The problem with bass traps is that most of them are plain ugly. Here is a DIY diffuser. It does not actually trap sound-waves but reflects them.



I have used an 8mm thick sheet of native timber. It is rimu and comes up very well when polyurethaned with a matt polyurethane. It also responds well to varnishes and oil.



The raised pieces are 18mm dowel that I have had cut to length and angled at about 20 degrees. The pieces have been glued on with PVA.



I have spaced all the dowel 27mm apart. When it comes to materials you don't have to use dowel but I chose it because it is round.



From the closeup you can see that the cuts are not perfect. If you want to try this then after you have done your calculations for the amount of dowel you require it would pay to order one and a half times that amount. Partly to allow a margin for error, but you will find that a lot of the cuts are not perfect and so there will be quite a few rejects.



The rear is not a thing of beauty but just to show a couple of horizontal pieces of wood. They are added to strengthen the board but also to make it easier to hand on the wall.
Although this was a DIY project I have had a joiner make the cuts. He will need accurate measurements. I tried hand-sawing them but it takes too long and is not as clean a cut. It cost only $20 to have the joiner do the cuts.
Final tip: do not be stingy when it comes to choosing your varnish or polyurethane. It is quite shocking how poor some products are, so get advice if you need it.

As for the size of the dowel, the angle you set it at (on both sides) - the world is your octopus. It is satisfying working with natural timber because its own beauty means that this sort of project can hardly be fouled up.