Experimental Broadband Absorber - Diagram included : )

[Ali Gator]

Hello everyone,

I am Ali and i'm from Chittagong (City), Bangladesh (Country). I have been lurking in this awesome website for quite a while now. Learning as much as i can in the shortest period of time so that i can do better recordings at home by treating for room acoustics.

In order to record decent acoustic guitars and vocals; i am in the process of making 8 Units of moveable 4.5' Long x 20" Wide "Broadband Absorbers" (Bass Traps for some lol..) that will be about 8" deep with an unconventional approach. Please bear with me because i do need your opinion on it to finalize it and begin putting it together asap. I have learned SketchUp, thought out and have drawn this plan in the last past 72 hours. Please note that unlike most of you we have limited resources here in our country. I have researched and sourced Plywoods for the main frame and the back panel.

For main absorption 4" thick Rockwool (Roxul Semi-Rigid with wire-mesh on the back) of 100KG/M3 Density are the only ones available so far :/ can't find rigid ones and none around 65-75KG/M3 of Density which would be more optimal for broadband absorption. Thus, for the sake of more high absorption i have also sourced 2" Thick Acoustic Foams (or so they say here lol..) of 45KG/M3 of Density to put on top of the Rockwool layer.

With my limited knowledge, i came up with this air gap of 1"+1" on the back-end of the unit with a 1/4" Plywood panel in the middle. I'll make these airtight with glue of course. If i had vinyl (MLV - Mass Loaded Vinyl) available in the market, i'd have used it here instead of the 1/4" Plywood panel as the membrane.


So to sum my queries up..

1. Since the 4" Thick Rockwool is of a higher density (100KG/M3 instead of 65/75) would the 2" thick Acoustic Foam of 45KG/M3 Density work for the higher frequency absorption?

2. Is it ok sound-wise to leave the Wire-Mesh behind the semi-rigid Rockwool that came with it (It's the only kind i could find here at the moment.

3. The 1"+ 1" air-tight gap with the 1/4" thick Plywood panel in the middle. Will it be as effective if not more for an overall lower frequency absorption instead of an additional layer of similar thickness of Rockwool?

Please feel free to give your opinion on the probable effectiveness of this concept. This drawing is just a prototype.. i will make alterations based on your advices, finalize it, repost it with local the cost into US$ for everyone along with a vdo of its making : )

[Ali Gator]

Hi guys, if there is any detail i may be missing; pls let me know. Sure am anxious to hear from any of you who knows better in this field and help another out.. regards..

[Ali Gator]

Wow, either the good folks here are too busy as of yet or just totally ignoring my queries for some strict rule i may have not adhered to. Never mind, i'm getting some help at gearslutz

Still love this forum btw : ) but someone needs to update the pic posting resolution of avatars (6kb really?) and diagrams of 750 pixels wide. Surely our monitors can do better than what it did back in 2006 right? : ) Do consider this John.

Sincerely,

Ali

[Soundman2020]

Hi Ali. Welcome.

8 Units of moveable 4.5' Long x 20" Wide "Broadband Absorbers" (Bass Traps for some lol..)

Those are too small to be true bass traps, but can still be very effective as mid-range broadband absorption.

100KG/M3 Density are the only ones available so far

It's a pity that that is all you can get, as it is a bit too dense for maximum effect at low frequencies, although it will work well for mids and highs.

Thus, for the sake of more high absorption i have also sourced 2" Thick Acoustic Foams (or so they say here lol..) of 45KG/M3 of Density to put on top of the Rockwool layer.

I'm not sure if you mean "high" as in high frequency, or "high" as in highly effective? Lower density is less good for high frequencies.

i came up with this air gap of 1"+1" on the back-end of the unit with a 1/4" Plywood panel in the middle.

so this is a combined panel trap and broadband absorber? That can work, but what frequency is is tuned to, and is that a frequency that needs treating in your room? You didn't mention the dimensions of your room, or post a REW data file showing the current behavior of the room, so are you sure you are targeting the correct frequency with that?

1. Since the 4" Thick Rockwool is of a higher density (100KG/M3 instead of 65/75) would the 2" thick Acoustic Foam of 45KG/M3 Density work for the higher frequency absorption?

The other way around: Higher density is worse for low frequencies, better for highs. And lower density is better for lows, worse for highs.

