Having been worried through research about the low-end problems of a small room, I looked at all possible avenues for appropriate solutions. Not being content with just one form of bass-trapping alone, I decided I would also add some acoustic hangers.
Acoustic hangers are known to provide a good amount of absorption, especially in the low frequencies. However, there is not a great amount of detail on how these types of absorbers actually work. The paper 'Sound Field Characterization and Absorption Measurement of Wideband Absorbers'
demonstrated that each panel creates a 'waveguide effect', meaning, as soundwaves travel through the hangers they are gently guided towards the direction of the hangers. If we hang the panels angled to the wall (rather than perpendicular), it will help stop sound reflecting directly off of the back wall, removing the likelihood of standing waves. However, this doesn't fully explain the great absorption coefficient that these hangers possess.
Discussions have appeared on many a forum with theories on how these might work, and it is typically considered that the size of the panel has an effect on the frequencies it is efficient at absorbing, with bigger panels absorbing lower frequencies. Therefore, having a range of sizes can help cover a broader range of frequencies.
One other aspect of the hangers is an observable one, as the acoustic hangers are... well, hanging, they are free to swing. As you push one panel it will begin to sway, then gradually the other panels will start swaying out of sync, creating compression and expansion of the air pressure between. From my understanding, if a decent amount of sound is generated from the speakers, the panels will begin to sway by themselves creating this change in acoustic impedance that the sound needs to travel through, and removing energy from the soundwaves.
To get the best performance out of these hangers, the material used needs to be taken into consideration. As per John Sayers's design (right), you want the panels to be light enough to freely hang and swing. So the core material to which the insulation will be glued is made from a light recycled material, similar to the material used in pin-boards. As I'm in the UK, I used a Sundeala 12mm board for the inner core and just a standard 25mm insulation roll (Rockwool is too dense for this job). Everything was cut to size, then the insulation was glued to the board with a spray adhesive.
Once the panels were made, I hung them one by one, ensuring the correct distance between each panel.
To hang the boards, I just bought some cheap screw-in hooks and small wire cables. The hooks were screwed into the plasterboard ceiling and holes drilled through the top of the panel for the wire to thread through.
To make use of empty space, I also hung more of these panels in the basement's 'bay window'.
Once all the bass treatment was in I could focus on mid and high-frequency absorption, fortunately, this doesn't require quite as much trickery to fix as the low-end. Strategic placement of some 100mm RW3 Rockwool around the room, namely the reflection points, and the back wall would help tame the mid-range. I was careful not to put absorption everywhere, otherwise, the high end can be sucked out of the room, leaving a dull, dead-sounding, lifeless space.
At this stage, the end was in sight... and I had some projects to work on so it was time to get my equipment and furniture down there! We'll see this temporary set-up next time.
Frank Leonard Walker
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