Using VPRs make sense when floor space is limited or when the VPRs absorption characteristics are needed.

In this 10,4m^2 [~112ft^2], 2,5m [~8'2"] height, pretty odd shaped room I have used 20cm [~8"] sandwiched VPRs (kind of ultra broadband absorber) in the back, deep porous absorbers in the front corner and some 2D-diffusorbers on the ceiling. It's a corner setup because of the room shape, doors/window situation.

The desk is non-reflective above ~500Hz. I would have liked to put some 10cm [~4"] behind the speakers but for now we opted for the additional space and positioned the speakers close to the wall.

Pictures show some preliminary measurements (L+R) without the final desk surface but some simple absorptive elements in place instead.

Take a look at another real world example of Shaping T30 with a Kind of Straddling VPRs showing off some before/after measurements.

Hi Gernot,

wonderfully educational posts, thank you.

Could you pls have a look at my measurements (center speaker with sub) and let me know your opinion?

They are from a 7.1 surround setup.
Mic position at the MLP (main listening position).

The room has ceiling and 2,5 sides fully covered with wooden panels in trapezoidal form (rural style ).

Although the room has the worst shape for acoustics: a square (4x4 m) I am quite satisfied with the sound.

Is that a bass mode with very low decay at 42 Hz?
I also see a dip at 70 Hz with a speak at 77 Hz.

Do you think a VPR (maybe tuned to 42 Hz) could help to even out the decay and/or the dips in the frequency response?

I included the measurement-file from REW.

Hi Gernot, have been reading about the enhanced broadband absorbers. Is there any reason for not working with Rockwoll as absorbent on them? I know you use Polyester fibre or CIB as absorbent, but none of these are available in my country. You think is reasonable to give it a try with Rockwool? thank you a lot for your contributions.

Hey!
I can confirm that mineral wool works with VPR. I've tried 100mm 40 kg/m3 Rockwool (Acoustic batts) + galvanized steel sheets 2,2mx1,25mx1,5mm on top. And it works from ~40 Hz and higher.
I haven't tried another layer of mineral wool on front side so far.
I can't compare it to CIB or any other suitable foam, because they are not available or cost a fortune here.

Also there is evidence in Fuchs book that he used mineral wool for some projects where other materials weren't acceptable for non-acoustic reasons. But there are no measurements and comparison.

Hey! thank you for your response. Do you have some measurements to share? i'm putting together some 100mm + 2.3mm steel + 100mm rockwool 32kg/m³ next week. The lowest mode is around 38hz. it's safe to assume it will work just fine, right?

Hey! Here are some old measurements before and after treating the whole back wall with 100mm rockwool ~40-45kg/m3 + 1.5mm steel - 2.2mx1.25m x3 sheets. Room is ~6x4x2.7m and is far from empty. Front wall is not treated at all with large window (which tends to resonate very much at 25 and 54Hz and I am going to get rid of it). ~1/2 of side walls are treated with 200mm air + 200mm rockwool, ~1/4 of ceiling is also treated with 200mm air + 200mm rockwool. (This is just for understanding that there is good amount of porous absorber already in the room).
Mic was placed at some random place in the middle of the room (not listening position), speaker was placed somewhere else as well, I can't remember now. (Of course both measurements were made with speaker&mic at the same positions).

As you can see 25 and 54 Hz aren't affected that much. But as I suspect from further experiments the main problem for these is large window and treating just back wall can't help eliminate that.

Also I did some more experiments with different rock wools and PU foam, different thickness, and different pressing on steel against rockwool. But after all that I understood that until I eliminate window problem all those measurements will not provide adequate information.

Hey guys,

I decided to implement the EPA's on a small listening room. All the info and measurements can be found on the following link:

Is this good enough for me to call myself an Acoustic Designer?

Cheers!

Thanks to GE and everyone else.

I have one simple question: in the case of for example the 2m x 1m Steel Plate EPA: is it important that the absorber behind it is one whole piece or can it be 2 pieces on top of each other?

The same for a plate of 1,50 m x 1 m: can the absorber behind it be made of 2 pieces of 1m x 0,75m polyester/CIB?

As long as the pieces are of the same thickness and quality I think you're good.

Very helpful, thank you.

Advertising ON. I'd like to use this post as a perfect example for what I'm gonna talk about (in German) at my acoustic workshop "Was du schon immer über Studioakustik wissen wolltest – Mythen, Meinungen und Messungen mit Gernot Ebenlechner" on September 8, 2022 at Fiakka Studios (Vienna/A). Over the years I got numerous PM-requests (that I preferred not to answer privately), so this is the perfect chance to ask all your acoustic questions about rooms, absorbers, diffusers, positioning, etc. in person. Advertising OFF.

I spotted something interesting in the results of measurements F and L, which doesn't quite seem to line up with the conventional wisdom regarding porous absorber bass traps:

When comparing the waterfall graphs to the modes of the room calculated with Amroc, the 20cm thick panel straddling a corner (measurement F) seems to be behaving as conventional wisdom would predict. The room modes between walls are absorbed effectively, because there is a good amount of absorber (in both thickness and surface area) a decent distance away from the boundary. On the other hand, the node between the floor and ceiling is relatively intact, because the absorber is simply not oriented to affect it.

However, when looking at the measurement L where the 20cm panel is along a wall, the results seem counterintuitive. The 42 Hz node is not affected very much, which is pretty much what the simulations would predict (2 sq.m at an absorption coefficient in the low 0.20s).

However, the short wall-to-wall node around 55 Hz and the floor-to-ceiling node around 76 Hz are absorbed relatively effectively, even though the panel only presents a 0.4 and 0.2 sq.m surface in the direction of those nodes.

The difference in the vertical node is particularly interesting, because the exposed area of the panel is the same as in measurement F, but somehow placing the absorber along the wall makes it dramatically more effective.

I can imagine some hypotheses for what might be going on, but I haven't seen this apparent phenomenon explained, or even discussed, anywhere before. Is there a known explanation for this that I've just missed, or is this a real blank spot in the popular understanding of how porous absorbers work at bass frequencies?