BASS traps with FRK (or something like it)

[Mr.Smith]

Quick question.

The room is about 10.1 x 11.2 x 7.4 (more or less). Its all I have to work with right now.

I am bass trapping the living daylights out of it (as much as I can anyway). I can't do superchunks as some would suggest because Its not practical at this time. But all my traps are approx 6" thick at 3.5lbs/cu ft.

I know the room is small but I dont want it to be completely dead (if I can help it). Mixing in a completely dead room is not my preferenece but would live with it, if I had.

Would putting a membrane on some corner traps work? Or would it not be practical and cause other unwanted issues? Is there specific places/traps to use it and get away with it?

thanks in advance

[DanDan]

Yes, a membrane will enhance LF absorption, particularly on corner mounted traps. The membrane is most effective facing out, but there will be HF reflection from it. I believe the membrane will still work facing in. Strangely this has not been tested properly, but I see no reason why the membrane would cease to work in the in position. You will find plenty on this at Ethan Winer - Home Page
DD

[Mr.Smith]

Hey.
thanks for the reply.
I guess I wasn't as clear as I should have been. The membrane would be facing the inside of the room and not the corner. I plan on doing it to keep some life in the room (upper mid/high frequencies). But if it is going to do some damage to the room (based on the small size), I would rather not place the membrane on the trap.

[tonio]

I have 4 corner traps covered w/ 4mil plastic over Rockwool SAFE-floor to ceiling 4"thick in a 12x13x8' mix room. I beleive the density is 4.5pcf

I plan to test the scrim on inside/outside, as soon as I can get Fuzzmeasure to not quit on me

T

[Mr.Smith]

Also, before I commit to it. Do bass traps have to be evenly distributed in thickness/width for optimal effect?? Ie, left vertical wall is 2ftx4ftx4in as well as right wall. Or Can they be different?? Example left wall is 2ftx4ftx4in and right wall is 4ftx4ftx6in.

The reason I ask is because I have one wall where I can make the width of the panel a lot wider (pushing 3 ft+) where as the other 3 vertical corners will be 2ft wide. I assumed it would be fine but Wasn't sure if it would effect the sound in a negative way. Just want to confirm before committing to the build.

[DanDan]

tonio, that test will be greatly appreciated. I have a feeling that 4 mil is way too thick though. We have seen very successful tests with FRK, which I would guess is less than 1 mil. Ethan is very experienced with this system. He has stated that it is necessary to bond the membrane to the fibre. This seems to be achieved using a light application of spray glue. SprayTac. This make your testing difficult. If you just want the result, I would be 100% sure that membrane facing the room will work very well.

Mr. Smith, absorption is ideally evenly distributed. However, in small rooms, more trumps that. Get as much Bass Trapping in there as you can.

DD

[tonio]

DD, my previous place I used .7mil, and it didn't do much of anything-but my purpose was to bring back some of the high end.
Currently working on a new room and by listening the 4mil is holding up to keep the high end intact though to a lesser degree. It could be that I still need a few more wall panels to bring up the sabin level. We shall see.

Found that Fuzzmeasure just quits on me if I use the device auto correction.
So , I'm rather at a standstill as far as the tests. Do you think it would make a huge difference if the correction feature is not used?

T

[Mr.Smith]

Thanks DD.

What if the FRK isn't bonded to the insul. but sandwiched between the fabric and isul. is there a negative effect? or just not as strong of an effect?

[DanDan]

tonio, there should be a considerable difference with the membrane. Perhaps your 0.7 mil was not bonded to the panel? Tests show quite a difference in LF performance with FRK. FRK looks less than 1 mil to me. The commercial products use thin membranes. Your 4 mil will be an experiment, an unnecessary one I suggest.
Turn off the Device Auto Correction. Room anomalies can be 10-30dB, so any little wobbles in your Device are completely insignificant in perspective.


Mr. Smith, Ethan has on several occasions said that the membrane needs to be bonded to the panel. When he speaks like that, there is always a real test behind the scene.

DD

[nms]

so is it correct then that the proper way to install the FRK is with the foil side spray adhesived onto the insulation? also going by the LEDE method, it would be good to use FRK on all receiving wall traps?

[DanDan]

I would buy it with the FRK on it. Yes I recommend FRK on all panels. Facing away for RFZ and Cloud. Chose between facing the corner or listener with Corner Straddling Traps. Facing the listener returns some HF to the room, and has a better LF performance. Facing the corner is softer sounding and more linear.
DD

[nms]

my issue is I'm shopping at home depot and to my knowledge all they have there is Roxul Safe n Sound.. which doesn't come with an option of FRK or no FRK.

With that in mind, what's the best membrane solution? Buy some FRK and use spray adhesive with the foil side against the insulation? Or perhaps just buy some poster board paper and adhesive that on? Maybe really thin plastic? I'm wondering what will get me nice low extension since I'm in a small 9ft 8" long room and this would be for the receiving wall.

[SAC]

I fear you are making this far too difficult!

Use the Safe & Sound if you need to.

