PVA glue on broadband absorber

[jonc19]

Hi,

I am attempting to acoustically treat a roughly 9m x 5m x 2.2m room with both bass traps and broadband absorbers.

I've read that diluted PVA glue is often sprayed onto Rockwool/OC703 panels to keep the fibres in - if used for a broadband absorber will this reflect the higher frequencies?

I am conscious of keeping the fibres in for the panels that will be moved about - so Rockwool + PVA glue solution + 4oz Polyester wadding + hessian is the plan.

I looked at using cling film or plastic film instead of polyeseter wadding and/or PVA glue, but apparently this would reflect high frequencies, is this true? (and seems like it would be a fire hazard also?)

Thanks

[Soundman2020]

Hi "jonc19", and Welcome!

I am attempting to acoustically treat a roughly 9m x 5m x 2.2m room with both bass traps and broadband absorbers.

That's a pretty large room with a low ceiling. What are you planning to use it for?

I've read that diluted PVA glue is often sprayed onto Rockwool/OC703 panels

OC-703 is made by Owens Corning, not by Rockwool. You seem to be mixing up your manufacturers! 703 is a fiberglass product, and Rockwool makes mostly mineral wool products. They are very different products, both physically and in acoustic application.

... to keep the fibres in ...

OC-703 is semi-rigid fiberglass panels. It already contains a bonding resin that is used to bind the fibers together, and the surface is smooth-ish. The fibers are usually only a problem when you are cutting and installing the insulation in the actual panel, but a properly built panel should not have any further issues.

You can also order it with a tougher, more robust ASJ polymer facing, that binds the fibers even better, but that does affect the acoustic properties.

if used for a broadband absorber will this reflect the higher frequencies?

Yes. It will affect the high frequency absorption / reflection for all panels, not just broadband panels. For bass traps, that's a good thing, but not so much for first-reflection points.

Covering the panel with glue is not a good idea.

...4oz Polyester wadding...

Why? What is the acoustic purpose of that?

I am conscious of keeping the fibres in for the panels that will be moved about - so Rockwool +

Why Rockwool here, and not OC-703? Why the switch? And which particular Rockwool product are you planning to use? They make many different products. Also, how thick are you planning to make this panel, and what frequency range do you need to absorb? How big will the panel be? Will it be flat against the wall, or spaced away? Parallel to the wall, or angled? Why do you need to "move it about!?

There are many variables that you have not mentioned yet.

For broadband panels, the general recommendation is to just put a breathable black fabric (reasonably fine weave) over the front of the panel, then put the final finish fabric over that. The black fabric does two things: it retains any fibers that might happen to still be a problem, and it blocks light, so you won't be able to see the ugly interior of the panel through the weave of the final finish fabric.

I looked at using cling film or plastic film instead of polyeseter wadding and/or PVA glue, but apparently this would reflect high frequencies, is this true?

Yes, correct, and for bass traps that's a GOOD thing! You WANT to reflect the highs! In small rooms, bass traps take up a large proportion of the total wall area, and therefore over-treat the high end. So in order to keep the acoustics as neutral as possible, some type of selectively reflective surface is needed, that reflects back the highs very well, the mids to a certain extent, but does not touch the lows at all. Plastic is pretty good for that, but you could also use OC-703 with the FRK facing, or the AJS facing. The advantage of using plastic (or other similar foils) is that you can tune the trap to reflect only the frequency range that you want to reflect, as needed.

(and seems like it would be a fire hazard also?)

So is hessian!

- stuart -

[jonc19]

Hi,

Thanks for your reply.

The material I have is Rockwool RW3 - 8 x 4" panels and 16 x 2" panels (60kg/m3 according to Bob Golds). I have Rockwool instead of OC703 as the latter doesn't seem to be available in the UK.

So, first intentions were to (where possible) treat the corners with 6" (2"+4" panel) bass traps, and then use the remaining 8 x 2" panels as broadband absorbers. I was thinking of making two of these movable, to put behind the mic(s) - again I will get a photo/diagram of where I was intending to position the mics. I've also purchased some adhesive-backed foil tape - for the bass traps I was thinking of taping this on the room-facing side in an attempt to reflect some of the mid- and high-frequencies.

I intended to use the 4oz polyester wadding to keep the fibres in - so this would hopefully be in place of the black breathable fabric you mention - would this be suitable? If I went for the black breathable fabric instead, what material would this be - cotton?

I've purchased some fire retardant spray for the hessian so hopefully this does the trick - the polyester wadding I was looking at here is already flame retardant.

