Bass traps - the more the merrier??

[kraskland]

Originally Posted by Brian Brock

in general, the denser materials seem to be more absorptive of bass frequencies than the lighter, up to a point, after which greater density creates greater reflectivity

I've read this elsewhere, but do you (or anyone) know at what density sound is reflected more? And does this effect some frequencies more than others?

[Weasel9992]

Originally Posted by kraskland

I've read this elsewhere, but do you (or anyone) know at what density sound is reflected more? And does this effect some frequencies more than others?

It depends on the material, but if you're talking about mineral wool/rigid fiberglass type stuff, look here for information on different densities.

Frank

[Ethan Winer]

Originally Posted by kraskland

do you (or anyone) know at what density sound is reflected more? And does this effect some frequencies more than others?

As far as I know this has never been tested. If a material is made dense enough, eventually it will be as dense as stone. What you want to know is the "turn-over" point, and so do I.

--Ethan

[kraskland]

Thanks, Frank. I've looked at that site but there doesn't seem to be enough data to make good comparisons within the same product range. But maybe there really isn't a real difference between say, Roxul Safe, Roxul AFB, and Roxul RHT other than densities and the name.

[kraskland]

Originally Posted by Ethan Winer

As far as I know this has never been tested. If a material is made dense enough, eventually it will be as dense as stone. What you want to know is the "turn-over" point, and so do I.

--Ethan

Yes, thanks for clarifying my question!

[avare]

Originally Posted by kraskland

I've read this elsewhere, but do you (or anyone) know at what density sound is reflected more? And does this effect some frequencies more than others?

The amount of sound absorbed is dependent on the gas flow resistivity and thickness of the material. The two together give the acousitic resistance of the material. For complete absorbtion, the resistance should equal the acousitc impedance of air. Practical studies have shown that a wide range of resistance is quite usable.

In the above paragraph, I never mentioned material density. We (Gearslutzes) use material density and material type as indicator of what the material resistivity is. There is an excellent, but unforunately short section Marshall Long's Architectural Acoustics detailing the effect of material impedance on absorbtion. It references research by Ingaard (I think, I don't have my copy handy) for the source.

The long and short for us is that around 3 lb/ft^3 glass type mineral wool and 4lb/ft^3 rock type mineral wool are best at thicknesses up to 4" or so. Thicker materials absorb better with lighter material. Cotton type material shows a flattening in absobtion from 4" to 6" implying internal reflection at low frequencies. There are slight variations between different suppliers due to specific inulation material, fiber lengths, amount of and type of binder.

If you are trying to get a good handle on the subject, think in terms material gas flow resitvity and thickness, NOT density.

All of this off ignores the effect of location in comparison to sound wave particle velocity, just to keep things interesting.

And you thought it was simple, hah! it's acoustics,
Andre

[andrebrito]

You can calculate this theoreticallly but it is a pain in the ass since most rockwool and fiberglass manufactures dont provide the needed information in terms of porosity of their products

[kraskland]

Thanks so much for the explanation Andre, that helps quite a bit.

As Andrebrito mentioned, it is quite difficult to get data on the various products though. It took me two weeks and a dozen emails just to find out the density of Rockwool 212!

Originally Posted by avare

The long and short for us is that around 3 lb/ft^3 glass type mineral wool and 4lb/ft^3 rock type mineral wool are best at thicknesses up to 4" or so. Thicker materials absorb better with lighter material. Cotton type material shows a flattening in absobtion from 4" to 6" implying internal reflection at low frequencies. There are slight variations between different suppliers due to specific inulation material, fiber lengths, amount of and type of binder.

Absent any data on gas flow resistivity, do you have any experience with higher density material, say 7+ lb/ft^3 Rockwool?

[avare]

Originally Posted by kraskland

Absent any data on gas flow resistivity, do you have any experience with higher density material, say 7+ lb/ft^3 Rockwool?

??? Absent any data on gas flow resistivity, no. Here is data.

98 kg/m^3 has an airflow resistivity, of around 58,000 mks rayls/m as opposed to around 15,000 for 48 kg/m^3 mineral wool (AKA Rockwool).

Andre

[DanDan]

I used Rocksil 100 KG recently for the walls of a booth. 50mm. It is certainly dead, as it should be in a vocal booth. However an unexpected bonus is that singers who have been entombed there find it 'airy' and pleasant to be in. I can't isolate the specific reasons, there are a few very small hard areas, but overall this is a fluffy womb. So, I begin to wonder does extensive use of a particular material have a 'sound' or sonic signature?
In any case I would reinforce the already stated wisdoms-
A wide range of densities will work just fine. Sometimes ease of handling is a decider. RS100 at 50mm is very nice to work with. It doesn't itch as much as lighter fibreglass, and it doesn't fall apart like lighter RW.
At greater depths, lighter performs as well or ultimately better, plus it is very much cheaper.
DD

[kraskland]

Originally Posted by avare

??? Absent any data on gas flow resistivity, no. Here is data.