If you want to add a barrier that functions similarly to the FRK barrier, you can simply securely attach a plastic membrane to the back of a frame (which you will need anyway with the non-rigid S&S material), and fill the panel frame with sufficient absorbent material to establish a firm physical interface with the covering both front and back - something you should be doing anyway!

No need to make this more difficult than necessary...

[PaulP]

Adding FRK or any other membrane will seriously affect the absorption properties
in a completely unpredictable fashion. Instead of broadband absorption you'll
get a hump and what if the hump isn't in the right place ? Are you prepared
to build a few, measure them in place, and start all over again if you don't like
the results ?

From what I've read what you want for very low frequencies is a very thick
amount of lightweight attic insulation (8" and more).

See BBC RD 1992/10 The design of a modular sound absorber for very low frequencies.

EDIT : I just went back and noticed you're looking to add back some highs but I don't
think you can just add a membrane and expect it to do something predictable.

Paul P

[DanDan]

Paul, with respect, I think it is pretty predictable, extrapolating from these tests.
Rigid fiberglass density tests
Note Ethan uses a different membrane these days, some sort of very thin plastic film. He also is very strong that it needs to be bonded to the fibre.
Paul, I took your view on this for a while. I was very convinced that membrane clad panels were resonant in an undesirable non linear way.
I removed a whole bunch of them. Tests before and after showed a slight deterioration in LF, actually VLF modal control. Thus I now recommend membranes on all panels, albeith facing away, as mine were.
To be honest though, I would buy insulation with FRK. I am not sure that I would go to all the trouble of sourcing a membrane and spray gluing it on.
My test wasn't that spectacular!
DD

[SAC]

Gee Paul! Such melodrama! And more reasons to simply sit and do nothing more than contemplate disaster!
As if skinning (or un-skinning) a panel is SOOOO complex!

Here's a radical idea sure to confuse many!!!

Make measurements of the room so that you know its quantified behavior. Easily done.

Isolate and note the behavior of the region of the room which you will treat - Easily done.

Make a panel and install it. Easily done.

Remeasure paying attention to the specific response variables (reflections, modes) limited to that particular spot. Easily done.

Modify as necessary in an informed manner. Which is how ALL informed tuning should be done! Easily done.

And if disaster strikes, and you manage to survive, you can simply remove the attached layer from the back of the panel and re-staple the backing material with only a few people and pets (and let's not forget houseplants, they have feelings too!) being irreparably harmed and traumatized.

Or you can just sit and contemplate imminent destruction until someone comes and magically does everything for you.

[PaulP]

Originally Posted by SAC

As if skinning (or un-skinning) a panel is SOOOO complex!

Who said anything about being complex ? I've been saying time consuming.

I may be wrong, but I have a feeling most people here are not interested in
turning them rooms into laboratories in which they spend the rest of their
lives conducting acoustical experiments.

This limits which forms of treatment we should consider, and will no doubt
result in a less than perfect solution, but a practical one.

I vaguely recall seeing Ethan's experiment a while ago but re-reading it
it does indeed look like frk can help in a broadband fashion for low
frequencies. The mass of the FRK surely has something to do things so
it would be interesting so see a comparison of different membranes stuck
to the fiberglass.

It would have been nice to see the effect on the higher frequencies as
well. I'm not crazy about the idea of adding FRK just "to get back some
highs". Which highs and by how much ? How does it relate to what is needed ?

Paul P

[SAC]

You also said: "Are you prepared to build a few, measure them in place, and start all over again if you don't like the results?" Which is a completely absurd statement. As we pointed out, all that is necessary to return them to their un-skinned state is the literal removal and reapplication of a few staples.

Let's see, pulling some staples and re-stapling would surely waste at least 15 minutes of someone's life!

Which ironically is Much less time then that which is wasted reading the myriad posts regarding how much time would be lost in verifying results you assert without any data to verify said results!

And the suggestion that one could easily try this hardly constitutes the utterly absurd statement that anyone has proposed that "people ...turn them rooms into laboratories in which they spend the rest of their lives conducting acoustical experiments."
Nope, instead what 'many' want are to behave as little birds in the nest expecting to be spoon fed by others who have taken the time to learn and experiment with such technologies by those too impatient or unwilling to do things themselves. And we had better hurry up in providing such information!

And heaven forbid you actually take all of that time to actually make measurements to quantify your myriad questions.

With all due respect, I am glad to try to help anyone who has both an interest and the wherewithall to actually try to increase their knowledge by trying. But I feel no obligation to help someone who does little else but simply sit on their posterior and demand quick answers based upon others efforts!

[PaulP]

Paul P

[Mr.Smith]

lots of contradictory answers. The only way now is to do some testing and possible reconstruction of panels. What I may do is leave the fabric off the front of all traps I decide to put the FRK on, test the living daylights out of the room in all possible scenarios (frk facing in, facing back, and none) and then fabric accordingly.

Thanks for everyones responses.

[nms]

They use "a very thin plastic" over Realtraps.. so what about the idea of using the thin packing plastic that comes on a roll used for shipping? That's pretty thin and I have some kicking around.

