Another Gas Flow Resistivity Question - difference between "facewise" and "edgewise"?

[Max Dread]

Hi

Sorry for all the questions - the more you think about all this the more questions seem to arise!

I was wondering whether the gas flow resistivity of a given product will be different depending on its orientation? For example, if we take a commonly used mineral wool such as Rockwool RW3, would the gas flow resistivity be different when in is used in a panel (facewise) to when it is used in a superchunk (edgewise)?

My guess would be yes, but I'm not certain nor am I sure of how much of a difference there might be.

Any thoughts?

Cheers

Max

[Sebg]

Pretty sure Rockwool fibres are random orientation. Other products such as polyester batts usually have a "lay". Some RW products may also be produced in layers, in which case flow resistivity may vary slightly according to direction but shouldn't be by much - pretty sure the published articles talk about predicting flow resistivity based on fibre diameter and no. fibres/unit volume.

[Max Dread]

Cheers for sharing your thoughts.

By visual inspection and feel alone, there seems to be a difference. The front face seems denser and is ribbed, whereas the edges/sides seems fluffier.

I'm wondering whether there are different amounts of binder (or even a different binder altogether) used on the front and back. But that's a guess!

Would be interesting to hear other thoughts....

[Sebg]

Some products do indeed have a "face finish" either by design or by "accident" due to the manufacturing process.

[Hannes_F]

Light glass wool products that I use like Isover Acustic TP1 have a fibre orientation. Additionally they are more contracted at the surface with a bonding glue. All in all their gas flow resistivity is anisotropic (depending on the orientation).

My guess is that the gas flow resistivity is about half or less than that if the sound is fired along the fibre direction.

[Max Dread]

Interesting comments guys.

Especially yours Hannes RE: which way around things are.... Just to clarify, are you saying you think that Frontwise orientation would give 100%, and edgewise around 50% of that?

With my RS60,

- IF it has a gas flow resistivity of around 22,500 (cannot find data so have to guess)

- IF it has a different binder on the front face or is subjected to a different process in the manufacturing on the front face, either leading to a different gas flow resistivity

....one would have to ask whether the 22,500 relates to "frontwise" or "edgewise". If the latter, would that then mean that in a superchunk arrangement this denser (60kgm3) higher resistivity (22,500) mineral wool is actually closer to to fluffy?

I know that's a lot of IFS and speculation!!!

Incidentally, I would say that the generic fluffy loft insulation I have come across seems much similar frontwise and edgewise when compared to things like RS60.

Cheers

Max

[SAC]

Max,

Be careful, as you are coming very close to where the complex acoustical impedance of a material is precisely useful = along with some very complex relationships.

You have raised a very prudent point, but one that does NOT lend itself to simple linear statements of relationship assumed to be uniformly distributed. And as such you are not going to get a simple answer - if such were even available from a manufacturer.

[johndykstra]

Interesting.

Had never even considered this, but now that you mention it, my 703 is certainly softer on the edges than the face... sort of like natural hair versus Aqua net.

Perhaps this goes to explain (among other factors), the efficiency of superchunks.

[Max Dread]

Originally Posted by johndykstra

Perhaps this goes to explain (among other factors), the efficiency of superchunks.

My thoughts too.....

Originally Posted by SAC

Max,

Be careful, as you are coming very close to where the complex acoustical impedance of a material is precisely useful = along with some very complex relationships.

You have raised a very prudent point, but one that does NOT lend itself to simple linear statements of relationship assumed to be uniformly distributed. And as such you are not going to get a simple answer - if such were even available from a manufacturer.

Yes - I'm being very careful! In fact, I put on a helmet before writing that post!

I've now turned around to go home and am retracting my head back to my shoulders after having stuck it out a bit.

I've also had a few glasses of wine!!!!!

Thanks guys for the repies and input - I really appreciate it.

And SAC - I totally agree. I've not been able to find out the gas flow resistivity figures for the products (the guy from Rockwool said: "the airflow resistivity [of our generic loft roll] is 1, like wise for the RW3."!!!!!!!!!) - so I'm not expecting them to be able to comment in any understandable form with regards this question!

Max

[Max Dread]

Originally Posted by Hannes_F

My guess is that the gas flow resistivity is about half or less than that if the sound is fired along the fibre direction.

