Absorber question, why a damping mat? (ceiling cloud, wall, GOBO)

[Mister Fox]

Dear Sirs and fellow audio engineers,

I hit the search function really hard but was unfortunately not able to answer my question. So I hope you don't mind me posting yet another "fundamental question", should this be one.


A little bit of backstory:
I'm building several kinds of absorbers for my studio at the moment: clouds (4cm rockwool, aluminum frame, hanging 10cm from the ceiling), gobo's (10cm rockwool, size 200x62,5x15 cm) and wall absorbers (4cm rockwool, size 100x62,5x10 cm)

The wooden frames are made of 18mm plywood, glued together with dowels rather than screwed together, covered with a transparent glaze. The wooden frames are made so that the inside fits an in cloth wrapped rockwool panel from Daemmisol (firm from Berlin), almost similar to Owens Corner.


The thing is, I know that I can broaden the efficiency of my wall absorbers, if I use some sort of spacing between the rockwool absorber and the wall. Hence the depth of the frames of about 10cm. With 4cm rockwool panels I have a 6cm spacer and therefore a more broadband absorbtion on the walls.


The only thing I don't get is the following:
Years ago I was watching a Sound on Sound Studio SOS video about building and optimizing absorber in not so ideal studio environments. One thing that always stood out to me was that both Paul White and Hugh Robjohns always seem to add some sort of heavy mat to a wall absorber to make it "broadband".

Here is a video example: Studio SOS with Midge Ure - Part.1 - YouTube
(at about 8 minutes into the video)


The questions are:
Do I need that mat? What kind of mat is it anyway? And what is it for?

I always thought having a spacer adds as much efficiency as having a heavy mat that throws back the not-absorbed frequencies. This is at least what I got out of several posts here on KVR, and (I have to admit) from videos from Ethan Winer as well.

Am I on a wrong track here?
Thanks for reading and commenting.

[kasmira]

That is whats called a "mass-loaded vinyl barrier" (or it could be some other sort of limp mass barrier)

Now that you know what its called, search that in the forums and you'll find all sorts of threads about em

[Mister Fox]

Soooo... a quick search revealed that "mass-loaded vinyl barrier" mats are basically for further insulation. Speak: making stuff more soundproof towards the outside world.

While I see this as an indeed good aproach towards walls (and neighbouring apartements), I ask myself "why with wall absorbers, clouds and GOBO's?".

If I look at it, these absorbers are there to reduce the reflections of a room, not necessarily "killing" everything towards the outside world.


Can somebody (from the longtime builders) please tell my why I should add these mats anyway if I added to the efficiency with spacers (so to speak) already? When are these mats practical and when not?

Thanks.

[kasmira]

I am under the impression that these mats when laying limp against the back of your panel will not do anything significant, and although spacing traps away from the wall generally extends low frequency absorption - it wouldn't have nearly the same effect if there was a backing on the panel. You would probably be better off just leaving the back open / covered with fabric.

If you read this thread it shares some good info on someone asking the same question. Granted, its not exactly a "do it or don't do it" type of answer, but if you read the whole way through the thread, someone tried making an absorber with it, and barely had any success with it at all. Yes, vinyl barriers can help with isolation, but not when you just have a couple of 2x4 slats hanging a couple inches away from the wall.

Sorry, I'm no expert. Perhaps one of the experts here could give some reasons as to why it won't work well, or if it would (and let me know if I was wrong in any of my statements).

Humbly awaiting correction,
Alexander

[Mister Fox]

Hm... I digged through the linked thread and it made me think...

From the reports and from reading the linked papers (I knew about the BBC Gobo design before), it looks like these "membrane" systems are either really for tuning only (if connected right and tight), or for total isolation/soundproofing (as these things are intended to be).


