21 It Was All a Fabrication! Finishing the Walls
Now we can begin to make the studio look nice...ish!
As I was using the original brick walls as the boundaries of the studio, I wouldn't be finishing off my treatment with plasterboard. Firstly, this would undo some of the acoustic treatment, and it would, in part, make my room acoustically smaller, which would throw my initial calculations out the window. Secondly, it would add back in further mid and high frequency reflections back into the room. So how do we cover up the acoustic treatment without hindering its performance?
With acoustically transparent fabric of course!
You've probably encountered the same or similar fabric before, as it is frequently used as covers for speaker cones. These fabrics, of course, aren't 100% transparent, but they absorb very little, as seen by their absorption coefficient graphs:
Above, we can see three different models of acoustic fabric: the Lucia, Cara, and Blazer Lite. If you've not seen an absorption coefficient graph before, the numbers on the Y-axis show how much sound is absorbed, with 1.0 being 100% absorbed, 0.5 being 50% absorbed, and 0, no absorption at all. The X-axis shows what frequency, as the absorption rate can differ greatly between frequencies within the same material. It may be important to note here that it is a LOT easier to absorb higher frequencies than it is lower frequencies, as we can see from the graph. Even fabrics that have been designed to be 'acoustically transparent' still absorb more of the higher frequencies. Now, a low absorption coefficient will mean one of two things: either the sound is being 'passed through' the material, or reflected back. We can determine which is happening by knowing the density of the material, a solid concrete wall would also have a very low absorption coefficient, but very little sound is making its way through to the other side!
Looking at this graph, and knowing the density of the material, we can determine that, for the vast majority of the frequency spectrum, we can see that the fabric is allowing sound to pass through, mostly unabsorbed. Then a slight increase in absorption past 5kHz, which is hard not to absorb or reflect back! Incidentally, I opted for the Cara fabric due to 4 factors; absorption coefficient, price, material, and colour options.
For comparison, let's look at the absorption coefficient of the 100mm RW3 (60kg/m3) Rockwool that I used as my absorption panels...
This is the response if this panel is mounted directly onto a wall. We can see that from around 10kHz up, we're absorbing almost 100% of the sound, but it clearly dips off as we get to the lower frequencies. With only around a 30% absorption at 100Hz. However, if we mount the panel with an air-gap away from the wall we can get an improvement in the low end absorption. With a 50mm air-gap we can increase the absorption around 100Hz to nearer 50% without any additional materials, as seen below:
Just before we move on, I want to quickly highlight the uselessness of those thin acoustic foam panels that are glued directly onto your wall. When I was younger I was totally sold on these, and had them plastered around my room - but they really don't make much difference to your room acoustics, as shown in the graph below. The green line is an approximation of the absorption coefficient of a foam panel, which isn't doing any meaningful absorption below the mid-range, where most acoustic problems occur, so save your money and your walls!
As we can see from all these graphs, the high frequencies are so easy to absorb and stop dead in their tracks. A common problem in studio design, as a result of this, is that the high frequencies are absorbed to a much greater extent than the rest of the frequency spectrum, making the space sound dull and lifeless. A useful trick, before covering your absorption with fabric, is to add a thin layer of plastic to reflect some of those higher frequencies. I considered doing this to my room, but before I did, I took some acoustic measurements and saw that absorption was consistent through the frequency spectrum. This was due to actually not much rockwool being exposed, mostly being covered by a vinyl in the diaphragmatic absorbers, the front wall was a hard wall, there was a diffuser on the back wall, plus the ceiling and floor were reflective.
For the ceiling, I went with a classic off-white, and the walls had a soft, purple colour. I wanted muted colours to create a relaxed atmosphere in the studio. Colour choice can not be overlooked in studio design, as it can really affect your mood, and besides, you may be sitting in this same room day after day, so you want to be happy with the choices you made!
The fabric was applied in stages, depending how far through I was on other aspects of the build. The first part to get done was the front wall, which would be covered by wood panelling, but hints of colour would still peep through the gaps. Application was fairly simple; starting by cutting the fabric down to size with plenty of excess, then attaching to the surface with a staple gun, keeping the fabric taut as I went. I made sure to only staple at the parts I knew I would be covering with wood, so no staples would be visible in the finished build.
One thing I skipped out on, was painting any framing black. Partly due to my laziness and impatience, partly due to the fact most of the framing would be covered by wood panelling in the end. But, if you don't paint the frames black, they can show through the fabric, as highlighted below by a strip of white acoustic sealant on the ceiling showing through the fabric, next to the vent.
Once the diffuser was in place I could cover the back wall in fabric. Here you can see the excess before trimming. I do like to cut my margins fine!
Time to cover the side walls, and cut out the holes for sockets...
The finished fabric on the side walls below.