The problem with small rooms is that they tend to exhibit disparate low-frequency energies, with large nulls and peaks up and down the low-end. With bass frequencies having the longest wavelengths, they typically require a large amount of space to be absorbed sufficiently to avoid these issues. Herein lies the quandary, how much of the already small room are we going to take up with large amounts of acoustic treatment?
There are several different approaches to treating the low-end, which I won't go into now, but for my room, I relied on a three-pronged approach that complimented my room shape to maximize space and absorption. These were diaphragmatic absorbers, acoustic hangers, and corner bass traps. For this post, I'm going to focus on the central prong - the diaphragmatic absorber.
Diaphragmatic absorbers are essentially sealed boxes, partly filled with Rockwool, and covered by a membrane. The advantage of this design is that it takes up less space than other traditional low-end treatment, and are relatively broadband in comparison to Helmholtz resonator style absorption.
The membrane of a diaphragmatic absorber moves when exposed to sound pressure, creating air displacement within the sealed chamber. This forces the air particles to interact with the porous acoustic treatment inside, fostering low-end absorption. Obviously, there are some factors that lead to the effectiveness of the absorption - the size of the chamber, what density of Rockwool is used, what material is used for the membrane, for example. I mostly used the specification laid out in RA: The Book.
The material I used for the membrane was the Acoustic Barrier Mat BM1060, which you can only get from two places in the UK! Mine was supplied by F T Medcalf, it comes in rolls and is extremely heavy! It has a vinyl-like texture with a hessian backing on one side. Rockwool was 100mm thick RW3 60kg/m3 density.
To further save on space, I built the 'cabinets' into the internal framing of the studio. This resulted in varying cabinet sizes, diversifying the absorption range throughout the room. The largest two covered most of the back wall, and I'll use these as the example for the building process.
The first step was creating the frame, this was made out of 150mm deep timber, 100mm would be filled with the Rockwool, the remaining 50mm for air space. Make sure you don't exceed the width of the barrier matting, as this needs to be attached as one single piece. The back panel was made of 15mm thick MDF.
Once constructed, all gaps needed to be filled with acoustic sealant, as these cabinets, truly needed to be sealed!
They could then be filled with the Rockwool, pushed up against the back panel.
Then came stapling the barrier matting to the front of the frame, again, acoustic sealant was used between the frame and the matting to ensure the cabinet was sealed.
Diaphragmatic absorbers were also installed on the side walls...
With the initial trapping in place, I could continue with further framing work...
And voila! Bass trapping installed, as I mentioned in the beginning, there are still a few more solutions to install to improve the low-end response of the room but these will be looked at in a later post. Next time we turn our attention away from the back wall and to the front wall.
Frank Leonard Walker
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