It looks to me that these kind of designs are aimed at broadband absorbers like (tuned) Helmholtz Resonators. Something similar can be seen in the following videos as well:

The Studio SOS team build a vocal booth - Part.1 - YouTube
The Studio SOS team build a vocal booth - Part.2 - YouTube

Though here, they add the mat "above" the rockwool, which is probably due to the desired effect that the fabric on top of it eats the higher frequencies, while the vinyl mat reflects some mids and blocks certain low frequencies with the added 4cm rigid rockwool in the back.


Conclusing:
It "might" work for DIY bass absorbers (10cm rockwool in a 15cm deep framing to have a spacer of 5cm, maybe even 10cm if 20cm deep frames are used), but the reports simply don't convince me. I still think a proper design with a resonating wall (thin plywood) and air pockets in between the absorbtive material (in this case, the 10cm thick rockwool) might work just as good as super chunks or whatever is out there.

Or, these mats are really only there to block the soundwaves towards another room - but if that is not needed/desired?


Good questions, interesting theory stuff...

[DanDan]

Happy New Year Mr. Fox!
The BBC used Limp Membranes in several of their designs.
They are not soundproofing.
They used asphalt felt, as in roof membrane. As you will see below they later found a simple wooden panel to be more effective and reliable at LF.
But the idea was used successfully in hundreds of BBC local studios for years.
The modern version using MLV or 'Deadsheet' has fans in the UK in particular. Munro, Newell, and the SOS people for instance.
http://downloads.bbc.co.uk/rd/pubs/reports/1992-10.pdf

You could include this on your cloud or side reflection traps, if you angle them to direct HF reflections to the rear of the room.
Plse note that they are the front skin of a sealed box trap. It needs to be airtight.


DD

[Mister Fox]

So... in general these "membrane absorber" are indeed mainly aimed at low frequencies. Now I get the concept of the build in the later SoS YouTube videos I linked to (the vocal booth build). But unfortunately not the first (the ones where the SoS crew fixes Midge Ure's studio).

In other words, it's a different form of a tuned panel absorber/resonators (e.g. Helmholtz Resonators). At least if it's (somewhat) sealed.


In other words... it's worth a try to built such an absorber if I have very low frequency nodes on top of the reduced/optimized reverberation time with standard absorbers and QRDs. But I get off fairly well if I simply create wall/ceiling/corner absorbers with enough spacing as Ethan Winer said over and over in his presentation videos on the "tube" as well.

Specifically like this one:
The Ultimate Home Studio - YouTube


I really hope it's that simple, else I really don't see a reson to create MLV absorbers unless you have some real issues in your room.


Thanks and happy new year as well.

[DanDan]

I haven't watched the videos. The membrane traps are for LF, although it is worth noting the BBC ended up using a wooden panel eventually.
Copies of the BBC design should be very effective in treating modal problems, as they were designed to do. Figure on using quite a number of them though, just like any other LF trap.
It will be interesting to see more VPR results as more DIY builds happen. My Experiment with a Metal Panel Absorber
DD

[Mister Fox]

I digged myself through the treads a bit with the keywords FSK/FRK (which I found in a broadband absorber discussion), and MLV again.

To my understanding now (and after reading some posts from both Ethan Winer and Glenn Kuras, even if the vinyl or plastic sheet layer is "not" stretched and tacked to the frame, it adds to the LF absorbtion.

I think this is why the SoS crew added a MLV to the wall absorbers (facing the wall in the Midge Ure built) and one MLV to the vocal booth built (facing the room, in front of the absorbtive material). They tried to create a broadband absorber with some LF enhancements.


But I still ask myself...
Is this practical on the long run if you have broadband absorbers (rockwool) already with 4cm and 10cm (gobo, bass traps) plus planned-in spacing (additional 6cm for the 4cm rockwool and 5cm for the 10cm rockwool) to tune/dampen the room?

Aren't these membrane "enhanced" (tuned) absorbers more aimed at certain LF nodes and very small rooms (2-4qm for example)?


From reading the threads, I got two impressions:
Lab-wise it's working and I should always go for it. From DIY tests it's not working and it's not worth the effort and might add more problems than that they're being helpful.