Very bad room acoustics (40hz resonance, bad reverbation time), what would you recommend?

[atomek]

I would like to improve acousicts of my living room, as of now the REW measurments are terrible, and I think the biggest problem for now is the bass, escpecially 40hz (with highest pressure marked as red dots on the room plan).

Bass issue: Would you recommend any tuned bass traps, if so how many of them I need and where to place them? What do you think of Gik Scopus T40?

Sound clarity / reverbation times: My left side of the wall are window with very light curtains, would you recommend any good curtains to lower reverbation time? What do you think of HOFA Studio3? Would you put bigger carpet, a felt underlay? Maybe other type of panels that could reduce RT60?

Below is the table of the curves recommended to me for my room dimensions (20m^2, 2.5m high), as you check diagram from REW my results are 2-3 times higher than recommended.

Octave-band centre	63 Hz	125 Hz	250 Hz	500 Hz	1 kHz	2 kHz	4 kHz	8 kHz
EDT (first 10 dB)	0.40 s	0.34 s	0.26 s	0.23 s	0.21 s	0.19 s	0.17 s	0.16 s
T20	0.42 s	0.36 s	0.28 s	0.25 s	0.23 s	0.21 s	0.19 s	0.17 s
T30	0.46 s	0.38 s	0.30 s	0.27 s	0.25 s	0.23 s	0.21 s	0.19 s
Topt (curve you EQ / treat toward)	0.44 s	0.37 s	0.29 s	0.26 s	0.24 s	0.22 s	0.20 s	0.18 s

Plan of the room

 

[Keith_W]

Attempting to fix bass problems with room treatment is not easy because of the size of the absorbers required. The easiest and least intrusive way to get rid of that 40Hz peak is to chop it off with DSP. But looking at your measurements, it looks as if your subwoofers are 10dB too loud. Dropping the sub by 10dB should help a lot. And I would be more concerned about that huge dip between 110Hz - 160Hz. You may or may not be able to fix it with DSP, but I wager likely not. You may be able to fix it by moving speakers or subs around. Unfortunately, a room like yours can not be simulated in REW so the only way to find out is to move the subs and re-measure.

Using your room dimensions I calculated your RT60 target according to the DIN 18041 standard and drew it into your graph with white lines. It looks as if you would benefit from some room treatment.

BTW, remember that in a small room like yours, you are not looking at reverberation decay. Anything below the Schroder frequency is a room mode, and if you chop off the room mode you will also reduce the decay. For the higher freqs, it is more accurate to call it "reflection decay" rather than "reverberation decay". The difference is that reflections are specific to microphone position, but "reverberation" is the same everywhere in the room. You will find that changes in microphone position will affect the "decay" pattern.

Your ETC is zoomed out way too much. You need to zoom in to the first 30ms. I can see there are a few loud and early peaks there but can't tell since it's not zoomed in enough. You can use the timing of the peaks to tell you which surface is causing the reflection, but my guess is that the wall behind the listening position is likely responsible for the earliest peak. PROVIDED you took your measurement properly, i.e. with the mic on a boom stand and not on a mini tripod.

In the meantime, please read this thread and choose proper settings for posting graphs.

 

[smygolf]

Helmholts (big) or quarter tube (high) would help a lot in this case
3 tubes can lower a peak with 9-12dB but it comes down to wave pressure/speed at the placement.

 

[atomek]

Thank you very much for this comment, @Keith_W. The 40 Hz problem is awful, and asking me to move the subwoofer was adding insult to injury I have only Dynaudio S25 loudspeakers, and I think 40hz is a mode between two side walls. I've experimented with DSP (miniDSP), but regerdless of the optimization I made I wasn't happy with the sound at the end, it looked better on the REW, but the music become less enjoying, without soul, no details improved.

I've measured with boon, umik-2, but pointing to the ceiling. I've made another measurments pointing to speakers like in the guide. Filter IR zoomed-in to 60ms. I've also attached photos of the room.

 

[atomek]

Helmholts (big) or quarter tube (high) would help a lot in this case
3 tubes can lower a peak with 9-12dB but it comes down to wave pressure/speed at the placement.

Could you recommend a specific product, how many of them, where to put? What do you think about this? https://www.gikacoustics.com/product/gik-acoustics-scopus-tuned-bass-trap-t40/

 

[smygolf]

Could you recommend a specific product, how many of them, where to put? What do you think about this? https://www.gikacoustics.com/product/gik-acoustics-scopus-tuned-bass-trap-t40/

Products like the one in link will not help you.
Quarter wave is diy and Very easy to build.
The bigger paper tubes that are used for concrete are Perfect.

