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Created by chris, on April 26, 2013 21:03

Music and Beyond: DIY Panel Absorber

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chris
almost 4 years ago
Low Cost Bass Treatment

Recently I started to read Mixing Secrets. After a chapter about monitor speakers, the author comes to the other important point of monitoring: studio acoustics. Somethings that is easily overlooked, but of really big importance. Good monitor speakers won’t shine in an untreated room. As commercially available acoustics treatment is quite expensive, I tried to do as much as possible on my own. Here is the story so far.

It is more than just helpful to have – at least – a basic understanding of room acoustics in order to treat a room properly. A few of the following resources are German, but you will find a bunch of books and websites in different languages.

This thesis [1], (German) gives a brief introduction to studio acoustics.
There are different ways to treat problematic frequencies. However, for now I am going to focus on the bottom end – the part of the frequency spectrum panel absorbers are made for.
To deal with low frequencies is especially challenging, because here is the most energy. Why do problems arise? Well, room modes are our enemies. Each room has certain resonance frequencies. You can calculate them as shown in [1] on p. 16. My room has the drawback that it has almost quadratic shape. This means that modes in two directions are overlaying. However, I was quite shocked, when I did a first hearing test. The effect of room modes is much more extreme than I thought. You can easily reconstruct the effect with test signals. E.g. this frequency generator is of great use: (be careful with the initial volume!)
My room is ~4.8m (as mentioned before – in two dimensions). This results in a mode at ~38Hz, and the next at ~76Hz. You bet that sine tones at these frequencies feel as if the room is exploding, while e.g. 60Hz at the same volume setting is really quiet. Another effect that is noticeable are areas of maximal pressure that manifest at certain positions of the room. With a 76Hz tone I get maximal pressure at both walls (for the moment we take only one dimension into account) and in the middle of my room. My monitoring position is almost in the middle, but when I move away from this position, say to 0.25* 4.8m, or 0.75* 4.8m, there is almost no sound – these are the positions of minimal pressure. Notice that for each mode an area of maximal pressure will manifest at both walls.

If you mix music, you want it to sound good at the speaker of the listeners. Your listeners will of course have room modes too. As we cannot know any of the room characteristics of our listeners, a preferably neutral room is what we want as studio. There are several ways to deal with low frequencies – panel absorbers are only one of them. So lets have a brief overview:

  • Helmholz Resonator : Very high Q, because of that, they must be fitted to a certain room (room shape) and they most likely catch only one mode.
  • Corner Bass Traps : Effectiveness depends very much on the size, Q does too; but corner traps are not tailored towards a specific frequency like helmholz resonators; take rather large space; depending on the front material, corner bass traps will absorb mids and treble or not. It is important to take this into consideration in combination with other (broadband) absorbers
  • Panel Absorber : Rather low Q, do not take up large space; can be built so that they are movable (with an attachment system that makes them removable), work only for low low frequencies (if good or bad), because they have a hard front
  • … and some more (e.g. PVC flooring hanging from the roof at a certain distance to the wall)

Well, I don`t think helmholz resonators are a good start for any room treatment. They might be the right choice, if the room is already treated, but there are still problems with a specific mode. If space is not a too big issue, corner bass traps should be a good choice. If you have the luxury of having a dedicated sound room, and you do not plan to move soon (why should you, you`ve got a sound room  ), permanently installed corner bass traps are almost a must. Everybody else – let`s build panel absorbers   Well, of course they are combinable with the other two bass treatment variants and this makes them a ‘long term investment’, I`d say.
My absorbers are mostly based on this HowTo [2], (German).
Also see this gallery of the building process of my absorbers.

The idea of a panel absorber is to ‘redirect’ pressure in a 90° angle. They have to be in an area of maximal pressure. Well, a wall seems to be the best place for such ‘boxes’ anyway   The pressure that impacts on the absorber will initiate a comparatively light an flexible front plate to pulsate. The pulsation is ‘along the plate plane’ (think of a drum membrane). This ‘redirected’ pressure will not be reflected back – it is absorbed.

I built four of the absorbers described in [2]. Here are some facts:

  • back plate is 13mm MDF
  • sides are 19mm MDF
  • front is 3mm colored laminated MDF
  • size: 100cm * 73.8cm; height is 10cm (+13mm back plate). This results in a corner frequency of (theoretical) 70Hz
  • a 6cm thick glass wool panel is attached inside
  • sides are painted
  • mounted to two hooked screws (per absorber) in the wall, see gallery for more details
    One absorber is roughly 40€, including paint and mounting. It needs quite some time to assemble them, but that will depend a lot on your Tim-Allen-Factor and how much space/bar clamps you`ve got.

Let me share some experiences with you: Is it worth the effort? Yes! Well, I am wondering if there is a better alternative to laminated MDF. First, the specific weight is slightly different from non-laminated MDF, which changes the corner frequency. But I think that is neglectable. Well, when I painted the sides, of course I masked the front. When I removed the tape, quite a few small pieces of the laminating came off, d`oh. You could use pint, but then the change of the weight most likely won`t be neglectable. You would have to estimat the weight of the paint in advance (or gather some experience) and take that into account when calculating the height of the absorber (see [2]). You also could use a wood, that dosn`t suck from a design point of view when it is unpainted (well, of course you should somehow treat it to make it sustainable). But it will be difficult to determine the corner frequency. Let me know if you have any experience with different woods, or if you found good resources about this.
If I would do it again, there is especially one thing I would do different. I used bar clamps to fix the position of the sides when I glued them to the back plate. I wanted no visible screws, but what you should do (I guess), is the following. Drill holes in the back plate and put screws through them into the sides. This will save some c-clamps (you still should use some), make the whole thing even more stable and most important, it should help to get the sides into a perfect 90° towards the back plate. If they are not, you will get into trouble when you attach the next side plate.
My side plates are 100cm and 70cm. When you look at my base plate dimensions, you`ll see that the short sides are ‘in between’ the long sides. If you start to glue a long side and it is not in perfect 90° angle with the back plate, you cannot glue the next short side to it without any free space (holes). So, another takeaway is: start with the ‘in-between-sides’ – the short ones in my case.
Of course there are better ways to do this, but you will need tools then. And then I most likely will not have to tell you how to assemble such a box  

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chris
almost 4 years ago
Remark/Update

I found this very interesting article from Ethan Wiener (he is making a lot jokes about the name himself  ), a well known expert in acoustic treatment. He is used plywood for his absorbers, which might be the better choice over MDF. Ethan uses rigid fiber glass, which has a much higher specific weight than the stuff I used. The US guys all stick to owens crowning (mostly 703). It turns out that it is not easy to find a comparable material in Germany. I`ll keep on checking for something.

A very nice alternative to a colored front plate and painted sides is to cover the whole thing with cloth (acoustic neutral cloth).
Further, I realized that the comparison of the absorbers limb mass (the front plate) with a drum membrane is maybe somewhat misleading because the the absorption also (and mostly) comes through the damping which is caused through the plates mass.

However, if you got interested in the topic: here is a bunch of further reading.

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