Are my bass traps reasonable or am i doing something wrong?

[slacki]

Hi, i was thinking about to buy the new Marantz Cinema 40 or 50 for my home cinema. That started my cinema hobby so i thought i do some more improvements.
I bought the Marantz SR7011 a couple of years ago and had some room modes with my Arendal Sub2 and the Tannoy XT8F (FL/FR). So i built these gigantic bass traps - in hope they would fix anything.
At that time i had no idea of REW or anything, i just did it and thought "yeah, sounds better". Now i can do REW measurements but i am probably lacking of interpreting it correctly.
As i am planning to replace the Arendal Sub2 with 4 smaller subwoofers (the Cinema 40 and 50 can handle 4 subs) to get a better bass, i thought i do some measurements regarding the bass traps.
My flat is the living room and a sleeping room, the kitchen is just behind the tv and the separation closet:

The sub is behind the green towel, have to protect it from my cat. Same with the Marantz. The towel below the center does not "close" the marantz, the back is completely open, so no problem with heat.

The couch was replaced with a much smaller one. So i have now one single sitting position in the sweet spot with a relax chair and a 2 seater couch at the right side.
In the left and right corner are 2 bass traps with size 120x50x50 cm (48 x 20 x 20 inch).

I moved the sub for testing from center left to between the printer and the bass trap (close to the Tannoy XT6F) and did a first measurement:

after that i removed the upper bass trap and did a measure again

I have a heavy drop in 70hz. So i thought i remove the lower bass trap too (so complete left side without bass trap in the corner) and check what happens

tbh: i dont understand it
I thought a bass trap "eats" peaks to prevent room modes, but this looks more like it makes a better curve in general and improves drops. Without the left bass trap i have a drop in 60, 70, 85, 92 and 100hz.
I am wondering now if i could remove the bass traps when using 4 subs* in the corners. I really liked them but they are just really... big. I have no girlfriend/wife so its not that big deal but, if possible, i would like to get rid of them. Or should i keep them? Or do i interpret the diagrams wrong?

*4) if so, what would be recommended?
ps.: sorry if my english is not the best, its not my native language

 

[alex-z]

The impact of bass traps depends on their placement in your room and their absorption curve.

The strongest room modes are axial modes which existing between 2 flat surfaces, like floor/ceiling, left/right side wall, and front/back wall.

Corner traps are going to have minimal impact on your axial room modes, but will improve the tangential and oblique modes, which is why your overall response is smoother. They also make bass energy decay quicker, so you will see improvements in time domain, not just frequency response.

Using multiple subs is the best way to manage axial modes. By exciting a mode from two different directions, the inherent phase difference cancels out the peak or null that would be caused at the listening/measurement location. The most simple example is dropping 2 stones in water and watching their ripples interact. With the right placement+timing you can enhance or reduce the waves.

https://www.harman.com/documents/multsubs_0.pdf

Using multiple subs + bass traps + EQ you should be able to achieve an extremely smooth bass response.

Rather than downsizing your subwoofer, I would get another identical model. Using small subs you will cost some bass extension, and 4 subs is overkill for most people. 2 good subs will be enough to smooth out your response.

You also don't need an AV receiver with multiple sub outputs. Subs can share a single output, unless you want to apply EQ + delay to each sub individually. If you do want such features, you can achieve that with a miniDSP 2x4HD, much less expensive than a new AV receiver.

 

[abdo123]

Corner traps are going to have minimal impact on your axial room modes,

Why?

 

[slacki]

alex-z: just to be sure i understood you correct: i should stay with the bass traps and buy another gigantic arendal sub2?
My problem is, that i can locate the sub when it is behind me and when i position one between FL and C (guess best location when i check REW), i need to have one behind me right in the corner where the bass trap is, or move it further to me (than i can locate it easily). I tested that yesterday when i was moving it around behind me, guess it is just too close. Its also a bit annoying when the sub is so hardcore heavy (e.g. when i move to another city).
And the next thing is, it eats a lot of space. I like it when its "free" in the room, know what i mean? Thats why i thought i should replace the arendal with 4 small ones and place each one in a corner - then i could get rid of the gigantic bass traps and have a better bass. And have no problem when i move the city in a couple of years.
I do not buy a new AVR only because of the 4 subs (my SR7011 can handle two), i also would like to get newest features (HDMI2.1, trying out Dirac, etc.), the 4 sub out are just an advantage that marantz/denon use now. I really like the Arendal Sub2, its just too big :/ And getting a new one might be problematic, they are not sold anymore. Guess the best would be to switch to something like KEF KC62 or SVS 3000 Micro, then its no problem to get 2 or 4.
Placement in the middle of the right/left side is unfortunately not possible. On the right side it would work, but on the left is the couch. And not sure how it is with locating the sub then.

