The perforated panels already built are, looking from the listening position:
Perforated MDF panel
4" airspace
2" fiberglass panel
1/4" MDF panel, against the wall.
In the calculation of these there was no other airspace considered.
The new absorber I'm contemplating for the corner would be:
1/8" plywood or MDF
2" fiberglass panel
4" airspace
Wall or back panel against the wall.
While all absorbers would contribute, my question is what would be most effective to use there, within my constraints. For example I could just use 4" of fiberglass panel against the wall, covered by cloth. But I understand that wouldn't be most effective in absorbing low end energy.
Ok a few things to discuss here:
Pressure absorbers (Reactive absorbers as you called them) operate best at the pressure maximum of the mode. That means, at the wall typically. As close to the wall as possible. Increasing the airspace lowers the tuning of the device, but simply moving a sealed pressure absorber away from the wall makes it less effective. With the really low modes that these panels are designed to treat, hitting the next pressure maximum would mean like 1/3 into the room or even half way into the room, not exactly convenient.
For velocity absorbers, porous fiberglass panels, these work better at the velocity maximum, which is away from the wall. This is why they work so much more efficiently the farther they are from the wall (thought eventually if the distance from the wall relative to their thickness and flow resistivity becomes great you get comb filtering. These should be spaced away from a wall (and they need to have an open back) but maybe not 1 foot from the wall unless they are built for that.
Adding a membrane of low mass to the front of a velocity absorber only slightly changes their mode of operation. At low frequencies, the added mass causes a greater reactive force against the LF wave than the fiberglass would otherwise have. Think of it as the equivalent of stiffening the spring of a suspension, with the fiberglass managing the damping still. At certain frequencies, the low mass means sound still mostly passes through the membrane and hits the fiberglass being dissipated. At higher frequencies, it reflects. This is still primarily acting like a velocity absorber and they have wide bandwidth.
Place a high mass in front of the fiberglass and suddenly that mass will dominate in the system, making it act better on the high pressure of the wave, rather than the velocity. The fiberglass is still acting as damping and it is still a spring-mass system, but the mass is now that front membrane or panel. The panel is also self-damping, which is why it works with no fiberglass as well.
Keep in mind that a Helmholtz is still a spring-mass system. The air in those holes is the mass. That mass is no different than the placement of a physical mass such as a sheet of plywood on the front.
All of this operates by converting energy from one form to another. From sound vibrations to heat.
Placing a 4" cloth covered panel against a wall isn't that effective at LF's, it just isn't efficient. That is a velocity absorber attempting to convert sound energy to heat energy where the conversion method is least efficient. Think of it as a car, you want to get the best gas mileage and you are cruising along at 55 mph in 3rd gear with the engine turning at 3500 rpms. The efficient zone is 1700rpms, you need to shift to a lower gear to get good gas mileage or slow down. With a velocity absorber, it isn't efficient against the wall, so you need to either move it outward or accept its absorption only at higher frequencies. Increasing flow resistivity increases efficiency for shallow depth panels to a point, but it has its limits.
http://www.acousticmodelling.com/
Playing around with these apps can be helpful to see all of this. I've always loved this site. It has great free tools. It doesn't model multi-layer panels correctly, but everything else is close enough.
