aesthetic fabrics for covering broadband absorbers

I finally got round to making some broadband absorber panels for my music room which is 13'x13'x10' room in a house. It's a struggle to convince family that these panels deserve to stay. Need to maximise visual aesthetics. They are 4" mineral wool. 2' wide and either 4' or 8' high. Although I know the standard acoustic fabrics like GoM, I had heard that for covering broadband absorbers it wasn't so essential to use these, and really a lot of fabrics will do as long as they are not shiny or reflective. So for my first panel I used a plain fabric of unknown material which is not especially shiny or reflective, but isn't especially loose-woven. I will profile again when I have finished the rest of the traps, but has anyone any advice on narrowing the range of fabrics? These panels are the only absorbtion in the room (other than a sofabed, curtains etc). Before absorption I had some big peaks, biggest at about 160Hz. So bass is my main problem but it would be nice to soak up some higher frequencies too. The room will never be great but I want it to be as good as practical for my home projects.

Harry wrote:

a lot of fabrics will do as long as they are not shiny or reflective. has anyone any advice on narrowing the range of fabrics?

First, stand in front of a plain slab of rigid fiberglass or rock wool or whatever you're using, and "talk into" it so you can hear what it sounds like. Now, hold any proposed fabric in front of the absorber and listen if you hear some of your voice being reflected. If it still sounds totally dead, like you're talking into a total void, the fabric is fine. --Ethan

Thanks. I was visiting the site this morning to say that I thought of something like that just after I posted and tried it out. I brought the fabric up towards my mouth while singing a tone. This did differentiate the fabrics - a fine slightly transparent material made hardly any difference but a thicker material made a lot of difference so I ruled it out. But I will try your method too, which should be a better test.

I found the talking test quite hard to judge - I tested 3 fabrics, and I could detect a little more treble bouncing off when the fabric covered the wool, but hard to tell how significant it would be. I also did a test where I pointed a mic right at the wool slab, just about 5mm away, and played a slow sweep through some speakers in the room. The mic was about 135 degrees to the speakers, maybe about 8 feet away. I recorded the signals and then looked at the amplitudes. I tried uncovered, with 2 fabrics, and also with the slab moved away completely. This last one was far from an ideal control as the mic was now much further away from any reflective surface. For my biggest bass peak the thin fabric was about 1db higher than uncovered, and the thicker closer-woven fabric about 1db higher again. The without-slab case was about the same as with the thicker fabric, but as I say this is not a fair comparison. For the room peaks further into mid-range e.g. 220-880 Hz the differences were more noticeable. The uncovered and thin-covered signals looked similar, while the thicker covering was noticeably reflecting more - sometimes several db although I didn't check the exact numbers. I was slightly disappointed since the thin fabric has proven not strong enough to stretch nicely over the panel while the thicker fabric was definitely reflecting more. Will have to keep searching for something nice in between. Any comments on the (in)validity of this test would be interesting.

Harry wrote:

I also did a test where I pointed a mic right at the wool slab, just about 5mm away, and played a slow sweep through some speakers in the room. The mic was about 135 degrees to the speakers

Much better is to actually measure the comb filtering, on-axis, as shown in Figures 1 and 2 here: http://www.realtraps.com/art_spaces.htm Whichever fabric yields the least deep nulls wins. --Ethan