Updated: Feb 13, 2020
Sound likes to reflect around whatever space it's in. It's main mission is to transfer around the space. In an amphitheatre this is ideal and allows for natural amplification of the source. However, whether you're in a recording studio, edit suite or dubbing stage, it's really important to control the sound otherwise your audio mix will never translate properly to the listener, whether in their lounge, car or especially a cinema!
Yes, egg boxes on the walls have remained in our consciousness, it's often said jokingly, but I have met more than a couple of people who would classify this as a real option - while it can offer some help, this is not going to be an effective solution. This is where it gets a little more complicated... Humans hear a range of frequencies (or pitches) from really low rumble at 20Hz to the almost (and genuinely for over 12's!) Imperceptible 20,000Hz. Dealing with high frequencies is relatively easy, a lightweight cotton material will help with echoes (as will egg boxes dare I say), but the low mids and low frequencies are a lot harder to deal with. There is one thing that is key here - Mass/Density. The more dense an absorptive material is the better it will be able to trap lower frequencies. So, with that little bit of context, let's get on with the build.
You don't need any specialist tools or expensive materials to make these, just some patience!
Tools List - Tools:
3mm Wood Drill Bit
PPE - Gloves, Mask, Goggles (Fiberglass)
Materials List - Materials:
Wood planks for the frames
Rockwool Insulation (more on this below)
Hessian/Acoustically transparent covering fabric
Wood screws - Different lengths
One of the first things you are going to need to decide is how thick the absorbers are going to be - this relates directly to the ability to absorb low frequencies, so the bigger the better generally. I went for 75mm thick RW60 Rockwool, RW60 is dense in Rockwool's range so it should be ideal, I'd of gone heavier if not for the extra cost. Other insulation can be fine, but try and get the most dense you can. It can be useful to look at the absorption co-efficient diagram for the material you are going to use, this will give you an idea of how much of a frequency band is absorbed (the scale is from 0 - 1. 1 is 100% absorption & 0 is 0%). Below you can see the chart:
So for our RW60, it absorbs approx 95% of the 250Hz frequency, not bad at all. But notice how the efficiency dramatically improves when it is mounted 300mm off the wall - this where we can really save some money! I mounted all of mine 100mm off the wall, as 300mm was far too intrusive, but if you have a bigger room it may be worth going all the way to 300mm. But as a rule, the larger the room, the less problems there will be with low frequencies anyway. In summary - we are mounting 75mm RW60 Rockwool 100mm off the wall.
Building the Frames
To keep cutting of the Rockwool down to a minimum (to reduce horrible fibreglass particles and save time) I opted to build the frames around the existing sheet size, which is 1200 x 600mm. I wanted a tight fit to avoid having to fix them in the frame, so I made the external size of the frames 1220mm x 610mm. I used softwood for lightness - my timber was 22mm wide and 75mm deep to match the Rockwool. To reduce cutting time I got the timber yard to cut the timbre to 1220mm lengths, so I just needed to cut them in half to make the end pieces. I made 12 of these in total and therefore had 36 lengths - at my local yard it was only 78p a meter!
Start by cutting the lengths ready to construct the frames. I held these in place when assembling with a nail in each corner to hold it ready for screwing together. I drilled a 3mm pilot hole for the screws, which was fine for the 4mm thick screws, I'd have used a 2.5mm drill bit if I had one, but the 3mm worked fine.
Once assembled, I removed any roughness from the outside of the frames with 120grit sandpaper so that the fabric might catch on, I left the inside rough to help keep the Rockwool in place. That's the hard bit over.
Covering with Fabric
When choosing fabric to cover the frames and insulation, use something that sound can pass through otherwise they won't work! I found it quite expensive to buy speaker cloth, as I needed nearly 17 meters! So I found dyed jute/hessian fabric instead, it's also great as it's available in a range of colours - I used orange as it's my company's house colour. It comes in 1 meter wide lengths, which is ideal for covering our 610mm wide panels. When working out how much you need, make sure your account for the depth of the wood as you'll need to staple it to the back of the frames. So for our frames, I worked out that each frame requires at least 1400mm (1220mm frame + 2x75mm end depth + 50mm for safety/stapling). I got mine from eBay.
When it came to the fitment on the frame, there were a couple of important points - keep the fabric parallel with the frame, as the hessian I used had clear lines and would look wonky otherwise. Also keep the fabric tight at all times - I found it useful to a) get help from the Mrs. when she was around or b) keep the edges between my feet and work from one end to the other. I found it best to start at the longest ends first and then do the sides.
The most difficult part here was folding the edges over so they looked neat, when I started it took a few goes, but it became second nature after a while (small caveat from experience - avoid trying this after a pub visit!). I ended up using a lot of staples - probably at least 40 in each panel, but this was necessary to keep it from creasing or sagging at the edges. I got 1000 staples for £5 so no probs there!
Finally, I just pressure fit the Rockwool (use gloves if I were you, the fibreglass is super itchy and irritating).
I also used an iron with lots of steam over the front and sides of the panel, as I found that there were creases from shipping that were still visible even after stretching. It also helped reduce any fuzzy bits from the fabric.
This is the part that I had to plan/fiddle/replan the most. I originally was going to screw them onto a frame 100mm from the wall, but then realised there was no way that I would be able to get the screwdriver around the back to actually fit them. So I decided to hang them and re-purpose some old brackets I had.
I'd advise to drill the pilot holes for the bracket/fixings when you make the frames, otherwise you'll have to take the Rockwool out and risk tearing the fabric.
Then it's just a case of cutting a slot in the top of the mounting wood on the wall - this will allow the metal mounting bracket to slot into cut and hang away from the wall. I then added some staples to keep the wood from splitting (just in case). All that is left to do is put the Rockwool in the frames and then iron the fabric and then hang them!
Share your pictures of your build and let me know how you get on in the comments!