It targets 1/4 wave length and it easy to make them tuneble ( a must to dial them in)
A bit of filling to get a wider Q and a thin cloth over the hole.


Not much of info to find these day, where popular in the 90-20:s then dsp took over.
I have a spread sheet to help with basic length.
Compared to Helmholts they does not take much space, easy and cheap to build.

*edit*
Min 200mm in dia

 

[smygolf]

40hz = a 200cm/30cm tube.

Source tubes near 30cm in diameter
Cut or glue 2 tubes so length is ~210 - 220cm.
Cut of a bit so total length is 190cm.

The part you cut of, on the length cut out a thin splice.
Then you can press it together and fit inside the other tube - moving it up/down changes the tuning.
Plug one side - glue it o a wooden plate or such.
Throw some T-shirts in the tube.


Cut som bedsheet to use over the open hole.
It shall be as thight as you can get it - a small coin shall bounce on it.
Rubber bands works good

Use REW to play your trouble freq and move the tuning until the top cloth starts to vibrate violent.
Fix the tuning part with glue so it is air tight.

You can fill the tube with speaker filling but it shall be Really loose and max 1/3 of the length.
A couple of shirts are normaly enough - to wide Q takes down the efficiency.

 

[Matias]

I got rid of my 41 Hz peak with a simple EQ, and large panels and columns for attenuating reverberation.

 

[smygolf]

One more tip to pin point the issue

Helmholtz Calculator

 

[Keith_W]

I would like you to examine closely the difference between these two measurements. It should very quickly tell you that the RT60 is not a reliable indicator of decay in small rooms.

As for the ETC, the vertical scale is too stretched. It should be between 0dB and -30dB or so. In any case, I can see that you measured both your speakers together (initial twin peaks) and there is a very strong reflection at 4ms, likely the rear wall. If you do deploy acoustic foam, the rear wall should be the priority.

I wouldn't be wasting your time with Helmholtz resonators. For them to be effective, they need to be carefully tuned. They are fussy about placement, and if you can't place them properly, they need to be large. It is difficult to predict how well they will work until you have them in place and you can take measurements, and by then ... you have wasted money on them. DSP is the way to go when tackling bass problems.

 

[atomek]

I never did any DiY

40hz = a 200cm/30cm tube.

Source tubes near 30cm in diameter
Cut or glue 2 tubes so length is ~210 - 220cm.
Cut of a bit so total length is 190cm.

The part you cut of, on the length cut out a thin splice.
Then you can press it together and fit inside the other tube - moving it up/down changes the tuning.
Plug one side - glue it o a wooden plate or such.
Throw some T-shirts in the tube.


Cut som bedsheet to use over the open hole.
It shall be as thight as you can get it - a small coin shall bounce on it.
Rubber bands works good

Use REW to play your trouble freq and move the tuning until the top cloth starts to vibrate violent.
Fix the tuning part with glue so it is air tight.

You can fill the tube with speaker filling but it shall be Really loose and max 1/3 of the length.
A couple of shirts are normaly enough - to wide Q takes down the efficiency.

I havent thought about making myself a bass trap, but you made me think about. I did a bit of research, and found a PVC pipe (315mm) that I could use and asked GPT to design me a trap tuned for 39hz, could you review its response if that makes sense? Would you change something?

ChatGPT - Bass Trap Design 39Hz

Shared via ChatGPT

chatgpt.com

 

[atomek]

As for the ETC, the vertical scale is too stretched. It should be between 0dB and -30dB or so. In any case, I can see that you measured both your speakers together (initial twin peaks) and there is a very strong reflection at 4ms, likely the rear wall. If you do deploy acoustic foam, the rear wall should be the priority.

Wow! I didn't realise that I could get this information from those measurements. They are spot on: the rear wall is about 30–50 cm behind and untreated. What kind of acoustic foam would you recommend and how thick should it be? I'll try to fit it behind the picture.

 

[smygolf]

I never did any DiY


I havent thought about making myself a bass trap, but you made me think about. I did a bit of research, and found a PVC pipe (315mm) that I could use and asked GPT to design me a trap tuned for 39hz, could you review its response if that makes sense? Would you change something?

ChatGPT - Bass Trap Design 39Hz

Shared via ChatGPT

chatgpt.com

Should work
My own tests gave a better result with a top cloth and just some minor filling.

