Reducing room decay times (bass ringing)

[unmutual]

I have some low end issues remaining in my music listening room , after putting in as much porous absorbtion as I can reasonably fit. Plus I have a very large subwoofer that only handles the sub bass region below 35 Hz. Although I rarely notice that the sub is working, it is helpful to not have my main speakers (floorstanders with good low end extension) handle the lowest frequencies, to avoid exiting the fundamental room mode which is around 36 Hz. I have tried other, higher, crossover frequencies, but with the current placement of the sub, I then get a major cancellation in the 50 Hz region instead. 35 Hz seems to give the best response overall. Multi sub set up is not an option with my current equipment (I would not be able to set different crossovers or do individual time/phase alignment).

As you can see from the waterfall image below, there are ringing in the following regions:

1. below 30 Hz - not so important as there is typically little musical content here in most program material.
2. Around 36 Hz - the fundamental room mode, will be difficult to treat as I have limited amount of space left. And as mentioned above, I have already improved this region through speaker/sub integreation.
3. Around 68 Hz - lower peak than 36 Hz, but more noticeable

So my working strategy will be to target the 68 Hz ringing with a tuned membrane bass trap. I have not found a step by step DIY tutorial for this, but instead have used ChatGPT as a design tool. The recommendation from the algorithm is to make a box of 18 mm MDF panels, with 6 mm MDF as the membrane, and fill the box appr. 50% with fibreglass/rockwool.

I am curious if anyone has tried a similar membrane design, and if so, how effective was it?

Also wonder what you think of my overall strategy of ignoring the deep bass region. I think I would have to use an extra 10-15% of the room space if I were to attempt to improve the sub 40 Hz region, and I'm just not sure it's worth it - but would appreciate all informed opinions on this matter.

By the way, the waterfall is with one EQ filter, but this is outside of the frequency regions mentioned above.

 

[alex-z]

You don't need different crossovers or time alignment for a basic multi-sub setup.

Measure your current sub at multiple locations and use REW to sum the various responses. High chance you can find two sub locations that when utilized together solve your 36Hz issue.

You can build a membrane trap but in terms of space that is less efficient than the second sub.

 

[Mort]

The DiracArt army will be here in 3..2..1...

 

[Oddball]

Second sub might smooth out the frequency response as noted, and provide some headroom to the system for lower distortion.

But decay is a different animal. Room treatments could help, but not very familiar with that area. Multiple subs in double bass array that is set up correctly could reduce decay times by cancelation. And then yes, there is Dirac ART that does cancelation (and other things) in software using all available resources.

 

[dfuller]

Pressure absorbers like membranes are your best friend here. Nothing else really works, save for the PSI active bass traps.

 

[NTK]

So my working strategy will be to target the 68 Hz ringing with a tuned membrane bass trap. I have not found a step by step DIY tutorial for this, but instead have used ChatGPT as a design tool. The recommendation from the algorithm is to make a box of 18 mm MDF panels, with 6 mm MDF as the membrane, and fill the box appr. 50% with fibreglass/rockwool.

I am curious if anyone has tried a similar membrane design, and if so, how effective was it?

BBC Research came up with a modular low frequency absorber design, and is described in their report below.

The design of a modular sound absorber for very low frequencies

Report 1992-10

www.bbc.com

Page 10 of the report shows the construction details, with measurement results.

 

[kyuu]

Can you please show the frequency response measurement?

 

[Keith_W]

A few points to note.

1. The noise floor rises in the bass region in all listening rooms and CAN give the appearance of increased bass decay. Not to mention: ground loops, vehicles driving past, etc. I make this point over and over. You can not rely on ONE waterfall graph, ESPECIALLY if you don't know if specific precautions were taken to increase the SNR. All measurements looking at bass decay need an excellent SNR - take a long sweep (512K or 1M), and take a few measurements, preferably at different times on different days, and compare them. If you see the same pattern of decays in the same locations, then it's real. If not, you are looking at an artefact.

2. Bass naturally has longer decay anyway. This is because: the period of bass frequencies is longer (20Hz = 50ms!), subwoofers ring and have increased group delay, especially ported subwoofers, and more bass energy is pumped into the room. Room treatment will do diddly squat if your subwoofer is ringing, and results will be disappointing, to say the least.

3. Individual peaks in decay are probably less audible than you think, unless it is very pronounced. You should ask yourself whether it sounds OK. And if it does, then nothing needs to be done.