For the most part I record vocals and acoustic guitar, but sometimes electric guitar (when I can make enough noise) - this is a rented flat (on one side of the house it is partly connected to another house, so this is the only room I can reasonably use (as its on the opposite side of the house), the floor is carpet covered concrete - not sure I can do much about that as it is rented - when I get home I'll get a photo of this as well as the exact dimensions of the room - the ceiling is definitely only 2.2m I'm afraid - I checked this to see if two panels would go on top of each other). I also have 2x monitors and a movable laptop stand, so if I could possibly create a space that can be used as either as an acoustic guitar recording or monitoring area that would be great.

Thanks very much for your help.

Cheers

[jonc19]

Sorry Stuart my original guess for the room size was way off!

It is indeed 2.2m high, but the length is 5.5m and the width 3.5 to 4m (excluding kitchen area - the kitchen is not quite as wide as shown in the diagram below - it's about 1.8m x 3m).

It's a bit odd as there's stairs in it and a kitchen coming off the side (with no door) - it looks something like this (S = stairs, K = kitchen entrance, D = door, W = window):

Code: Select all

----SSSSSSSWWWSSS--------
-   SSSSSSSSSSSSS       -
-   SSSSSSSSSSSSS       W
-                 K     W
-                 K     W
-                 -     -
W                 -     -
W             -----------
W             - 
-             -    
-             -   
-             -                             
----------dddd

FYI - at the bottom of the stairs is the door to the outside. It will be difficult to put a bass trap on the stairs corner without being able to move it - same deal with the bottom right-hand corner, as there is a door there. There is a fifth corner next to the open kitchen entrance.

The attachment shows what I mean about being connected to the house on the left - the above room is on the right in that photo, with the kitchen on the near side.

I figured that a recording/monitoring area near the bottom left hand corner might be the best spot - but really unsure at this stage.

Hopefully this makes things a bit clearer.

Thanks again
Jon.

[jonc19]

For the most part I record vocals and acoustic guitar, but sometimes electric guitar (when I can make enough noise) - this is a rented flat (on one side of the house it is partly connected to another house, so this is the only room I can reasonably use (as its on the opposite side of the house), the floor is carpet covered concrete - not sure I can do much about that as it is rented - when I get home I'll get a photo of this as well as the exact dimensions of the room - the ceiling is definitely only 2.2m I'm afraid - I checked this to see if two panels would go on top of each other). I also have 2x monitors and a movable laptop stand, so if I could possibly create a space that can be used as either as an acoustic guitar recording or monitoring area that would be great.

Any ideas on where the best place to record in the room might be? - In the past I have recorded facing away from the bottom left corner into the room, with duvets and pillows etc. piled up behind me and whatever else I can find, sheets etc. behind the mic. It is a bit of an odd shaped room so not too sure this is the best option. I can produce a better diagram if required.

I plan to build the panels this weekend, so almost ready to go. I've gone for Rockwool RW3 as it's all I could find, with polyester batting to stop the fibres escaping, and the final covering in hessian. For the bass traps, I plan to put foil tape on the front room-facing surfaces to reflect some high frequencies, tape for 60mm, then no tape 60mm etc. alternately (I was thinking to put this directly on the RW3 surface, but alternatively could put it on top of the polyester batting).

Thanks for your help,
Jon.

[Soundman2020]

Any ideas on where the best place to record in the room might be? -

I really can't make out much from that highly simplified diagram! Please draw up something a little more accurate on paper, and post a photo of that.

But in general, for a small room you should face the shortest wall, with the longest walls to your left and right, and you need to be set up symmetrically, so you are an equal distance between those side walls, with your head on the center line. Your speakers should go tight up against that short wall (which is the "front wall"), your head needs to be about one third of the length of the room away from the front wall, and the speakers need to be aimed at a point about 16" behind your head. The speakers need to be placed on heavy stands, such that the acoustic axis is 47-1/4" above the floor, or maybe a bit higher (up to 49" is probably OK, but not more).

I can produce a better diagram if required.

And a couple of photos of the room itself would help too!

I've gone for Rockwool RW3 as it's all I could find, with polyester batting to stop the fibres escaping, and the final covering in hessian.

sounds good.

For the bass traps, I plan to put foil tape on the front room-facing surfaces to reflect some high frequencies, tape for 60mm, then no tape 60mm etc. alternately

Sounds reasonable, but what is the density of that tape? How much does each square foot weigh?

You'd probably be better of with just ordinary plastic for that. They type that comes in rolls, for all kinds of home projects. Find out what thickness/density they have at your local hardware store.


- Stuart -