 

[olieb]

40Hz is a range where it is more than problematic to improve acoustics even with a tuned resonator/absorber like a Helmholtz resonator. You probably have to throw everything at it that you can afford the effort for. That is absorber AND EQ.
It might be interesting to have a closer look at the resonances/room modes around 40 Hz.
This might help: https://amcoustics.com/tools/amroc-pro
I do not know how the lower right of your room is built, but with some wild guess it might look like this.
If modes similar to those two below are excited at 40Hz you will get pressure peaks in the corners you marked.

 

[janbth]

40Hz is a range where it is more than problematic to improve acoustics even with a tuned resonator/absorber like a Helmholtz resonator. You probably have to throw everything at it that you can afford the effort for. That is absorber AND EQ.
It might be interesting to have a closer look at the resonances/room modes around 40 Hz.
This might help: https://amcoustics.com/tools/amroc-pro
I do not know how the lower right of your room is built, but with some wild guess it might look like this.
If modes similar to those two below are excited at 40Hz you will get pressure peaks in the corners you marked.
View attachment 458818 View attachment 458819

I think this is a good analysis, showing that there may be a degenerate resonance at about 40 Hz. This is similar to my own situation, where I have a peak at 45 Hz. I combat this peak by running two subs in mono, positioned diagonally. This also has other positive effects on the smoothness of the frequency response in the bass region. More info in this post, for those who are interested:

Wanted: Proof of multiple subs and sub EQ

How can I make myself invisible? Blindfold us. :)

www.audiosciencereview.com

 

[raindance]

Your speakers are in the front corners and you're sitting closer to the rear wall, so you've created a worst case scenario. Try moving the speakers out of the corners and a little away from the wall behind them.

 

[atomek]

40Hz is a range where it is more than problematic to improve acoustics even with a tuned resonator/absorber like a Helmholtz resonator. You probably have to throw everything at it that you can afford the effort for. That is absorber AND EQ.
It might be interesting to have a closer look at the resonances/room modes around 40 Hz.
This might help: https://amcoustics.com/tools/amroc-pro
I do not know how the lower right of your room is built, but with some wild guess it might look like this.
If modes similar to those two below are excited at 40Hz you will get pressure peaks in the corners you marked.
View attachment 458818 View attachment 458819

Thank you for this analysis, I don't know how to interpret this diagram, but walking around with mic and REW generating 40hz I've found the peaks like in the room plan red spots. I think I'll give it a try and set bass traps at least in top right corner and see how it will improve.

 

[atomek]

I would like you to examine closely the difference between these two measurements. It should very quickly tell you that the RT60 is not a reliable indicator of decay in small rooms.

As for the ETC, the vertical scale is too stretched. It should be between 0dB and -30dB or so. In any case, I can see that you measured both your speakers together (initial twin peaks) and there is a very strong reflection at 4ms, likely the rear wall. If you do deploy acoustic foam, the rear wall should be the priority.

How do you see the RT60 is not a reliable indicator? I don't know how to interpret REW results, I guess that in that small room I should go for a sound absorbers rather than diffusers? What would you recommend to put as a absorber on the rear wall? I'm thinking of 8 60 cm x 60 cm modules for absorbtion, maybe the top 2 of them on a sides with build-in scatterer? I'm affraid that with added curtains and carpet I'll absorb too much of highs.

What do you think of placing such absorbers in the middle of rear wall, between bookshelves like this?

S A A S
A A A A (head level when sitting)

 

[olieb]

Thank you for this analysis, I don't know how to interpret this diagram, but walking around with mic and REW generating 40hz I've found the peaks like in the room plan red spots.

If my "analysis" is somewhat correct (it depends on the volume/geometry in the part not shown in your plan) then there are two room modes near to 40Hz. If a 40Hz tone is played both of these modes will be excited and the pressures of the modes will add.
With the phases (red/blue) shown you get red+red =strong red in bottom left and blue+blue=strong blue in top right.
In top left and bottom right red+blue=weak signal.
So that would fit with your observation.

I think I'll give it a try and set bass traps at least in top right corner and see how it will improve.

This should be a (tuned) bass trap that works on pressure (resonating type: Helmholtz or plate absorber). Porous "bass traps" work on velocity and that will be rather low in corners.

 

[atomek]

With the phases (red/blue) shown you get red+red =strong red in bottom left and blue+blue=strong blue in top right.
In top left and bottom right red+blue=weak signal.

Yes, that makes sense, thanks!