High Frequency Absorbers

Home Studio Acoustics Part 2 – Making HF Absorbers

The absorbers I built are basically wooden boxes lined with Rockwool with a frame across the front over which is stretched some acoustically transparent material, in this case hessian. You need some basic tools and very basic DIY knowledge, e.g. how to use a saw and screw two pieces of wood together. Expert DIYers and carpenters can probably ignore the instructions and will probably do a much better job than I did, however I’m very happy with the results which look good and most importantly – they work very well. The sides of the boxes are timber 4″ x ½”. The back is hardboard with a couple of battens across for strength, although the backs could just be thicker wood or MDF. I chose hardboard and battens for lightness. Here I shall outline how to build a box 2’ x 4’

UK SUPPLIERS

Rockwool is better than foam as it absorbs more frequencies. You need the rigid stuff that comes in 2 inch thick slabs. Available from the insulation section of any builders merchant or large DIY store, e.g. Wickes or B & Q. You must get the compressed insulation that comes in slabs, not the soft stuff that rolls up. 

Materials

• Back: hardboard 2’ x 4’
• Sides: timber 12’ x 3½” x ½”
• Rockwool 2’ x 4’ x 2″
• Wood batten 1½” x ¾” x 12’
• Very small battening, approx 4’ x ¼ ” x ¼ “
• A few 1½” wood screws
• Some ½” panel pins
• Wood glue
• Hessian 4’ 6″ x 2’ 6″
• Polyester batting 4’ 6″ x 2’ 6″
• Paint (optional)

You will also need at least a saw, a sharp Stanley knife and straight edge, a staple gun and staples, a drill, sandpaper and a screwdriver. I have suggested starting with this size as it’s easy to get hardboard already cut to these dimensions. You can make the absorbers whatever size you want of course. If you need to cut the hardboard you can either saw it or cut with the Stanley knife by scoring a few times along a straight edge.

Making The Box

1. Cut the sides and top of the box. Two long bits for the sides, 4’ long and two shorter bits for top and bottom (23″). (As the top and bottom fit inside the sides, which are ½” thick, the external dimension of the top will be 2’). If your timber is slightly thicker than ½” you need to make the top slightly shorter or else you will need to adjust the size of the hardboard for the back.
2. Drill holes in each end of the 4’ side pieces so that you can screw into the shorter top & bottom pieces. You need to be quite accurate so the screws go into the middle of the top and bottom bits. As the screws are going into and along the grain of the top & bottom they should not need a guide hole to be drilled, but there is no harm in drilling a small guide hole if you like. A countersink can be used so the screw heads do not protrude.
3. Glue and screw the sides, top and bottom. At this stage it will be quite flimsy so be careful. A professional carpenter would have used a clamps and a set square but the next stage will firm it up and ensure square corners as long as you do it before the glue sets
4. Lay the hardboard back over the four screwed pieces, make sure the corners fit squarely.
5. Glue the back on and pin at approx 6″ intervals
6. Once the glue has set, sandpaper the box to smooth off the sawn edges.
7. Paint or varnish the box to taste. (Bare wood can look good depending on the quality of your carpentry)
8. Lay the rockwool inside. It’s easy to cut with a sharp knife but keep it away from bare skin as it is an irritant. (Ideally wear a mask). Cut it just slightly larger than inside of the box so it fits snugly. If the boxes are going on the ceiling it would be a good idea to glue it in with some contact adhesive.

9. 

   Lay some polyester batting over the top. This is what people make duvets out of and is much nicer stuff. I use it because its HF absorption is also very good and it will stop any stray fibres from the rockwool getting into your room. If you do this you can either tuck it in between the rockwool and box sides, or staple it to the sides. If it all fits snugly it will stay in place, although you might want to fix it in place with staples for the ceiling boxes

The Front of the Panel

You could of course leave it like it is, you now have a fully functioning HF absorbing panel, but it will look much better with a screen on the front. This is basically a frame made from wood battening with an acoustically transparent material such as hessian stretched across. The screen can other be the same size as the box and screwed onto the front (as in this illustration of the ceiling panels), or it can be made a bit smaller and set just inside, which I think can look just a tad more professional: To do this you need to make the external dimensions of the frame the same as the internal dimensions of the box, minus a little bit to allow for the thickness of the hessian. As the exact thickness of timber you used for the sides of the box may vary depending on your supplier, I won’t give the exact dimensions here, but once you have the material you can experiment to work them out. The method below is for the screens mounted inside the box.

1. Cut the four pieces of battening for top, sides and bottom, glue and screw. As with the box this will be flimsy until it’s finally mounted so be careful. You can add an extra batten across the middle for strength.
2. While the glue sets, sit the frame inside the box, wedged tightly with card so that it sets nice and square. Wood glue takes only about an hour to set.
3. Once the frame has set, Lay the hessian out and place the frame on top
4. Wrap the hessian around and staple the top two corners. You must stretch the hessian so that it doesn’t sag, but be careful not to pull to tight or it will pull the frame apart which is still quite fragile.
5. Stretch again and staple the bottom two corners.
6. Staple all the way along each side at intervals of 1″ to 2″, stretching gently and carefully as you go.
7. Apply some wood glue along the hessian where you have stapled it and allow to dry. (Use enough to soak through the material to the wood beneath)
8. Pin and glue some thin battening inside the box as supports for the frame (see illustration).
9. Drill two holes in each side of the box and screw the frame in securely. If you find the hessian is too tight and is pulling the frame inwards, more screws from the sides should straighten it up.
10. You can make these look a bit nicer with some plastic screw caps that fit into cross headed screws. Available from all good hardware stores.

Mounting the Panels

You could just screw them to the wall but it’s advisable to mount them at least 2″ from the wall as they will work much better. To do this I fixed some angle brackets onto a piece of batten along the back of the absorber. This is fixed about 6″ from the top of the panel, glued to the back and screwed into the sides. I then bent over the ends of the brackets with a pair of pliers to hook over some battening on the wall. This has a couple of bits of smaller battening glued on top as lips for the brackets to hang on. My walls were brick, but if you have a stud wall you will need to find a timber stud to screw into or use plenty of plasterboard fixings.

I then put another batten at the bottom of the panel and used a piece of 2″ wood as a spacer which does not support, it just holds the panel away from the wall. Mounting the ceiling panels was a bit tricky as you must find a joist to screw into, I wouldn’t trust a plasterboard fixing on a ceiling – if you do on your own head be it. I used a hook in each corner and some strong link chain. The panels above the mixing desk are angled slightly upwards towards the middle of the ceiling, and have some extra rockwool and batting laid on top for extra absorption. The mobile high frequency absorber: To be continued…(Bass traps and diffusion panels).