I've built numerous porous absorbers, membrane absorbers, Helmholtz absorbers, and VPRs within the last years always wondering which bass trap type would be the most efficient at the "last octave" where music key tones (~40Hz-80Hz) happen to match dominant room modes of "small" acoustic spaces (control rooms) resulting in standing waves.

Now that I have an empty room with different types of bass traps (see above) and a ready-to-go test setup available I kindly take your suggestions until the end of the year 2011 to compare different configurations directly for further public reference.

How to "apply" if you are interested:

1. Check if a similar idea to yours has already been suggested.
2. Use the attached Google Sketchup 8 file to "build" your setup with what's available focusing on the corner opposite the microphone (see example).
3. Disable unused layers.
4. Save your file with a unique filename (with date and your Gearslutz user name in it).
5. Upload your suggestion as a Google Sketchup file to this thread together with at least one image for visual reference.

Some "rules" to keep the process transparent:

1. No PM in regard to suggestions.
2. Testing can be attended (5 seats).
3. Suggestions may be rejected.
4. Test results of approved suggestions will be published.
5. No publishing of test results for commercially available 3rd party bass traps.

I'll play Monday when I'm back at my drawing computer!

To validate the test setup variations of the whole chain (except the room itself) are possible prior to the final test run:

• speaker
• positioning
• microphone
• soundcard
• sound pressure level
• software
• impulse response measurement excitation
• modal analysis method

Speaker choices are from 5"/40W up to 15"/1000W, all in sealed boxes for extended (smooth lowcut) low range:

1. Speaker_A: Auratone 5C Super Sound Cube*
2. Speaker_B: HELI K 12 (vintage active 45W rated speaker with a single 12cm [~5"] driver used in GDR broadcast studios)
3. Speaker_C: Custom made bipolar woofer with dual 10" Visaton WSP 26 S drivers in a closed cabinet*
4. Speaker_D: Blue Sky Sub 15 Universal powered woofer (1000W)

*) powered with the t.amp PM40C 40W power amp module

The bipolar woofer with a push-pull configuration inspired by the Blue Sky SUB 212 woofer measured accordingly to the specs (about minus 12dB down from 100Hz to 20Hz) outside (some walls near). It's been designed to have:

• ... optimized geometry for corner placement
• ... smooth frequency response roll-off down to 20Hz
• ... minimized cabinet resonances under 100Hz (lightweight and stiff enclosure)
• ... theoretically no outer forces due to drivers movement

Wow, thanks for linking to this in the other thread. I hadn't seen it and since I have several weeks still, I will definitely have a look at it and play.

Regards,
John

* fully alert *
this gonna be huge!

The room is 4,055m x 3,13m x 2,26m [~ 13'4" x 10'3" x 7'5"] with solid concrete floor and walls. The ceiling is made of steel beams and hollow blocks bond with solid concrete. There's a small (usually closed) window and a simple wooden door.

What makes this room convenient for the test method suggested by the original inventor of VPRs Helmut V. Fuchs in "Schallabsorber und Schalldämpfer", S. 6ff, is the quite even distribution of the lowest modes. Checking with Bob Golds Room Mode calculator gives a room ratio: 1 : 1,38 : 1,79 and shows that the room meets the criteria for preferred modal distribution.

I own these two mics with an omnidirectional pattern (well, and a Neumann U87A):

1. MIC_A: Audix TR40A SDC (Small Diaphragm Condenser measurement microphone)
2. MIC_B: AKG C414B-TL2 LDC (Large Diaphragm Condenser microphone, transformerless)

Usually I go for the rather affordable Audix but I want to see what difference it makes to use a LDC.

My mobile setup incorporates either:

1. SOUNDCARD_A: Digidesign Mbox 2 Mini
2. SOUNDCARD_B: RME Fireface UFX

For this test one needs 1x line output and 1x mic input (48V phantom powered). Mbox specs should be good enough but again it's interesting how the test results hold up to a higher quality interface.

For consistent "absolute" levels I got a

1. SOUNDLEVELMETER_A: AZ Instrument Digital Sound 8922 sound level meter

All this because:

1. nonlinearities (which we want to track down) especially in resonant absorbers might lead to different test results due to variations of absolute sound pressure level
2. checking different traps at a consistent level
3. repeatability of the test setup

I'm thinking of configurations incorporating the same amount of material with different geometry as well...

In the spirit of the protocol here, I might suggest outdoor ground plane measurements and in-room nearfield measurements of the speakers in addition to testing the room without treatment as it sounds like you have planned.

Among other things I want to show that the test results for the traps are quite independent from the speakers (as long as they're able to drive the deepest modes). It's about "trapping traps"...

Great, can´t wait!
Thanks for sharing...

I've never touched mineral wool for acoustics so far. Anyone recommend a setup with mineral wool to compare with the same amount of CIB (Caruso Iso Bond) and Thermohanf?

The recommended mineral wool type (for example from Isover) should be readily available over here in Vienna (Austria).

I will happily share any results tested with setups suggested by you (and other gearslutz).

The metal panels may be incorporated with the porous absorbers even in vertical configurations.

I agree, if measurement results are presented as difference "before" and "after" traps... it is no needed to waste a time with precise subwoofer calibration.

It is only needed a precise and repeatable position of microphone AND subwoofer for every different measurements which are compared.

G. E., this is a great idea, thank you for your effort and will to share measurement results.

I'd love to see your suggestion for a perforated panel absorber for this room. There's one enclosure in the Sketchup file. I could do a new frontplate according to your specs. Plus there is one more enclosure (not mentioned in the Sketchup file, 20cm [~8"] outer depth) that I could use additionally.

would be great to see wall > vpr > air gap > isover
sorry i`m not able to use sketchup.

... makes sense if you want to build a truly broadband absorber full way up from 40Hz. I have often seen that pressure based bass traps (partly) covered with porous absorbers or panels don't degrade the bass traps's performance.

This thread is focusing on comparing bass traps and I only want to touch mineral wool for this purpose -- so what do you compare with?

Depending on the date I would really love to attend that session.

Depending on your suggestions I would really love to run that session.

I often wondered as the discussion with ceiling mounting options was running if you simply could hang (or put on a stand or whatever) Isobond with a small distance under the ceiling and lay a steel plate on top of it. I tought that from below the soundwaves should see a bka. I know you wrote you don't want to compare broadband solutions, but I think it would be interesting to compare the effect of these two setups on the low frequencies. Perhaps putting Isobond on the plate on the floor makes the result more comparable but also more broadband.
And of course I know that the plate won't fly in the air and a framework for it to rest on would be needed.

With this setup there's little to none absorbent where air velocity is high so I don't expect usable absorption under 100Hz. I guess the plate itself makes no big difference in this setup (ceiling), but who knows... I'll see if I find an easy way to mount it as you suggested.

wouldn't the plate resonances be roughly the same?

Yes and we need absorption of the moving air that those resonances create. I think I get your idea and the ceiling would be a great place if it works. On the other hand absorbent in front of the steel never enhanced the bass absorption in my experience. We'll find out...

I cincerely appreciate your offer to be a part of your research, but the first results of your research and tests of VPR inspired absorber construction are more than enough for my needs and from my experience. It will nicely fit in the small room, at first absorbing layer, close to the wall, with Newell's panels at second position ("cascaded" with VPRs), behind air transparent diffusers, in MyRoom Design principle. And this will work.

Now, it is more important for me to be sure how to technically realize it (VPR inspired absorber), without possibility to decompose after some time (especially at ceiling), so, I'm not suspicious whether they will work or not.


Thank you very much, anyhow.

This result came from a corner mounted trap, call it an enhanced porous absorber.