2. Is it ok sound-wise to leave the Wire-Mesh behind the semi-rigid Rockwool that came with it (It's the only kind i could find here at the moment.

No problem.

3. The 1"+ 1" air-tight gap with the 1/4" thick Plywood panel in the middle. Will it be as effective if not more for an overall lower frequency absorption instead of an additional layer of similar thickness of Rockwool?

That is a tuned panel trap. It will absorb at the frequency it is tuned to. The frequency is set by the surface density of the panel itself (mass per unit area), and the depth of the cavity.

Please feel free to give your opinion on the probable effectiveness of this concept.

It should be effective as a general broadband absorber plus tuned panel trap. The mix of different density insulation helps to get broader coverage: the lower density on the front is better for low frequencies, and the higher density at the back will be good for highs. There isn't a lot of difference between the two, but it does help. The only change I would make is to swap the rear panel for something much thicker and more rigid: maybe 3/4" plywood. If not, that panel will flex and vibrate, reducing the panel trap efficiency (lower, broader Q).

too busy as of yet or just totally ignoring

Nobody is ignoring your thread: it has had over 30 views in the couple of days since you posted it. Sometimes it can take longer than that to get a response, if people are busy.

Surely our monitors can do better than what it did back in 2006 right?

Have you tried viewing the forum on a smart phone, as many members do? We cater to as many screens as possible.


- Stuart -

[Ali Gator]

Stuart, i'm grateful for your response friend Note my main objective with these units are for recording acoustic guitars and vocals. Pls see my responses in blue..

Hi Ali. Welcome.

Thanks man

8 Units of moveable 4.5' Long x 20" Wide "Broadband Absorbers" (Bass Traps for some lol..)

Those are too small to be true bass traps, but can still be very effective as mid-range broadband absorption.

Really? even at 6"-8" deep at 4.5 feet tall and 20 inches wide? What would you say is optimal then? These are of course more of a broadband absorbers

100KG/M3 Density are the only ones available so far

It's a pity that that is all you can get, as it is a bit too dense for maximum effect at low frequencies, although it will work well for mids and highs.

Yeah i just sourced 95kg/m3 density this week. But you mean it is too dense for highs n mids right? Isn't denser better for low frequency absorption?

Thus, for the sake of more high absorption i have also sourced 2" Thick Acoustic Foams (or so they say here lol..) of 45KG/M3 of Density to put on top of the Rockwool layer.

I'm not sure if you mean "high" as in high frequency, or "high" as in highly effective? Lower density is less good for high frequencies.

For High Frequency absorption ofcourse sorry for the confusion. So what do you think abt this foam of 2" thickness at that 45KG/M3 density? Fingers crossed..

i came up with this air gap of 1"+1" on the back-end of the unit with a 1/4" Plywood panel in the middle.

so this is a combined panel trap and broadband absorber? That can work, but what frequency is is tuned to, and is that a frequency that needs treating in your room? You didn't mention the dimensions of your room, or post a REW data file showing the current behavior of the room, so are you sure you are targeting the correct frequency with that?

I scrapped this membrane with airgap idea moving forward for now. Glenn Kuras from GIK acoustics and few other gentlemen at Gearslutz convinced me that it is indeed risky and may require a lot of testing. I'm still a noob in learning afterall and yes, i will provide the specs of the room soon, my wife is working on it (she's an interior architect), we just moved to our new house and she caught the chicken pox..

1. Since the 4" Thick Rockwool is of a higher density (100KG/M3 instead of 65/75) would the 2" thick Acoustic Foam of 45KG/M3 Density work for the higher frequency absorption?

The other way around: Higher density is worse for low frequencies, better for highs. And lower density is better for lows, worse for highs.

Wait.. did i get it all wrong then? I totally thought higher density of rockwools would reflect back high and mid frequencies.. i mean low frequencies afterall goes through most objects and barriers like wood, plastic and sorts right? I am confused..

Please feel free to give your opinion on the probable effectiveness of this concept.

It should be effective as a general broadband absorber plus tuned panel trap. The mix of different density insulation helps to get broader coverage: the lower density on the front is better for low frequencies, and the higher density at the back will be good for highs. There isn't a lot of difference between the two, but it does help. The only change I would make is to swap the rear panel for something much thicker and more rigid: maybe 3/4" plywood. If not, that panel will flex and vibrate, reducing the panel trap efficiency (lower, broader Q).