98 kg/m^3 has an airflow resistivity, of around 58,000 mks rayls/m as opposed to around 15,000 for 48 kg/m^3 mineral wool (AKA Rockwool).

Andre

Excellent, thank you!

Plugging these numbers into the porous absorber calculator, I see that 58,000 mks rayls/m performs quite poorly compared to 15,000 (at 100mm). This is at all frequencies. Am I reading that correctly? Also, at 58,000 there doesn't seem to much of a difference if there is a gap or not.

Where do you find this data? Architectural Acoustics as you mentioned earlier?

[avare]

Originally Posted by kraskland

Excellent, thank you!

You are welcome.

Plugging these numbers into the porous absorber calculator, I see that 58,000 mks rayls/m performs quite poorly compared to 15,000 (at 100mm). This is at all frequencies. Am I reading that correctly?

With knowing exactly what parameters you plugged in, yes.

Also, at 58,000 there doesn't seem to much of a difference if there is a gap or not.

Correct.

Where do you find this data? Architectural Acoustics as you mentioned earlier?

The lighter material's data from memory, and the denser from NRC IC IR-693. It shows the lighter material as around 16,000, but the difference is of little practical significance.

AT times it takes a fair of acoustic detective work find such data. I think you experienced the same thing looking for a material's density.

Andre

[AwwDeOhh]

Originally Posted by avare

material All of this off ignores the effect of location in comparison to sound wave particle velocity, just to keep things interesting.

And you thought it was simple, hah! it's acoustics,
Andre

OUCH! Muh Brain hurts!

Very interesting and informative discussion! Thanks for sharingthumbsup

....now, where did i put that Asprin?...

[kraskland]

Thanks again Andre, this has been very educational. Looks like I should have stuck with the lower density material. At least I know not to waste my time with an air gap if I use the remaining high density. That porous absorber calculator is a neat tool.

[kraskland]

Originally Posted by DanDan

I used Rocksil 100 KG recently for the walls of a booth. 50mm. It is certainly dead, as it should be in a vocal booth. However an unexpected bonus is that singers who have been entombed there find it 'airy' and pleasant to be in. I can't isolate the specific reasons, there are a few very small hard areas, but overall this is a fluffy womb. So, I begin to wonder does extensive use of a particular material have a 'sound' or sonic signature?

How large of a vocal booth is this? Interesting thought on the sonic signature of different material.

[DanDan]

Hi K, I am not sure about that air gap and high density comment. It is my understanding that an air gap will always shift the effective frequency spectrum downwards.
My booth is approx 2.5 x 2.45 x 2.9 Metres. I am a bit intrigued about the sonic signature or other factors. Whatever is happening in there, they like it. Everyone notices how dead it is, the usual 'unnatural' 'weird' 'claustrophobic' and so on, but they very quickly move on to the ' actually quite nice' 'airy' and so on. I wish I could patent it!
DD

[kraskland]

Originally Posted by DanDan

Hi K, I am not sure about that air gap and high density comment. It is my understanding that an air gap will always shift the effective frequency spectrum downwards.

Except for really dense material, I think that's true. In this case I've got 100mm thick material at 58,000+ mks rayls/m. According to the porous absorber calculator, adding an air gap of any size doesn't help. In fact, at 100mm air gap it slightly lowers absorption between 80 and 300 Hz. This is assuming I'm using the software correctly (a big assumption) and the software is correct.

Originally Posted by DanDan

My booth is approx 2.5 x 2.45 x 2.9 Metres. I am a bit intrigued about the sonic signature or other factors. Whatever is happening in there, they like it. Everyone notices how dead it is, the usual 'unnatural' 'weird' 'claustrophobic' and so on, but they very quickly move on to the ' actually quite nice' 'airy' and so on. I wish I could patent it!
DD

I wonder what else is going on. Maybe your insulation is reflecting some of the frequencies at those vocal ranges that account for airiness. Whatever the case, looks like you've got a nice setup! thumbsup

[DanDan]

I am not familiar with that Calculator. It is my understanding in general that there is a lot more to most than meets the eye. Does that calculator deal with normal, random, field, or what type of incidence? In any case I am convinced that for practical densities, an air gap has a huge effect. I don't think we are ready for a new wheel right now!
DD

[Brainchild]

According to the porous absorber calculator, adding an air gap of any size doesn't help.

I am not familiar with that Calculator. It is my understanding in general that there is a lot more to most than meets the eye. Does that calculator deal with normal, random, field, or what type of incidence? In any case I am convinced that for practical densities, an air gap has a huge effect. I don't think we are ready for a new wheel right now!

