a "cabin" for the wool to place in the corner. something like the device in THIS thread.

[Exhale]

Hello.

I have a question about building superchunks.
I have a very dense rockwool 125 kg / m3 lying in the storage.
It is left after I made a floating floor.

Is there a chance to make good corner superchunk bass traps with it ?

For some reason couple of acoustic engineers here in Russia told that the best way is to use 100-120 dense wool.
But actually most opinions at american and european forums are opposite.
Everybody suggest using no more than 50 kg density.
Also software like Zorba and other absorption calculators are showing that below 250 hz 125kg is very resistive and
has worse specs in terms of absorbing low freqs.

So, who is right ?

[Exhale]

Anybody ?

[Soundman2020]

There is an announcement at the top of the forum about what to do to assure getting as many responses as possible.
The announcement leads to this post (click here). Actually, several people, who are experts on this forum, will most likely not reply if you don't do what is written in that post. Many others who are very helpful, will most likely not reply out of respect for the moderators' wishes.

It is left after I made a floating floor.

Floating floor? That was probably a mistake. You should read this: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173

To summarize: Floating floors are almost always unnecessary, and very often are built incorrectly. If you did not use concrete for the upper leaf of your floor, then you did it wrong, and it won't work. You wasted your time and money, and you lost a lot of room height, and room volume.

Read the thread to understand.

So firstly, please provide details of how you built that floor, then we can tell you if you need to take it out again.

For some reason couple of acoustic engineers here in Russia told that the best way is to use 100-120 dense wool.

That's a commonly believe myth. People who don't really understand acoustics assume that low frequencies need more density, and very low frequencies need very high density. In fact, the opposite is true.

In the real world of real acoustics, it works like this: For porous absorbers (such as mineral wool and fiberglass) the factor that you need to know is not the density, but rather is something called the "Gas Flow Resistivity", abbreviated "GFR". GFR refers to the manner in which the material resists the movement of air through itself, and since a sound wave is basically just molecules of air moving back and forth, that's what actually controls how a porous absorber panel absorbs sound. GFR is measured in the units of "kPa.S/m2", which can also be written as MKS Rayls, and each different type of porous absorber material has a different range of GFR. Here's a graph that shows the ranges for the two products you mention: Mineral wool and fiberglass insulation:

Gas-Flow-Resistivity-chart.jpg

As you can see, there is a relationship between the density of each material, and the GFR: That's a good thing, because most manufacturers do not ever measure the GFR for their products, since it has no meaning for the primary purpose: thermal insulation. It only matters for acoustics, and since most manufacturers don't make their products specifically for acoustics, they don't measure it: Some do, and there's a lot of information available on that, but it's not easy to find.

However, that graph is idealized and theoretical. In reality, the relationship is not truly linear. Here's a graph created by a manufacturer of acoustic insulation, that shows the real, measured relationship:

gas-flow-resistivity-vs-density-graph-GOOD!!!.png

As you can see, the GFR rises faster than the density.

So, to summarize: what you need to know is the Gas Flow Resistivity, not the density, and there is a relationship between them, but it is not totally linear.

So far so good.

So, moving on to the next point: Even though the relationship between density and GFR is sort of linear, the relationship between GFR and acoustic absorption is NOT linear! Here's the graph that shows the actual relationship:

gas-flow-resistivity-bell-curve.gif

You can clearly see that there's an optimal GFR number for each type of material. If the GFR is too high, then the material does not absorb sound very well, and if the GFR is too low, it does not absorb sound very well. Only when you have the optimum GFR, or density, do you get optimum absorption.

On to the next point: absorption is not the same at all frequencies either, and it also changes depending on the thickness of the panel!

That curve above shows the overall absorption, generically, but in reality the graph below shows it really works:

GasflowResistance1a-T.png

As you can see, there is a different absorption curve for each thickness of the panel, and the curve for each panel shows variation for different frequencies.

So it's a lot more complex than simply saying "use 120 kg/m3"! It depends on many, many factors, not just the density. But the key factor is GFR.

Here's the absorption curves for three different densities, where the GFR is 9,000 rayls, 16,000 rayls, and 28,000 rayls

Porous-absorption-graphs--3-GFR.png

The blue curve is the absorption curve vs. frequency for the dense material, 28,000 rayls. The Green curve is for normal density material with GFR of 16,000 rayls, and the red curve is for a lighter, low density material with a GFR of 9,000 rayls. All of them are 100mm thick.

As you can see, the high density material has WORSE performance in mid and low frequencies, whereas the other too are quite good. Below 50 Hz, they are all pretty much the same, but in the range 80 Hz, to 200 Hz, which is where most of the problems occur in small rooms, the optimal GFR is around 16,000 rayls, or a bit less. Around 14,000 in fact.

As you can see from thee Isover graph, for mineral wool, 14,000 rayls is roughly in the range 20 kg/m3 to 40 kg/m3, and for mineral wool, it is in the range 45 to 100 (off the right edge of the graph).

And that's how it REALLY works, in real life.

Mineral wool at 120 kg/m3 is too heavy (too dense, GFR too high) for good absorption in low frequencies. It would be fine for high frequencies, or even mid-frequencies, but not for lows.

Optimal density for mineral wool is roughly 50 kg/m3, and for mineral wool it is roughly 30 kg/m3. You could go up to 60, 70 or maybe even 80 kg/m3 for mineral wool, but not much higher. And you could go up to maybe 40 or even 50 for fiberglass, but not higher.

Everybody suggest using no more than 50 kg density.
Also software like Zorba and other absorption calculators are showing that below 250 hz 125kg is very resistive and has worse specs in terms of absorbing low freq

Correct. The software is telling the truth: high density is not good for low frequencies, because the GFR (which is roughly the same as impedance) is too high. 50 kg/m3 is about optimal.

One more graph:

GasflowResistance5b-T.png

That shows the direct relationship between density and absorption, for all frequencies, for panels that are 200 mm thick mineral wool. As you can see, the higher the density, the WORSE the acoustic absorption. The dotted light blue line in the middle of the graph is for 55 kg/m3 mineral wool, and it turns out that densities that are LOWER than that have even BETTER absorption: the curves for 50, 45, and 40 kg/m3 show improved absorption.

So I'd be VERY careful about listening to the people who told you that 120 kg/m3 is good for low frequencies! They are clearly wrong. You probably should not listen to them about other things, either...

- Stuart -

[Exhale]

Thanks for response.

Actually I did a concrete layer.
So it was constructed like this:

First - the priming, to provide a safety from the moisture.
Then 4cm of Rockwool FloorBatts 125kg/m3.
Then a layer of polyethylene.
Then steel carcass to provide rigidness for concrete.
Then 8-9cm layer of concrete with fibre.
Concrete layer is totally isolated from the walls by Roockwool stripes and polyethilene.
Finishing auto leveling layer. About 1-2cm.

I collected an information during one year before proceeding.
So everything was done professionaly.
I don't think I should take it out or crash it.

Here's some plan:

[Exhale]

And I did not lost much height. Maybe 1-2 cm.
Because I removed an old wooden floor that consisted of blocks and cheap old mdf.

[Exhale]

I followed this guide.

http://www.acoustic.ua/foto/744.html