 

[Webninja]

How did you build those traps? From the little I have researched, you can’t just use any foam

 

[alex-z]

Why?

Because the pressure differential exists across the entire surface. You would need to treat a substantial amount of the surface area to significantly impact axial modes. OP has bass traps which cover 15, maybe 20% of the "rear" wall. It will certainly have some impact, but is mostly treating the tangential and oblique modes.

Also, axial modes are lower in frequency, so the effectiveness of porous absorbers goes down.

 

[slacki]

How did you build those traps? From the little I have researched, you can’t just use any foam

i used caruso isobond. i told them that i want to build bass absorbers, and they told me which one to use.
should be the same: https://www.don-audio.com/CARUSO-ISO-BONDZ-WLG-040-100mm-Schallabsorber-Akustikvlies
each "box" (4 in total) contains 5 of those, i just cut about 10 cm off, because one box is 50x50x120cm

edit: sorry for delayed answers, my posts must be allowed by moderators - not sure why

 

[abdo123]

Because the pressure differential exists across the entire surface. You would need to treat a substantial amount of the surface area to significantly impact axial modes. OP has bass traps which cover 15, maybe 20% of the "rear" wall. It will certainly have some impact, but is mostly treating the tangential and oblique modes.

Also, axial modes are lower in frequency, so the effectiveness of porous absorbers goes down.

but you're also forgetting that putting the traps in the corner is basically coupling the trap to the air particles of the entire room (similar to how a horn works) which increases their efficiency dramatically in bringing down the reverberation time across a wide spectrum.

That's the main problem (that is not really solveable by EQ or multi subs) in most small room acoustics.

 

[sigbergaudio]

So your first graph (with all traps) clearly looks best (or should I say most EQable). But your preference here is to remove all the traps and you want advice on what to do to achieve good bass without them? Or?

And do you not have any EQ? Is this without Audyssey activated in your Marantz? How does it look after Audyssey?

 

[sigbergaudio]

tbh: i dont understand it
I thought a bass trap "eats" peaks to prevent room modes, but this looks more like it makes a better curve in general and improves drops. Without the left bass trap i have a drop in 60, 70, 85, 92 and 100hz.

I would say your result (eating dips not peaks) is to be expected. And also mostly fine, because the peaks can be reduced by EQ.

EDIT: You also don't need to replace the Marantz to handle more subs. Even though it only have two physical outputs, you can split the output and connect more subs, and it will still be able to handle that and EQ that.

 

[AdamG]

edit: sorry for delayed answers, my posts must be allowed by moderators - not sure why

As a new member your first 10 or so posts must be manually approved by the Moderator Team. It’s a Spam precaution and we wish it was not necessary just like you do. It does save us from spammers dropping hundreds of posts in the blink of an eye. Then we have to go clean up the mess.

Welcome Aboard @slacki !

 

[slacki]

@AdamG247 i fully understand that, thanks for clarification!

@sigbergaudio correct, i would love to have a bit more "room" available to me. The room is a bit overwhelming with all that stuff inside, so i thought maybe i can replace the bass traps somehow. And also the gigantic Arendal. I mean i really love that sub. And if i somehow could remove the bass traps, it would be fine for me to buy another one. But i guess (i am a noob when it comes to audio, so maybe i am wrong) it would be more reasonable to go with 4 subs in each corner that are smaller (also because the arendal sub2 is not available anymore, so i must use another one anyways).
The measurements now are without any room correction applied. This is directly from: laptop -> REW -> Scarlett 2i2 USB -> (unbalanced) Cinch L -> Arendal Sub2 (XLR)
I did not measure it with Audyssey enabled. Should i try to measure that? Thought best would be to improve the room without any EQ and then use EQ in the last step. Currently i am looking for the "best" position for that one single sub. Why i dont like the traps/big stuff: now i have the problem that i can not get the second box up to the other (stacked). I was able to get it down but now, because its just too heavy, i dont get it up
Need to call now a friend that he comes to me and helps me. Its just a bit annoying with those big and heavy things.

