Stuck with RS80 - is this a bad thing?

I am an experienced amateur musician, recording my own material, but I have only recently begun to pay attention to acoustics. I bought some 100mm slabs of Knauf Rocksilk RS60 (=RW3 i.e. 60kg/m3 rock mineral wool) to make bass traps (of the wood-framed fabric-covered vertical variety, not superchunks). However, what the company delivered was an equivalent quantity of RS80 (the 80kg/m3 version). I also made the mistake of not realising that a backing can improve the bass absorption and I have bought unfaced slabs. Looking at the absoprtion data for RS60 and RS80, it looks like RS60 should be the better absorber at 125Hz, although they are very similar above that. However, it's hard to be sure because the absorption results seem a bit arbitrary e.g. some thicknesses not obeying the general trends. (In general browsing I have come across the opposite suggestion - that the denser products can be better bass absorbers but are more relective at higher frequencies because they are more rigid). My room is about 14'x13'x10'8". It is used for both live & control purposes. It also has a shallow but wide bay window - about 6' wide, 1' deep, and full height, centred on a wall. The room has an 8" plaster cornice right round the wall-to-ceiling join. It has to double as a spare bedroom and contains a bed-sofa. 2 walls are external - thick sandstone on the outside and 19th century lathe & plaster on the inside. The other 2 walls are internal and are lathe & plaster. The ceiling is untreated at the moment. It has a very shallow smooth carpet - may eventually lay a wooden floor but only if I can tame the room. I can't put traps in all corners because the door is right in one of the corners. I was going to trap the "front" (console wall) corners, and then put panels on the back wall. The drum kit is currently near this back wall, facing out into the room. The single door is at the end of this back wall. I typically record drums, guitars (sometimes direct), bass (usually direct), keyboards (direct), and clarinet. The room design is a compromise between acoustics, functionality (packing in a lot of equipment) and has to pass a family design review! So the acoustics are a compomise but I'm looking for an improvement. I have profiled the room with a slow sweep and a sound pressure meter; this has revealed a big resonance about 160Hz, plus lots of smaller peaks and troughs. My questions are: - is it worthwhile swapping the RS80 for RS60 or there hardly any difference? - how should I go about adding a backing? - if I'm complementing the 100mm-thick corner traps with wall panels, should I go for a slightly different material and/or thickness for the wall panels so as to get a broader spectrum of absorption? Edit: sketch included - target asborption locations in red PS. This is a great forum, thanks!

I moved this thread to the Acoustics forum... Better fit. You're sure to get some good feedback here. --Keith :mrgreen:

Can I provide any more info to help anyone give an answer or a comment?

Harry, > Looking at the absoprtion data for RS60 and RS80, it looks like RS60 should be the better absorber at 125Hz, although they are very similar above that. < I'm sure what you have is fine. Believe it or not, acoustic data is not very useful to compare materials at low frequencies for many reasons. I use rigid fiberglass, not rock wool, but the principles are similar. See my Density Report for what I measured: www.ethanwiner.com/density/density.html This is a better way to assess materials than standard lab tests, though the results are not "portable," so you need to test all of them together in the same room at the same time. > However, it's hard to be sure because the absorption results seem a bit arbitrary e.g. some thicknesses not obeying the general trends. < Bingo. > I can't put traps in all corners because the door is right in one of the corners. < So put one trap on the door, and another on the adjacent wall. > is it worthwhile swapping the RS80 for RS60 or there hardly any difference? < Probably not. > how should I go about adding a backing? < Securely glue card stock or thin cardboard to one side of the panels. That's the side that will face the room. > should I go for a slightly different material and/or thickness for the wall panels so as to get a broader spectrum of absorption? < The way to make a material absorb over a wider range is to make it thicker. Even thin panels absorb well at high frequencies. So what's always needed is to extend the absorption to lower frequencies. --Ethan

Thanks that's very helpful, I will keep my RS80. I note Ethan's data showing improvements by adding the backing, especially in the 12x3" scenario. (I will have 9x4" at first) The other DIY corner trap posts I saw used fabric to cover the slabs and didn't mention a card backing. Some of the pictures taken during construction look like unfaced slabs. So is fabric achieving a similar purpose to the backing, or are people using fabric in addition to a backing?

Harry wrote:

The other DIY corner trap posts I saw used fabric to cover the slabs and didn't mention a card backing.

The use of cardboard is an original concept invented and patented by me. But I hereby grant you a one-time non-exclusive use of the idea. I'm kidding! :) > So is fabric achieving a similar purpose to the backing, or are people using fabric in addition to a backing? < Fabric is mainly cosmetic, and to keep the fibers in place. It's not the same as cardboard or FRK. --Ethan