Originally Posted by Max Dread

Interesting comments guys. Especially yours Hannes RE: which way around things are.... Just to clarify, are you saying you think that Frontwise orientation would give 100%, and edgewise around 50% of that?

Hi

Hannes - if you come across this - I wondered if you'd be happy to give this clarification as to what you were suggesting?

I was looking through some photos and they led me to think again about this issue again. I'll attach them to the post. I know most folk will be familiar with rockwool, fibreglass etc, but the photos really bring home just how different the front face and edges really are......


Cheers to all

Max

[Audioactive]

stupid question here, if edgewise the gas flow resistivity is only half than frontwise, if we use the same mineral fiber frontwise we only need half the volume to get the same result (?)..

[mhch]

Originally Posted by Audioactive

stupid question here, if edgewise the gas flow resistivity is only half than frontwise, if we use the same mineral fiber frontwise we only need half the volume to get the same result (?)..

Another linear extrapolation ... I doubt it is that simple specially for a superchunk, considering the various factors at play, flow velocity distribution, varying depth, etc ...

[ajbianco]

this whole gas resistively thing really confuses me. on one hand people want more resistively so that when sound makes it way into the absorber, more will be absorbed. on the other hand people are saying that you want a low resistively because it lets more of the sound into the medium to be absorbed rather than being reflected.

surely there is a point where these two factors combine to give an overall ideal outcome? does anyone know what the best value is for low end absorption?

im looking for documents online but haven't found anything as yet. just about to hit the IEEE journal articles but id be surprised if this wasn't already common knowledge in this forum?

[Max Dread]

I think it really depends on when you say "LFs" which are of most importance to you. And to find that out, you really need to measure your room.

Have you downloaded and tried the Porous Absorber calculator?

See Porous Absorber Calculator V1.58 if not.

Here's what I've done. I currently have two main LF problems in my room - which I discovered through measuring the room. In my case, these problems are:

- modal ringing at around 40Hz
- a big broad dip from 90 to 150Hz

Knowing what areas are free for bass trapping in my room (and knowing the dimensions of these areas) I then used the calculator to find the best GFR to use. The calculator is handy because it has spin buttons which lets you see the changes more easily. To put this into an example....... the back half of one of my side walls forms a large alcove with a chimney breast. The alcove is 40cm deep. Using the PAC, I put 400mm into the "absorber thickness" field. I changed the "Start Graph at" field to 30Hz to see what's happening at the lowest frequencies. I changed "Absorber flow resistivity" to 2000. I then used the spin button to increase the "Absorber flow resistivity" and watched the graph changing. For me it was about compromise. As absorption at 40Hz increased, absorption in the 90-150Hz region decreased. So I found the best range of GSR which dealt with my frequencies of interest by doing this.

And as you can see, there was no "ideal" for LF absorption. One end of the spectrum was best for one area of LFs, but at the cost of the other.

But I should add a couple of other things to consider:

- depending on where you live, you may or may not be able to find MW/GF which has a published GSR. And even if you do, there's no guarantee that the published spec is correct. (That's where I'm at. I have my chosen GSR figure, but no product to buy with that figure).
- Even if you did know the GSR and it was accurate, things like the "facewise/edgewise" question, how the trap is constructed (and therefore how much the fibre is compressed), etc. will have an effect on the figure.
- many people say that the behaviour of LFs in room corners is different and the PAC does not necessarily allow for this. If so, that obviously changes the significance and/or accuracy of the info obtained from the PAC. IN fact, I believe some folk on this forum compared the behaviour of SSCs using a low GSR mineral and a more dense product (RW3/RS60/OC703 I think) and found the results to be less divergent than one might have expected.

So, lots of things to consider it would seem. Enough to give you a headache!

And yet, a lot of people knowledgeable in all things acoustics will recommend in threads on here that if your traps are going to be large, and you want them to deal with the LFs in your room, generic fluffy loft insulation will be the best choice. That's the advice I'm going to stick by. For the reasons given above, I can't really see an alternative.

One thing I might do though if I can get enough spare RS60 together - fill my alcove with fluffy, measure, replace for RS60, measure, stick the results up on this forum.

Good luck

Max

[ajbianco]

Originally Posted by Max Dread

I think it really depends on when you say "LFs" which are of most importance to you. And to find that out, you really need to measure your room.

Have you downloaded and tried the Porous Absorber calculator?

See Porous Absorber Calculator V1.58 if not.

