refractioncat
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I've found a curious phenomenon in the measurements of the "Trapping Traps" experiment on Gearspace
The experiment was conducted by placing various configurations of bass traps (porous and VPR) in a 4.055m x 3.13m x 2.26m room, and measuring the decay times at low frequencies. The room has relevant axial modes at ~42 Hz, ~55 Hz, and ~76 Hz, corresponding to the long direction, short direction, and floor-to-ceiling. In the untreated condition all these modes ring for way longer than 1000ms.
The measurements F and L involved placing a 20cm thick 1m x 2m slab of Iso-Bond (around 12000 rayls), straddling a corner, and along the short wall, respectively. The absorber size and thickness being the same, only the placement differs.
The straddling measurement absorbed the 42 and 55 Hz modes quite effectively (roughly 300ms and 200ms respectively), and did not do as much to the 76 Hz mode (800ms).
This matches conventional wisdom on velocity absorbers; the panel presented a roughly 1.68 m^2 area to the modes' directions, and had a sizable air gap behind it, improving low-frequency absorption. The floor-to-ceiling mode was relatively unaffected, as the panel was not oriented to affect it.
However, when looking at the on-wall measurement, something strange happens: the 42 Hz mode is relatively unaffected at >1,000ms, but the 55 and 76 Hz modes are still relatively effectively absorbed (250ms and 600ms).
By conventional wisdom (i.e. acousticmodelling calculations at normal incidence) a 200mm absorber of 12k rayls should have a coefficient of absorption only around 0.3-0.5 at those frequencies, and the surface area presented in the direction of the modes is small (0.4 and 0.2 m^2 respectively). In comparison, 2 m^2 at a coefficient of around 0.2-0.25 should be significantly more useful at absorbing the 42 Hz mode, yet this is the mode that rings the longest in this experiment.
The difference in the 76 Hz mode (from around 800ms to 600ms) is particularly interesting, as the only difference between the straddling and boundary measurements is that the absober is against a wall. Somehow this makes an absorber in the "wrong" orientation unexpectedly effective anyway. The drop in the 55 Hz absorption is also less than one might expect, going from 200ms to 250ms despite the change in the orientation of the absorber.
I haven't found a clear explanation for why this happens, nor consideration of this effect in bass trapping recommendations, despite the apparent effect of it seeming rather significant. The difference in the 55 Hz and 76 Hz could suggest that the exposed edge may have something to do with it, as the 55 Hz mode has twice as much area/edge length in its direction. The position of this edge (starting at 1/3 from the corner, instead of filling 90% of the wall) may also be related.
Bjorn has mentioned some secret "new studies" in his posts and talked about the drawbacks of airgaps, which may be related to the phenomenon observed here:
This is highly interesting because, if we can work out a more exact cause of these unexpected measurement results, it could imply a new way to achieve effective bass trapping with more space-efficient or more conveniently shaped designs.
For example, enclosing fiberglass in a rigid frame on the 4 sides appears common in panel-shaped bass trap designs. However, if the exposed edges contribute a large part of the absorptive effect at these "grazing angle" nodes, this could be harming the potential absorption of the panel.
In comparison, gluing a self-supporting box from a sufficiently rigid foam (e.g. 50mm basotect) could allow a panel trap of comparable dimensions to have fully exposed absorbing edges, while weighing less and being thus easier to mount. The interior could then be filled with cheap fiberglass, with the foam enclosing the fibers for those who are worried about that stuff. Alternatively, sheets of foam could be simply glued together around the edges to make a simple DIY absorber open on 5 sides. In the UK, t.akustik melamine foam+glue is around £135 for a 100x100x20cm absorber, which places it somewhere between GIK panels and DIY fiberglass options in price. Using foam for the exterior only and filling the inside with cheap fiberglass would make two 100x80x20cm absorbers from the same amount of foam for around £140-150. Replacing the surface panel with a pyramid version at a slightly higher price is also an option.
From playing around with the calculator, it seems that an exterior of denser material with a core of the fluffy stuff performs similarly to a homogenous absorber of the denser material, or even slightly better. It also appears that higher-density absorbers perform better at oblique incidences, showing less of a dip at mid-bass frequencies compared to normal incidence.
The apparent ability of absorbing panels to have an effect "sideways" when placed along a wall could also mean that difficult, low-frequency nodes might be controllable with less thickness than traditional normal-incidence absorption would require, making the right kind of porous absorbers a more viable alternative to pressure-based absorption at the low end.
