Absorption coefficients and broad band absorption

[Glenn Kuras]

Please correct me but is foam a broadbrand absorption material? I assumed it was, since it covers a wide range. Same as fiberglass, but generally not as effective in absorbing the same range.

You are correct.

I was discussing this with someone the other day and he was claiming foam is not broadband but can be designed to target specific frequencies, like 200 Hz. Is that a load of nonsense?

I guess you could use it in a pressure based design like you would fiberglass.

What is the absorption coefficient of foam and can it be called broadband absorption?

It would depend on the foam (thickness and so on).
Comparing Foam to GIK 244 Bass Traps

[goneten]

How do I prove foam is broadband? Does one just need to take a look at the absorption coefficient? Just want to nail this one in the bud.

[goneten]

These is what was said :

"Foams don't have a very large bandwidth and are therefore used to target specific frequencies. There are even companies that make composites to suit your needs. I.E. if you have a 200Hz peak they will make up a composite of various foams and sometimes fiberglass to absorb that frequency only.

The tricky bit is to find where that frequency is at its highest peak in the room and then place that panel there. This is a more complex way of doing things but it yields far better results. The reason it yields better results is because when you use broadband absorption you will absorb a lot of the total acoustical energy resulting in an increase of the volume control to get back to the same dB level as you had prior to the paneling.

So if you take a measurement before any paneling and then target only that which is needed then you are balancing the room to gear with less acoustic absorption yielding the same result with more head room but more time spent."

[Jens Eklund]

Originally Posted by goneten

How do I prove foam is broadband? Does one just need to take a look at the absorption coefficient? Just want to nail this one in the bud.

Does the wall gap apply to foam?

[Glenn Kuras]

Originally Posted by goneten

These is what was said :

"Foams don't have a very large bandwidth and are therefore used to target specific frequencies. There are even companies that make composites to suit your needs. I.E. if you have a 200Hz peak they will make up a composite of various foams and sometimes fiberglass to absorb that frequency only.

The tricky bit is to find where that frequency is at its highest peak in the room and then place that panel there. This is a more complex way of doing things but it yields far better results. The reason it yields better results is because when you use broadband absorption you will absorb a lot of the total acoustical energy resulting in an increase of the volume control to get back to the same dB level as you had prior to the paneling.

So if you take a measurement before any paneling and then target only that which is needed then you are balancing the room to gear with less acoustic absorption yielding the same result with more head room but more time spent."

Have your buddy read this. It is not about foam itself but talks about different kinds of bass traps (absorption). It should clear it up.
Understanding Different Bass Trapping - GIK Acoustics

[goneten]

Does he have an argument re "The tricky bit is to find where that frequency is at its highest peak in the room and then place that panel there. This is a more complex way of doing things but it yields far better results. The reason it yields better results is because when you use broadband absorption you will absorb a lot of the total acoustical energy resulting in an increase of the volume control to get back to the same dB level as you had prior to the paneling."?

[localhost127]

Originally Posted by goneten

Does he have an argument re "The tricky bit is to find where that frequency is at its highest peak in the room and then place that panel there.

the perceived response at the listening position (what you are likely concerned with) is the resultant summation (superposition) of the direct and indirect signals at that particular point in 3space (technically 4space).

how does one know that the particular "frequency at its highest peak" in one part of the room is causing the predominant issue in another part of the room (receiver/listening position).

small acoustical spaces deal with local areas of variable pressure - which means the response will vary as you move about the room. implying that the addressing of one particular problem in one particular point in 3space does NOT imply that problem is solved across/through-out the entire room (eg, at the listening position).

[Syncamorea]

To all the above, I'd add that foam is not going to be a very good narrow band absorber when just put up as a foam panel - in the frequency ranges that are the typically the main problems you are trying to treat in small to mid sized rooms. Configured as a VPR (which is a pressure absorber) the right kind of foam can be very helpful for taming room modes.

My Experiment with a Metal Panel Absorber

build a VPR bass trap.uk

Trapping Traps

Originally Posted by goneten

"Foams don't have a very large bandwidth and are therefore used to target specific frequencies. There are even companies that make composites to suit your needs. I.E. if you have a 200Hz peak they will make up a composite of various foams and sometimes fiberglass to absorb that frequency only."

Honestly, I think this person is feeding you a line of BS. The materials mentioned in this composite are foam and fiberglass. I've never heard of a narrow band foam/fiberglass composite.* Narrow band treatments typically involve a physical behavior such as resonance that couples with a specific frequency range due to the physical constraints of the design. So read about Helmholtz and LMV traps that are tuned to specific frequencies in order to target those frequencies specifically.

Tim's Limp Mass Bass Absorbers

Helmholtz Resonator

*: Given proper motivation, one could possibly take a very rigid closed cell foam and fashion it into a Helmholtz device and show some degree of narrow band performance. But I don't think this could be a cost-effective device to build.

[goneten]

It almost seems like he is trying to argue in favour of pressure-based absorption over velocity-based absorbers. Which I don't get, because time and frequency issues are normally wide-spread issues that aren't best addressed using narrow band absorbers.

Foam, as is commonly used in acoustic panels, are velocity-based, so I don't think this guy is making a whole of sense here. I mean, foam not having large bandwidth??? It absorbs over a wide range. Not just specific frequencies!