Noted, thanks. I'll redo the second and 3rd version of the design and post it here soon. I'm doing one with open front, sides and back. Another with a Slat Design on the front.

too busy as of yet or just totally ignoring

Nobody is ignoring your thread: it has had over 30 views in the couple of days since you posted it. Sometimes it can take longer than that to get a response, if people are busy.

I'm glad to hear it my friend, am new and was feeling neglected that's all

Surely our monitors can do better than what it did back in 2006 right?

Have you tried viewing the forum on a smart phone, as many members do? We cater to as many screens as possible.

Actually i do, but i guess i didn't think of it on a broader spectrum for everyone else on their cellphone side my bad Stuart.

[Soundman2020]

Really? even at 6"-8" deep at 4.5 feet tall and 20 inches wide? What would you say is optimal then?

Bass traps need to be big. Superchunks and corner absorbers, for example, are usually either 48" wide. or 36" wide minimum, and run from floor to ceiling (8', 10', whatever the ceiling height is). Also, since all room modes terminate in corners, that's where bass trapping should be, notably in the tri-corners.

But you mean it is too dense for highs n mids right? Isn't denser better for low frequency absorption?

No, I mean that it is too dense for the lows. It's a common misconception that denser insulation is better for lows: it isn't. Higher density absorbs low frequencies worse, because the fibers are too closely packed and cannot respond to the lower frequencies fully. In any event, it is not the density per sé that matters: the actual characteristic of insulation that absorbs sound is called "Gas Flow Resistivity", and since it is really impedance, it varies with frequency. High gas flow resistivity provides more impedance to high frequencies than it does to lows.

So what do you think abt this foam of 2" thickness at that 45KG/M3 density?

If it really is true acoustic foam, from a reputable manufacturer, then that should work, but 2" isn't enough so you'd need to double the thickness, or even triple it. But do be careful! There are many "not so reputable" people who sell ordinary packing foam and pretend it is acoustic foam: it isn't. Acoustic foam is open-cell foam with very precise gas flow resistivity characteristics. Packing foam, mattress foam, upholstery foam, and other similar foams are much cheaper, but are basically useless for acoustics. They look the same at first glance, but they are totally different, at the microscopic level. so be careful when you buy "acoustic foam"! If it is cheap, then it most likely is not acoustic foam.

Glenn Kuras from GIK acoustics and few other gentlemen at Gearslutz convinced me that it is indeed risky and may require a lot of testing.

Yep!

i will provide the specs of the room soon,

Great! I would suggest that you do it in SketchUp, which is sort of the standard here on the forum (and other forums too).

I totally thought higher density of rockwools would reflect back high and mid frequencies..

As I mentioned above, that is a very common misconception. Intuitively, you'd think that denser, heavier insulation would be better for lows, but there's a lot of things about acoustics that just are not intuitive at all!

The density has to be very high before it starts reflecting appreciably.

Basically, there is an optimum range of density (related to gas flow resistivity) for each type of material and each frequency range, and the general curve looks like this:

gas-flow-resistivity-bell-curve.gif

(That graph is from the University of Kentucky, but I don't recall which publication)

So there's a peak, optimum point of absorption, for each specific flow resistivity, but it turns out that this also varies by frequency. Here's a graph created by highly respected acoustician Eric Desart, showing exactly this point:

GasflowResistance5b-T.png

(That is from a series of studios that Eric did a while back, exactly on this point, and it comes from StudioTips, as far as I recall. I hope he doesn't mind me reposting it here, as I can't find where it came from originally, to link it.)

That graph compares the absoprtion vs. frequency for several different densities, using 55kg/m3 mineral wool as the baseline. As you can clearly see, lower densities are better at absorbing low frequencies, and higher densities are worse. From that graph, it would seem that your 100kh/m3 stuff would be only about 60% as efficient, compared with 40 kg/m3 which would be 120% more efficient (than 55 kg/m3).

Not intuitive at all, but true.

Another with a Slat Design on the front.

With a sealed cavity, slats can act as tuned Helmholtz resonators too, so be careful of the spacing and dimensions on those. Here too you want to make sure that you are absorbing the correct frequencies for your room.

- Stuart -

[Ali Gator]

Thank you Stuart, you are the man

So what do you think abt this foam of 2" thickness at that 45KG/M3 density?

