Diaphragmatic versus membrane bass traps

Looking inside some old literature about membrane bass traps I found that they were the chosen way to control bass in some BBC studios.

Although they use a similar principle, hard diaphragmatic traps have a different performance - yes, I have read the Jeff excellent summary at:

http://www.audioholics.com/education/acoustics-principles/bass-trapping-ideas

Can our resident experts advise on pros and against of each of them? Is there any range of preferred frequencies for each type?

You can find links to the BBC papers in this page:

http://www.hifizine.com/2011/09/bass-integration-guide-part-2/

JackD201 said:

Both can be designed and built tuned to target frequencies and Q but IIRC you have more play with membranes. Membranes however may detune faster over time depending on the materials used and environmental conditions. I went with hard almute and airspace for this reason but I also use tuned resonators. Soft membranes are the most space efficient for frequencies in the low 30's and below. Space isn't so much of a factor in my case so I passed.

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Jack,

Are you using sintered aluminum plates as a diaphragm in your bass traps? Is there any advantage of sintered aluminum over common aluminum plates? My bass traps need to cover frequencies centered around 33 Hz, problematic decays show bellow 60Hz, as shown in the decay times measured with REW with a Martin Logan Descent operated full range exciting the room. This is not the best frequency response, frequency response depends strongly on placement, but decays are stable.

JackD201 said:

Hi Micro,

The sintered aluminum makes up two of my bass traps. The 3mm variant I use has similar properties as 4" of compressed fiberglass. I sent the 3D CAD drawing to Japan for simulation given the air volume then had my acoustician work around the results. At that point I left the two acousticians to work out the math and construction for my desired curve and RT60.

Aluminum plates if curved will serve more as a diffuser and not do much more unless they are very large and mounted with an airtight cavity and even then will soak up little bass. It may even ring if not damped. Sintered aluminum lets air through and so it is a bit of a combination of a velocity trap with some aspects of a pressure trap due to flexure.

What decay time and what slope would be ideal for you? You look like a candidate for tuned traps.

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Jack,

The sintered aluminum plates seem great - but I did a quick search and they seem very difficult to source. As the RT60 on most of the spectrum in my room is 400ms and I am pleased with its balance I am targeting at this value, accepting up to 50% increase in the low bass frequencies. But I am not an acoustician. I measured a room of a friend who has an RT of 300ms, flat in the bass zone. I found it to dry - very detailed and dynamic, but not my taste.

One more beginner question. Ignoring price, what is acoustically the best material for a very low bass frequency (36Hz) wall bass trap - 3/8" MDF or 3/8" plywood? I can easily compensate the small difference in surface mass variation changing depth and they will hidden behind and acoustic curtain, so aspect is not relevant. They seem to have a very different acoustic properties as they "ring" differently.

After reading all I could get about bass traps, and finding that no two acousticians share the same opinion, I had to take a decision on how to correct the 36.5 Hz too long decay of my room. As it was due to a nodal axial mode between two large solid walls and I am generally pleased with the measurements and sound of my room, I choose a tuned device. Considering the availability of materials, the low frequency and my limited DIY wood skills I decided for the membrane absorber - a box fitted with slab of compact rigid mineral wool close, but not touching a mass loaded membrane reinforced with a thin interior fiberglass mat. In order to lower the Q of the absorber one third of the volume (at the back) was filled with fluffy mineral wool. After considerable design time I got the MDF panels cut to size last Thursday and assembled the bass trap today. I attach the pictures of the assembly phase. I moved it to my listening room and after such an effort, bias expectation was high, IMHO the sound had improved a lot - bass had more attack and was more defined, image seemed more "free", there was a general "cleaning" of the sound .

Happily the measurements confirmed that the bass trap was doing its job perfectly - the decay at 36.5Hz was now noticeably faster and the 53Hz mode also shows some improvement. Soon I will add one more similar bass trap, that is only waiting for the membrane to be fixed, and will add some more measurements taken with an accelerometer.
The decay plots show decays at 0, 150, 300 and 450ms before and after placing the bass traps.