Originally Posted by Tweakophyte
I would guess as low as 40-50hz.
One thing people that measure these things will tell you is any measurement below 125hz is very tough. You cannot compare one test to another (though I believe you can compare a relative number made during a single test).
That is a good guess I think... but a lot depends on what we mean by "effective".
If we are talking about the frequency range at which the device's performance drops out of the stratosphere of sabin numbers per unit we enjoy when we use corner mountings then I think you are dead on the money. At about 40 - 50 Hz we are going to see the a big droop.
But, you need to recognize that just because a device has dropped back to the more rational absorption performance range we might expect to see in the rest of the room - for example, the levels we see in a wall mount - that doesn't mean it isn't working anymore. To me that is just an indication that the "free ride" we get from corner mounting stops and we have to go back to normal performance expectations.
Look at the phenomena this way... a pair of 32" faced 703 SuperChunks running from floor to ceiling is going to add something on the order of half a dozen sabins per linear foot of corner treated at 100 Hz. That is a shocking number when you consider the device has 2.66 SF of face per linear foot and therefore 2.66 sabins would mean an "open window" of that size - no reflection of sound whatsoever. At first blush the idea you might get 6 SF or so of open window for every 2.66 SF of treatment seems to defy physics. Its an acoustical black hole, Wow!
But cool as that is, it aint magic. Instead its a byproduct of resonant behavior. Corner mounted porous absorbers typically exhibit a sharp peak in performance - typically somewhere in the 63-100 Hz bands. This little elf has been dubbed the "100 Hz peak" for lack of a better name. The 100HP moves around a bit, and gets broader or sharper, depending on the topology of the construct [wedge versus panel], the density of the foam or mineral fiber, whether you have upholstery and what kind of fabric is used, whether you space the devices or mount them edge-to-edge, whether you capture tri-corners, and the overall thickness and face dimension, etcetera.
The basic concept is however pretty straight forward, namely that porous absorbers in corners are a mass-spring resonate system. The air in the corner is the spring, held in place by the room's walls, and the panel or wedge of absorbent material is the mass. Mineral fiber or foam, naked or upholstered, panel or wedge, thick or thin, big face or small face... doesn't matter. They are all mass-spring systems with a resonance kick somewhere around 100 Hz, and, that kick has a front side and a backside on a measurement graph. You reckon [I think] that the backside of the free ride starts running uphill somewhere around 40 to 50 Hz - and from everything I have seen, that is a fair statement, but concluding that this means no work can be done using a porous absorber south of say 63 Hz does not follow.
Expressions of this peak can be quite large [on the order of a 50% increases over the median absorption outside the peak center band], and can appear very sharply expressed. Or, they can be so subtle as to not look "peaky" at all, but rather a gentle rise and fall. There are ways to soften the peak - and to broaden it - or you can even make it more "peaky" by adding a membrane [a generally nutty idea IMO - but it could fit the need for a particular application just so].
But, keep in mind that whatever you study as to this effect, to some degree the actual effect is masked by the 1/3rd octave band reporting technique. For example, a peak straddling a pair of bands could be quite "peaky" but not show up that way in the graphs - alternatively a peak of exactly the same magnitude but which falls completely in a single band will have a much more dramatic expression on a graph.
This is part of why it always best measure for low frequency performance in situ as the intrinsic properties of devices are hard to measure very low in the band, such measurements are hard to interpret [even if you know everything about how they were taken], and the results in any given room will vary enormously even using the same devices when you vary the placement. There are a number of "known good" devices, and known good treatment placement schemes, but nothing can beat verifying the installation with an on site measurement system [and a professional acoustician too if you can swing it].
PS: A couple of other comments:
The idea that foam doesn't work as well as mineral fiber is falsehood. Acoustic foam works great... non-acoustic foam does not. By the same token, there are thermal insulation materials that have excellent acoustic properties, and ones that do not.
Comparing absorption measurements at 100 Hz even or even lower, can be done both rationally and usefully provided you have all the information. It takes some intellectual honesty and a keen understanding of what you are looking at, but a lot can be learned from comparing measurements - even between measurements taken by different labs -if you have full details of the measurement procedures [sample size, sample placement, the % Uncertainty of the measurements, etcetera]. When measurements are proffered absent these details comparison becomes quite dicey stuff as you suggest, especially below about 80 Hz. This is not so much a limitation of the science - though physics does come into play - but rather, the key limitations are artificial constraints created by researchers and manufacturers who hide data.