If it really is true acoustic foam, from a reputable manufacturer, then that should work, but 2" isn't enough so you'd need to double the thickness, or even triple it. But do be careful! There are many "not so reputable" people who sell ordinary packing foam and pretend it is acoustic foam: it isn't. Acoustic foam is open-cell foam with very precise gas flow resistivity characteristics. Packing foam, mattress foam, upholstery foam, and other similar foams are much cheaper, but are basically useless for acoustics. They look the same at first glance, but they are totally different, at the microscopic level. so be careful when you buy "acoustic foam"! If it is cheap, then it most likely is not acoustic foam.

Yes i realize that.. i don't have much of a choice here bcus i only got this one source here in my city who are saying it is acoustic foam. I thought by density you could be sure if it would work or not for acoustic absorption.. how can i tell if it is open-cell or not? Is there any specifics i can ask them to verify that it is actually acoustic foam and not furniture foams? Check the picture below, they said it can be colored too if ordered.

i will provide the specs of the room soon,

Great! I would suggest that you do it in SketchUp, which is sort of the standard here on the forum (and other forums too).

Absolutely

Basically, there is an optimum range of density (related to gas flow resistivity) for each type of material and each frequency range, and the general curve looks like this:

gas-flow-resistivity-bell-curve.gif

(That graph is from the University of Kentucky, but I don't recall which publication)

So there's a peak, optimum point of absorption, for each specific flow resistivity, but it turns out that this also varies by frequency. Here's a graph created by highly respected acoustician Eric Desart, showing exactly this point:

GasflowResistance5b-T.png

(That is from a series of studios that Eric did a while back, exactly on this point, and it comes from StudioTips, as far as I recall. I hope he doesn't mind me reposting it here, as I can't find where it came from originally, to link it.)

That graph compares the absoprtion vs. frequency for several different densities, using 55kg/m3 mineral wool as the baseline. As you can clearly see, lower densities are better at absorbing low frequencies, and higher densities are worse. From that graph, it would seem that your 100kh/m3 stuff would be only about 60% as efficient, compared with 40 kg/m3 which would be 120% more efficient (than 55 kg/m3). Not intuitive at all, but true.

This diagram is a big help my man, thank you so much, i have no idea how i got it the other way around.. :/ I'll look into this in more detail and get back to you Sir : ) but for now just tell me this, how it is that a plastic sheet that some consider using to cover the rockwools or fibreglass to lock stray fibres from getting airborne; reflects back the high and mid frequencies while it doesn't affect the low frequencies from getting absorbed inside??!! Shouldn't that be the other way around as well then?

Another with a Slat Design on the front.

With a sealed cavity, slats can act as tuned Helmholtz resonators too, so be careful of the spacing and dimensions on those. Here too you want to make sure that you are absorbing the correct frequencies for your room.

I'll show you the progression of the design, i'm working on it Get back to you by tomorrow on this.

[Soundman2020]

how it is that a plastic sheet that some consider using to cover the rockwools or fibreglass to lock stray fibres from getting airborne; reflects back the high and mid frequencies while it doesn't affect the low frequencies from getting absorbed inside??!!

A sheet of plastic is not a porous absorber! It's a solid barrier to sound, a reflective surface. But also a membrane


Sound waves consist of two components: velocity and pressure. Air particles move back and forth as the energy waves travels through them, so the particles have velocity. If they hit a solid surface, obviously the stop and bounce back, and that is what we call "air pressure". So a sound wave consists of the combination of velocity and pressure, and the relationship changes at different points along the wave. Where the pressure is at maximum, the velocity is at minimum, and vice versa.

Porous absorbers work on the velocity component of sound waves: as the sound moves through the insulation, some of the particles hit the fibers, transferring some of their sound energy into the fibers, where it is converted into low-grade heat energy, but the rest of the particles carry on through the insulation. Some are deflected slightly their path, others are deflected more, some are not deflected at all. So the sound wave moves through the panel, losing some energy along the way.