This is from the Marantz App:

 

[sigbergaudio]

@slacki Right.

The reason I asked for a version with active EQ is that your first graph doesn't really show much problems from 20-100hz, so it would be interesting to see an EQd version of that. But if you're not going to keep the traps I guess we need to start with your last graph instead.

Also if I understand you correctly you now have a single sub? Then I would start with replacing it with two subs, and test that before considering if you need 3 or 4.

What is your crossover point? So how high will the subwoofers play? 80-100hz? (it would also be interesting to see how the speakers play in the room to help you decide that)

If we assume something like this is your target:

Then your problem areas without any bass traps is 60-70hz and possibly 80-100hz. It may also be that your speakers will fill out that dip if you cross over at 80-90hz. In that case you're left with only 60-70hz as a problem. It's very possible that can be remedied by adding just one more sub.

 

[slacki]

Yes i only have one Arendal Sub2 (thats the naming, it does not mean i have 2 subs ).
The thing is, the graph with those dips is only with the left side removed. This is how it looks right now:

I only removed the left two bass traps, not the right one. I guess, when i remove the right ones too, the graph will get much worse. But its too heavy and i have problems with my back, so i can not remove them alone again, need some help from a friend (getting them into my sleeping room and then back).

The crossover was 100hz for FL/FR/RR/RL and 120 for the heights. I just changed it to 80 for all (for testing)

 

[sarumbear]

i used caruso isobond. i told them that i want to build bass absorbers, and they told me which one to use.
should be the same: https://www.don-audio.com/CARUSO-ISO-BONDZ-WLG-040-100mm-Schallabsorber-Akustikvlies
each "box" (4 in total) contains 5 of those, i just cut about 10 cm off, because one box is 50x50x120cm

edit: sorry for delayed answers, my posts must be allowed by moderators - not sure why

They do not do work below 150Hz or so due to their low depth. They are not bass traps. They are wide range absorbers. Incidentally the link you send calls them just that. I’m afraid they will not make much difference within the range you are interested.

 

[slacki]

They do not do work below 150Hz or so due to their low depth

Thanks but i have 5 times those in one box. So they are 50cm deep.

Or is it because of the material, that they should not work below 150hz? If so, whats the reason why the flatten the curve between 50-100hz? Because the wall is overcast?

 

[sigbergaudio]

@slacki The marantz app shows a calculated "after", it doesn't actually measure, so you need to check with REW too. I suspect it won't change much below 100hz removing those last ones as Sarumbear is hinting at.

EDIT: I see now you have a pretty thick version. Perhaps then, you won't know until you remove and measure again

Also, one thing is the response, another thing is the decay, which you can see in the "waterfall" tab. It would be interesting to see if much has changed there from the first to the last graph.

 

[sarumbear]

Thanks but i have 5 times those in one box. So they are 50cm deep.
View attachment 243110
Or is it because of the material, that they should not work below 150hz? If so, whats the reason why the flatten the curve between 50-100hz? Because the wall is overcast?

Without any spec of your DIY absorbers I’m afraid I cannot comment. When there’s no data there’s no design. Luck comes to the forte.

Your best bet is to place it at different locations and see the reverberation waterfalls. They will show you the best location.

 

[slacki]

when i remove the traps on the left side, i have many dips between 50 and 100hz (see "komplett ohne trap links"). when i move the traps back and measure (notice: with REW, not audyssey app), the dips are gone (see the first measurement). so i am wondering if i measured wrong. because when sarumbear is right and they do not do anything with the signal <100hz, then... something is wrong or something else influences the room what gets fixed with those absorbers. i could create some more measurements when i ask my colleague to help me. So i will create one measurement without the traps (completely removed), then i move the left traps back, measure again. and then the right traps back, measure again. will this help in any way (the measurements) to get more information/an idea what is going on and if they are required or not?

 

[slacki]

@sarumbear unfortunately, this is in german, but he also improves his lower frequencies (<100hz) with caruso isobond: https://recording.de/threads/bassbehandlung-mit-poroesen-absorbern.142725/
Also, another one with a nice graph comparison: https://recording.de/threads/caruso-isobond-wlg-40-vs-35.214320/#post-2513161
The green one is without Caruso Isobond, the pink is with. Are you sure that it does not affect <100hz when they are stacked?

the colleague is here in 1h30min, then we measure.