Here's what I've done. I currently have two main LF problems in my room - which I discovered through measuring the room. In my case, these problems are:

- modal ringing at around 40Hz
- a big broad dip from 90 to 150Hz

Knowing what areas are free for bass trapping in my room (and knowing the dimensions of these areas) I then used the calculator to find the best GFR to use. The calculator is handy because it has spin buttons which lets you see
the changes more easily. To put this into an example....... the back half of one of my side walls forms a large alcove with a chimney breast. The alcove is 40cm deep. Using the PAC, I put 400mm into the "absorber thickness" field. I changed the "Start Graph at" field to 30Hz to see what's happening at the lowest frequencies. I changed "Absorber flow resistivity" to 2000. I then used the spin button to increase the "Absorber flow resistivity" and watched the graph changing. For me it was about compromise. As absorption at 40Hz increased, absorption in the 90-150Hz region decreased. So I found the best range of GSR which dealt with my frequencies of interest by doing this.

And as you can see, there was no "ideal" for LF absorption. One end of the spectrum was best for one area of LFs, but at the cost of the other.

But I should add a couple of other things to consider:

- depending on where you live, you may or may not be able to find MW/GF which has a published GSR. And even if you do, there's no guarantee that the published spec is correct. (That's where I'm at. I have my chosen GSR figure, but no product to buy with that figure).
- Even if you did know the GSR and it was accurate, things like the "facewise/edgewise" question, how the trap is constructed (and therefore how much the fibre is compressed), etc. will have an effect on the figure.
- many people say that the behaviour of LFs in room corners is different and the PAC does not necessarily allow for this. If so, that obviously changes the significance and/or accuracy of the info obtained from the PAC. IN fact, I believe some folk on this forum compared the behaviour of SSCs using a low









GSR mineral and a more dense product (RW3/RS60/OC703 I think) and found the results to be less divergent than one might have expected.

So, lots of things to consider it would seem. Enough to give you a headache!

And yet, a lot of people knowledgeable in all things acoustics will recommend in threads on here that if your traps are going to be large, and you want them to deal with the LFs in your room, generic fluffy loft insulation will be the best choice. That's the advice I'm going to stick by. For the reasons given above, I can't really see an alternative.

One thing I might do though if I can get enough spare RS60 together - fill my alcove with fluffy, measure, replace for RS60, measure, stick the results up on this forum.

Good luck

Max

When I say optimal absorption I mean of the absorber itself. I don't mean the net response of the room. That is very complex and application specific however i'm just trying to make sense of absorber materiel. I am aware of the modes of my room. I have a 30dB hump at 43Hz and a null at 110Hz but before i try to treat that, i need to pick the materiel that will suit my treatment strategy. right now that means the most effective low end absorption i can get reasonably priced.

The strategy chosen to flatten room Responce and the effectiveness of the bass trap material I'm looking at as 2 different things.

[ciro]

Originally Posted by Max Dread

IN fact, I believe some folk on this forum compared the behaviour of SSCs using a low GSR mineral and a more dense product (RW3/RS60/OC703 I think) and found the results to be less divergent than one might have expected.

I remember the thread (maybe the same) comparing 45 vs 60 fiberglass performance on big chunks? And the difference was sooo subtle.
If is the same comparison, although was done in a room with some previous treatment and most issues on lows (decay) basically "solved" before the chunks(I remember the waterfalls),and probably not a fair comparison...
...I agree, some difference would be expected, and no, zero ... "edgewise effect" maybe?

I think any test "face/edge wise" would be welcome!

Ciro

[Brainchild]

Depends on how thick and what orientation and cetera.

If you look at these side-view shots of a speaker firing into 700-series panels stacked against the side wall of the room, in orientation one the panels are just facing the room.

In orientation 2, they're facing the speaker.

In orientation 3, they're facing the floor/ceiling.

In orientation one, the soundwaves are hitting the front faces at all angles.

In orientation 2, they're hitting the edges at the surface, but as the waves enter the material they're hitting the front faces of the panels at most angles.

In orientation 3, they're hitting the edges and continuing to move through the "edge fiber orientation" as they move through the interiors of the panels at most angles.

So, orientation 3 would ostensibly give you the most uniform internal impedance for most angles.

Of course, the panel orientation could be flipped to the vertical toward the floor and ceiling very close to the speaker in order to get almost perfect internal impedance consistency for almost all angles.