But a sheet of plastic is not porous to sound waves: it works on the pressure component of the sound wave, not the velocity component. When a wave hits the plastic sheeting, ALL of the particles stop and bounce back. NONE of them make it through to the other side. Plastic is a solid barrier to air. But plastic is also a resilient membrane, so waves that have enough energy can cause the plastic sheeting itself to move and vibrate, thus transferring energy to the other side: particles with enough energy cause the plastic to move and vibrate, and as it vibrates, it bumps into other air particles on the other side, causing them to vibrate at the same frequency. But the key here is that the sound wave pressure component has to be big enough to make the plastic vibrate, and that only happens for low frequency sound waves: Since resilience depends on frequency (it is impedance, not resistance), the plastic reacts differently to different frequencies. It vibrates easily for low frequencies, less so for mids, and not at all for highs, depending on the characteristics of the plastic itself, an the thickness.

So it is two different principles of physics that you are looking at: porous absorption on the one hand for the insulation, and membrane resilience on the other hand for the plastic sheeting.

Shouldn't that be the other way around as well then?

No, because it is two different mechanisms at work. Porous absorbers are based on gas flow resistivity, but for a plastic sheet the gas flow resistivity is practically infinite! No gas can flow through it at all, so the equations for porous absorbers do not apply to plastic sheeting. On the other hand, plastic sheeting is a membrane (technically, it is a foil), so the surface density, mass, stiffness, resilience, and other characteristics come into play. Those simply do not apply to fibrous insulation at the microscopic scale. Each fiber has practically no surface density, no mass, and complex stiffness, so the equations for membranes simply do not apply to porous absorbers.

Two different materials, two different acoustic principles, two different effects.

(OK, caveat for the nit-picker physicists: The above explanations are somewhat simplified, to make it easier to understand the basic concepts, but they aren't strictly correct for the purist acoustician who loves perfect theory! They are just meant to give a rough idea of the mechanisms involved.)


- Stuart -

[Ali Gator]

Stuart, you are my best friend from here on out here thanks man, thanks for taking the time to clarify in such a way, really appreciate it

Also, if you could advice on the acoustic foam query i wrote later on, would be grateful.

Here is a draft of the scatter panel / slat panel i'm working on for the frontside of the absorbers.

This is based on the GIK Scatter Plate slat design. The measurements are for a 4.5 feet high, 20" wide and 6-8" deep panel as before. The back and the top side will be left open of course. Sides and bottom shall be kept closed. The rock wool or the additional layer of less density 2" acoustic foam would be placed right against this 1" thick Plywood slat frame or the backside, or maybe on both. Should there be an air gap between the slat frame and the 1st layer of absorbent material? Not sure.. Let me know what you think

[Soundman2020]

Here is a draft of the scatter panel / slat panel i'm working on for the frontside of the absorbers.

OK, that should work fine. The slat and gap dimensions are wide enough that there should be no resonance there, only scattering, diffusion and reflection. Looks good!

Should there be an air gap between the slat frame and the 1st layer of absorbent material?

Not necessary. Nothing is resonating here, so a gap at the front won't make much difference. You can leave a gap if you like the way it looks: it won't do any harm. Or not, if you prefer the way that looks...

how can i tell if it is open-cell or not? Is there any specifics i can ask them to verify that it is actually acoustic foam and not furniture foams? Check the picture below, they said it can be colored too if ordered.

Unfortunately, without looking at or testing an actual sample of the foam, it is hard to tell! That's why it is important to buy only form reputable manufacturers, who publish their test data from independent labs. If the manufacturer does not provide the results of certified acoustic tests done on his products by reputable independent acoustic laboratories, then you have to wonder what he might be hiding... All good manufacturers provide such data. The ones that don't, you should avoid.

But you don't really need foam at all: ordinary fiberglass and mineral wool insulation works very well, and is much cheaper, in general. Foam is fine, if you are certain about it, can afford it, and like the way it looks, but isn't the only way of doing things.


- Stuart -

[Ali Gator]

Thanks Stuart

I'll import materials if i have to for the major works. This is just the beginning of my projects.

No foams for now. I'll just use double wraps for the rockwools and make the panels. One kind with the scatter and the other without. I'll post the diagrams soon. Thinking of making them look very contemporary so need to think on this tonight ; )

[Ali Gator]

Ok, i disappeared for a while but only to get this cool design i had in my mind done in sketchup .

Let me know what you all think. I'll be making these very soon and post pictures. This can be made in reverse as well. Black Frames and white or any other color of fabric.

This is the Broadband Absorber with all open sides. And guess what, i found 65KG/M3 Density of rockwool at last

Next up is the one with the Slat/Scatter front panel. Cheers!!