The experiment was conducted by placing various configurations of bass traps (porous and VPR) in a 4.055m x 3.13m x 2.26m room, and measuring the decay times at low frequencies. The room has relevant axial modes at ~42 Hz, ~55 Hz, and ~76 Hz, corresponding to the long direction, short direction, and floor-to-ceiling. In the untreated condition all these modes ring for way longer than 1000ms.
The measurements F and L involved placing a 20cm thick 1m x 2m slab of Iso-Bond (around 12000 rayls), straddling a corner, and along the short wall, respectively. The absorber size and thickness being the same, only the placement differs.
The straddling measurement absorbed the 42 and 55 Hz modes quite effectively (roughly 300ms and 200ms respectively), and did not do as much to the 76 Hz mode (800ms).
This matches conventional wisdom on velocity absorbers; the panel presented a roughly 1.68 m^2 area to the modes' directions, and had a sizable air gap behind it, improving low-frequency absorption. The floor-to-ceiling mode was relatively unaffected, as the panel was not oriented to affect it.
However, when looking at the on-wall measurement, something strange happens: the 42 Hz mode is relatively unaffected at >1,000ms, but the 55 and 76 Hz modes are still relatively effectively absorbed (250ms and 600ms).
By conventional wisdom (i.e. acousticmodelling calculations at normal incidence) a 200mm absorber of 12k rayls should have a coefficient of absorption only around 0.3-0.5 at those frequencies, and the surface area presented in the direction of the modes is small (0.4 and 0.2 m^2 respectively). In comparison, 2 m^2 at a coefficient of around 0.2-0.25 should be significantly more useful at absorbing the 42 Hz mode, yet this is the mode that rings the longest in this experiment.
The difference in the 76 Hz mode (from around 800ms to 600ms) is particularly interesting, as the only difference between the straddling and boundary measurements is that the absober is against a wall. Somehow this makes an absorber in the "wrong" orientation unexpectedly effective anyway. The drop in the 55 Hz absorption is also less than one might expect, going from 200ms to 250ms despite the change in the orientation of the absorber.
I haven't found a clear explanation for why this happens, nor consideration of this effect in bass trapping recommendations, despite the apparent effect of it seeming rather significant. The difference in the 55 Hz and 76 Hz could suggest that the exposed edge may have something to do with it, as the 55 Hz mode has twice as much area/edge length in its direction. The position of this edge (starting at 1/3 from the corner, instead of filling 90% of the wall) may also be related.
Bjorn has mentioned some secret "new studies" in his posts and talked about the drawbacks of airgaps, which may be related to the phenomenon observed here:
This is highly interesting because, if we can work out a more exact cause of these unexpected measurement results, it could imply a new way to achieve effective bass trapping with more space-efficient or more conveniently shaped designs.
For example, enclosing fiberglass in a rigid frame on the 4 sides appears common in panel-shaped bass trap designs. However, if the exposed edges contribute a large part of the absorptive effect at these "grazing angle" nodes, this could be harming the potential absorption of the panel.
In comparison, gluing a self-supporting box from a sufficiently rigid foam (e.g. 50mm basotect) could allow a panel trap of comparable dimensions to have fully exposed absorbing edges, while weighing less and being thus easier to mount. The interior could then be filled with cheap fiberglass, with the foam enclosing the fibers for those who are worried about that stuff. Alternatively, sheets of foam could be simply glued together around the edges to make a simple DIY absorber open on 5 sides. In the UK, t.akustik melamine foam+glue is around £135 for a 100x100x20cm absorber, which places it somewhere between GIK panels and DIY fiberglass options in price. Using foam for the exterior only and filling the inside with cheap fiberglass would make two 100x80x20cm absorbers from the same amount of foam for around £140-150. Replacing the surface panel with a pyramid version at a slightly higher price is also an option.
From playing around with the calculator, it seems that an exterior of denser material with a core of the fluffy stuff performs similarly to a homogenous absorber of the denser material, or even slightly better. It also appears that higher-density absorbers perform better at oblique incidences, showing less of a dip at mid-bass frequencies compared to normal incidence.
The apparent ability of absorbing panels to have an effect "sideways" when placed along a wall could also mean that difficult, low-frequency nodes might be controllable with less thickness than traditional normal-incidence absorption would require, making the right kind of porous absorbers a more viable alternative to pressure-based absorption at the low end.
