Using different density materials for acoustic panels

[zicmaveric]

Hi all I have a multi-part question that I can’t find the answer to… hoping you can help!

This is often recommended: to design acoustic treatment panels (if space and budget allow) with a layer of high density material at the front (ex mineral wool) and a layer of lighter material behind it (ex fiberglass). In addition, materials with a density of 150 or more kg/m3 are not recommended because their higher density will reflect more of the sound.

Here are my questions about this:
1- Say I make some thin acoustic panels (due to space restriction) which can make use of a .5" a layer of 150kg/m3 polyester and a 2" layer of 20kg/m3 open-cell foam, what would be the ideal configuration?

2- When you use a higher density material, you typically trap more of the sound. But you also might reflect more of the sound too, compared to a lighter density material. Let’s take for example the combination of 2" 20kg/m3 open-cell foam and the .5" 150kg/m3 polyester… if I place the polyester sheet in the front of the acoustic panel, wouldn’t that make it reflect more of the sound compared to if I put it in the back of the panel (ie behind the light open-cell foam)?

3- When looking at a larger panel made from both mineral wool (32kg/m3) and fiberglass (7kg/m3) why would it be so often recommended to use the high density material in the front and the low density material in the back?

4- Given my goal of making small thin panels and the fact I can’t use the conventional mineral wool and/or fiberglass for this, would you recommend something different than my initial plan of combining polyester sheets (150kg/m3) with open-cell foam (20kg/m3)? Maybes using sheets of Rockwool ComfortBoard 80 (128kg/m3) along with a mass loaded vinyl MLV (24kg/m3) is better than what I was planning?

I would love your input on this! Thanks!

 

[somebodyelse]

What exactly are you trying to do? Different sorts of absorber are designed to do different things, and in different situations. You seem to be mixing up some of the concepts given the questions you're asking.

 

[Bjorn]

Density is a common misunderstanding. It's the air flow resistivity that matters. You can have two materials with same density and still very different acoustic properties.

 

[DVDdoug]

You should be able to find the acoustic properties of the panels (or material) you are using. For example GIK ACOUSTICS shows a graph of absorption vs. frequency.

What exactly are you trying to do?

Right! Are you trying to create a "dead" room or fix a particular problem? (Most people don't like the sound of a totally dead space but maybe you do.)

Did you measure the room? "Diagnosis before treatment".

Most big problems are usually in the bass range and that requires bass traps. If you install a bunch of regular acoustic panels and absorb everything except the bass you tend to get too much bass, and the uneven bass remains a problem.

...Bass traps absorb the bass that would otherwise be reflected and that smooths-out both the bumps and dips, plus it helps to even-out the bass at different listening locations. But they are large and you have to cover a significant percentage of the wall space so they are rarely used "at home".

Room Modes (standing wave bass problems) can also be improved with 2 or more subwoofers in different locations. That's more common than bass traps in home environments.

 

[zicmaveric]

Some people advise designing deep floor to ceiling panels with the first 6 inches made of a dense material like Rockwool mineral wool and using something like 24 inches of a lighter density material like Owens Corning Pink Fiberglas insulation. This would then be a 30" thick panel. This is supposedly very efficient but I wonder why and why is the denser material at the front and not the back. And of course I wonder if the principles at work here are transferable to differently sized panels made of other combos of light/heavy materials.

 

[zicmaveric]

Density is a common misunderstanding. It's the air flow resistivity that matters. You can have two materials with same density and still very different acoustic properties.

Not all materials out there have acoustic properties available so I am just using density as a shortcut here. I know for example the GFR is more important here than density but most manufacturers don’t have GFR numbers nor any other directly useful numbers for acoustics

 

[Cogito]

You should be able to find the acoustic properties of the panels (or material) you are using. For example GIK ACOUSTICS shows a graph of absorption vs. frequency.


Right! Are you trying to create a "dead" room or fix a particular problem? (Most people don't like the sound of a totally dead space but maybe you do.)

Did you measure the room? "Diagnosis before treatment".

Most big problems are usually in the bass range and that requires bass traps. If you install a bunch of regular acoustic panels and absorb everything except the bass you tend to get too much bass, and the uneven bass remains a problem.

...Bass traps absorb the bass that would otherwise be reflected and that smooths-out both the bumps and dips, plus it helps to even-out the bass at different listening locations. But they are large and you have to cover a significant percentage of the wall space so they are rarely used "at home".

Room Modes (standing wave bass problems) can also be improved with 2 or more subwoofers in different locations. That's more common than bass traps in home environments.

Just curious, can bass traps be tuned for specific bass mode frequencies?

 

[somebodyelse]

Just curious, can bass traps be tuned for specific bass mode frequencies?

It depends on the type of trap, and whether you mean 'tuned by the user' or 'tuned by design but fixed once manufactured.'

Helmholtz resonators will be tuned to a specific frequency. They may be fixed, or may allow user tuning by changing port size or enclosed volume. The level of damping will change how narrow the frequency range is.

Limp mass traps have a tuning frequency defined by the mass and enclosed volume, although the equation usually given for cuboid designs simplifies this to mass per unit area and depth. Recent metamaterial designs sometimes use multiple cavities tuned to different frequencies. You can pick these at design time to target specific frequencies, or a broad range of frequencies.

Cylinder traps (using a porous cylinder and enclosed volume) sometimes offer a degree of tuning depending by covering a portion of the cylinder with something non-porous.

A load of resistive material in the corner isn't tuned as such, but is only effective if the depth is significant compared to the wavelength. If you don't have much depth it won't be effective at low frequencies.

 

[DVDdoug]

Some people advise designing deep floor to ceiling panels with the first 6 inches made of a dense material like Rockwool mineral wool and using something like 24 inches of a lighter density material like Owens Corning Pink Fiberglas insulation. This would then be a 30" thick panel.

That should be effective at all frequencies, including bass. If you don't mind "shrinking the room" that should be OK. But there are thinner "membrane" bass traps and you can build them.

Some people advise

Different experts have different approaches. But random advice without knowing anything about the room is mostly useless. There is usually more than one solution.

If you were building a studio (or if you want your room to sound like a studio) you'd probably start with some acoustic treatment (not much need to measure the bare room), make some measurements with REW (or something similar) and a measurement mic, and then experiment with adding/subtracting/moving the treatments to optimize the results. Then, use EQ to fix-up rest.

In a normal home situation you probably should measure the room as-is first. After identifying what your trying to correct, try EQ first, especially if you want to be economical. If that doesn't get the sound you want, you can start start adding acoustic treatment and re-measuring. Or you can try adding subwoofers.



...Personally, I don't have any acoustic treatment in my combination living room/home theater. It's a "typical" living room with carpet and the usual furniture. I'm satisfied with the sound and I haven't measured, but every small room (smaller than a concert hall or theater) has room modes in the bass range so I'm sure it could be improved.

 

[zicmaveric]

What exactly are you trying to do? Different sorts of absorber are designed to do different things, and in different situations. You seem to be mixing up some of the concepts given the questions you're asking.

I’m just curious as to what results I should expect from the different configurations of different density material in building panels that include 2 different materials in it.

 

[Involver]

This website lets you model the effect of different layers.

Multi-layer Absorber Calculator