Apparently there was a problem with the wavenumber and air impedance implementation in v1.53. There is now v1.54 that properly reveals the effect of an air gap behind a porous absorber.

That calculator is based on fundamental laws of physics. Its results match perfectly with, eg., Zorba, as well as all the calculations I've ever done regarding these issues on my own using the well-established formulas.

DanDan, if you've never used it, I can't possibly recommend highly enough that you at least take a look at it.

The porous absorption is calculated [as is carefully noted in the calculator] at one specific angle at a time. The default is normal; you can sweep it through 89 degrees to see the effect of various incidences. There is no single "random" incidence visualization, but it is easy to extrapolate based on the totality of the individual angles' responses.

[Scott Mix]

where can i buy the 703s? or preferably the 705s? Also i have Auralex AudioTile ShockWave 48-Pack ... Auralex AudioTile ShockWave 48-Pack | Sweetwater.com ... in my very small 4x2 iso booth.. i was wondering if i should mix it up and put some 703s in there along with leaving some of the aurolex or should just take all of the aurolex out, fill the booth with the 703s and put them scattered in my control room with some more 703s.

[DanDan]

Brain, I took a look. Nice. It seems like a lot of work went into that. That guy Chris is surely generous with his time and knowledge. I have stored it with my other calculators, and will give it a spin. I tend to work in broad strokes. From experience with calculation and even measurements, I have developed a bit of a sceptical view. e.g. Room modes never coincide exactly with calculation. And e.g. here Acoustics Forum • View topic - Helmholtz formula, is this incorrect? I found a wrong formula, plus a whole wrong section in the Master Handbook of Acoustics. I have found that choice of fibre seems to be what is available. It is not easy finding rigid batts in many parts of the world.
Regarding my vocal booth. I did treat two corners with Superchunks before attending to the walls. Furthermore I put two thick HF Minitraps on the celing. The floor is carpeted.
For the walls, 50mm RS100 with no airgap has worked very well.
DD

[Gie-Sound]

Originally Posted by Weasel9992

I agree, but with small rooms "dead" (meaning appropriately short decay times across the spectrum) is usually the only option. The OP's room is just big enough that he might be able to get away with a more live sound.

Frank

What do you consider a "small room"

Would you consider this one small;
length is 5,03 m (16,5 ft)
width is 5,90 m (19,35 ft) in the back, 5,40 m (17,7 ft) in the front
hight is 3,73 m (12,23 ft) in the middle of the room

[Weasel9992]

Originally Posted by Gie-Sound

What do you consider a "small room"

Would you consider this one small;
length is 5,03 m (16,5 ft)
width is 5,90 m (19,35 ft) in the back, 5,40 m (17,7 ft) in the front
hight is 3,73 m (12,23 ft) in the middle of the room

No. The orientation is not optimal, but I suspect that you know that already.

Frank

[Arksun]

Originally Posted by avare

The amount of sound absorbed is dependent on the gas flow resistivity and thickness of the material. The two together give the acousitic resistance of the material. For complete absorbtion, the resistance should equal the acousitc impedance of air. Practical studies have shown that a wide range of resistance is quite usable.

In the above paragraph, I never mentioned material density. We (Gearslutzes) use material density and material type as indicator of what the material resistivity is. There is an excellent, but unforunately short section Marshall Long's Architectural Acoustics detailing the effect of material impedance on absorbtion. It references research by Ingaard (I think, I don't have my copy handy) for the source.

The long and short for us is that around 3 lb/ft^3 glass type mineral wool and 4lb/ft^3 rock type mineral wool are best at thicknesses up to 4" or so. Thicker materials absorb better with lighter material. Cotton type material shows a flattening in absobtion from 4" to 6" implying internal reflection at low frequencies. There are slight variations between different suppliers due to specific inulation material, fiber lengths, amount of and type of binder.

If you are trying to get a good handle on the subject, think in terms material gas flow resitvity and thickness, NOT density.

All of this off ignores the effect of location in comparison to sound wave particle velocity, just to keep things interesting.

And you thought it was simple, hah! it's acoustics,
Andre

It's a mind***k, I can barely get my head around all of it!.

I'm just wondering, is a combination of material types in single insulation a bad idea?.

For example, instead of an all-hemp insulation, one used a hemp/cotton mixed one which was 50/50 content.

[Gie-Sound]

Originally Posted by Weasel9992

No. The orientation is not optimal, but I suspect that you know that already.
Frank

Depends on what you mean with "orientation"...

[Weasel9992]

Originally Posted by Gie-Sound

Depends on what you mean with "orientation"...

Potential double entendre aside, I mean that he should be firing down the longest dimension, and not across the shortest.

Frank

[Gie-Sound]

ok, now I understand what you mean.
Yes, not ideal... but this is the only option.