It seems to me that throwing a membrane on there and spray adhesiving it on is pretty easy to do.. and as far as I know will give you more low end extension. Is this not a better idea for corner traps than just fabric alone?

[SAC]

Let me try to provide a 10cent summary of the options posited in the hopes that it might help you decide on a course of action...

FRK (or FSK for "foil-scrim-Kraft"(paper), and a few other descriptive variants depending upon the manufacturer) is a thin non-porous sheet coating applied to some of the various semi-rigid and non-rigid insulation normally intended for use primarily as a vapor barrier. Here it is proposed for use in select configurations of bass traps.

Let me explain... A trap sufficient to absorb low frequencies is also quite capable of absorbing the mid and high frequencies! But, some would prefer to have a more 'frequency selective' bass trap whereby the LFs were absorbed, but where some of the mid and high frequency energy was preserved in order to avoid as 'dead' a space and to employ the MF/HF energy along with diffusion to create a more diffuse soundfield - and thus augment the very small space with a sense of greater volume...

In order to preserve some of the mids and high frequency energy in this case, the front surface is covered with the 'FSK (film) which is acoustically transparent to the high energy LF waves, yet reflective to the lower energy shorter wavelengths of the mid and high frequencies.

So that is one approach utilizing the coating.
A very oversimplified explanation, but I think it may serve in order to create a comparison.

In addition, it has been proposed by a few reputable sources that a bass trap with the FSK film applied to the back surface, while not doing anything to preserve mid and high frequencies in this case, however augments the absorbers efficiency in absorbing and trapping LF energy. Thus for the small investment in materials and time, the efficiency of the bass trap can be improved.

Bottomline, in either case, the difference and the effort required to effect the changes (or to reverse them if you do not like the results) are quite minor compared to the potential gains.

So, if you plan ahead, it should be quite easy to try the version appropriate to your intended use, with the option remaining to further modify them if you do not like the results.

I hope that is sufficient to enable you to to sort the various primary choices out...

[tonio]

K then.

I took a few tests with scrim (4mil plastic sheeting) on the inside (toward the room )and outside (toward the corner) for 4 - 2x8' x4" Roxul SAFE (4pcf) obsorbers, simply by taking a measurement then flipping them and taking another measurement.

Result? Not worth phoning home about

With the scrim on the inside the high end (@ 10khz +) had approximately 3 dbs more amplitude and extended further up to about 18khz without dramatic drop vs scrim on the outside. Also there was @ 3-4db rise around 3khz , and @120hz. I do have a null @ 120hz which may help, however found that adding a 2x4' panel at the rear wall had a larger effect on the low end.

My room- 12' x12' 10" x8" is far from finished. Also the room is coupled with openings at the rear, so is my individual case.

T

[SAC]

When you say "took measurements", be aware that the primary differences with the scrim on the front will be seen in the ETC - NOT a frequency response curve. The purposes of the scrim is different for front and back. One is to increase absorption, the other is to increase reflection of energy of wavelengths shorter than the panel dimensions.

FR (and related) are great for LF modal issues, but near useless for the evaluation of reflections. That is where you want to use time domain measurements such as the ETC.

[DanDan]

Hi tonio. I presume you meant 0.7mil not 7.
Your test would seem to indicate that the LF performance is quite similar no matter what side the film is on. Exactly my point. It works. It would have been interesting to test without versus with. However Ethans test is pretty clear proof of the LF benefit.
I have another test here which I can't publish. It is Lab test. It shows that FRK facing the listener has stronger LF absorption than facing the surface/airgap behind it. However they too didn't compare without film.
DD

[DanDan]

Finally I found my test. The one which has me (somewhat controversially) recommending foil on traps facing the wall/airgap.
These graphs are from a small room, concrete walls and floor, lots of Bass traps. Six or eight other traps create an RFZ and Cloud. These are HF MiniTraps. I got the notion that foil on the back couldn't be helpful, was perhaps causing a resonant bump, removing warmth in the room sound. So I removed it. The graphs can speak for themselves, with one additional comment.
The panels plus FRK have an effect on modal activity below 100Hz.
DD

[nms]

What about using Saran Wrap all the way around corner traps and 6" bass trap panels?

That's a pretty thin plastic and seems like it won't reflect the mid/hi really at all?

[tonio]

DD, actually the scrim is 4mil, will correct original post.
In the past I asked a few forum boards about a room being too dead and inquired about diffusors. Due to the room being small many folks have suggesting to use scrim then fabric over BB treatment. The numbers suggested was 6mil. Not sure it was meant to .6 or not, however.
But that was before Ethan did the tests.


SAC, thank you in regard to information of using ETC for reflections. I have seen that mentioned before, however I am new to the ETC vs impulse responses. Actually I believe it is anew function in FM?
Could you perhaps give a run down on how to set up the response graph, and how to read/determine the responses?

I can post screen shot if that helps.

That said, I am more concerned about the low end being in a small room, those issues are a priority. Not sure sure if that would be inline with OP however. If not I can start my own thread as to not detract Mr. Smith.

T