[Syncamorea]

Originally Posted by goneten

It almost seems like he is trying to argue in favour of pressure-based absorption over velocity-based absorbers. Which I don't get, because time and frequency issues are normally wide-spread issues that aren't best addressed using narrow band absorbers.

Foam, as is commonly used in acoustic panels, are velocity-based, so I don't think this guy is making a whole of sense here. I mean, foam not having large bandwidth??? It absorbs over a wide range. Not just specific frequencies!

OK, I'm done adding links to my post above.

Jens has described using an EPS diffuser as the front panel of a pressure type device. I don't know what range of materials and/or configurations have been designed and tested but for example, you can buy LMV laminated to foam and I wouldn't be surprised if that kind of product could have utility in the right design.

I will disagree somewhat with your comment above about the need for narrow band. In fact, narrow frequency range devices can be very effective room treatments when attempting to solve modal issues.

[Glenn Kuras]

I will disagree somewhat with your comment above about the need for narrow band. In fact, narrow frequency range devices can be very effective room treatments when attempting to solve modal issues

Agreed.
GIK or Realtraps?

[goneten]

I will disagree somewhat with your comment above about the need for narrow band. In fact, narrow frequency range devices can be very effective room treatments when attempting to solve modal issues.

I don't disagree. What I tried to get across was that narrow-band treatment isn't, in my opinion, the best type of treatment to use for widespread frequency and time-related issues given the limitations involved.

Use a helmholtz resonator to address a very deep bass resonance or two, but all the other issues are, in my opinion, better addressed using broadband absorption.

[goneten]

Has anyone heard of narrow band foam treatment? What about closed-cell foam? Is that not a type of narrow band foam?

[Jens Eklund]

Originally Posted by goneten

Has anyone heard of narrow band foam treatment?

No, unless talking about thin panels that might be called "narrow band absorbers" (although "limited frequency range absorber" would probably be a more appropriate term) since they only affect the highest frequency range.

[goneten]

I thought thin foam (1" thick) extended to lower mid-range. But bass is a no-go, unless you use very, very thick, dense foam.

[Jens Eklund]

Originally Posted by goneten

I thought thin foam (1" thick) extended to lower mid-range. But bass is a no-go, unless you use very, very thick, dense foam.

EDIT:

Oh, and if you want to absorb low frequencies using velocity based absorbers (assuming very thick), you want to use a material with low flow resistivity (usually meaning light, not dense).

[goneten]

Originally Posted by Jens Eklund

Oh, and if you want to absorb low bas using velocity based absorbers, you want to use a material with low flow resistivity (usually meaning light, not dense)

So between 64 kg/m3 and 96 kg/m3 fiberglass or mineral wool the lighter panel will be more effective at absorbing lower frequencies? Why would people decide to use 96 kg/m3 or above?

[Jens Eklund]

Originally Posted by goneten

So between 64 kg/m3 and 96 kg/m3 fiberglass or mineral wool the lighter panel will be more effective at absorbing lower frequencies? Why would people decide to use 96 kg/m3 or above?

The internet is wonderful but unfortunately; it´s also very good at spreading misconceptions.

Use this tool to evaluate the effect of different flow resistivity:
Porous Absorber Calculator

[goneten]

The internet is wonderful but unfortunately; it´s also very good at spreading misconceptions.

I don't get why the higher density stuff exists if it worsens the performance at low frequencies. What's the purpose behind high density mineral wool/fiberglass?

[Jens Eklund]

Originally Posted by goneten

I don't get why the higher density stuff exists if it worsens the performance at low frequencies. What's the purpose behind high density mineral wool/fiberglass?

Acoustic treatment is an extremely small niche compared to general construction where you might need dense fiberglass for foundations etc.

[Syncamorea]

^^^ Jens nailed that. The oldest document I read about mineral wool talked about how great it was to get thermal insulation out of what was essentially a waste byproduct of the steel manufacturing process.

[Glenn Kuras]

Originally Posted by goneten

I don't get why the higher density stuff exists if it worsens the performance at low frequencies. What's the purpose behind high density mineral wool/fiberglass?

Rigid fiberglass density tests

[goneten]

Thanks Glenn. But according to your report the higher density fiberglass improved absorption. 705 cleaned up ringing over 703 and some of the dips had been filled in ...

So now I'm confused. So higher density does improve absorption.

[Syncamorea]

Up to a limit, yes. But I think two layers of 4" 705 has significant reflection of midrange. (going off of old memory disclaimer so I'll leave it to you to investigate)

[Glenn Kuras]

Originally Posted by goneten

Thanks Glenn. But according to your report the higher density fiberglass improved absorption. 705 cleaned up ringing over 703 and some of the dips had been filled in ...

So now I'm confused. So higher density does improve absorption.

Don't shoot the messenger.

If making the panel 4" thick higher density seems to work better. Keep in mind though that 705 is much more expensive so using 6" of 703 will cost less and works better.

[Ethan Winer]

Originally Posted by goneten

according to your report the higher density fiberglass improved absorption. 705 cleaned up ringing over 703 and some of the dips had been filled in ... So now I'm confused. So higher density does improve absorption.

That's my report.

Yes, higher density does absorb more bass, and adding FRK increases that much more. Of course, at some point too much density will reflect more than absorb. But for this range of densities (and panel thicknesses) 705-FRK is indeed best.

--Ethan

The Acoustic Treatment Experts