Help with room treatment

[GalZohar]

Hi,

Given my 4.15 long 4.5m wide 2.7m tall listening area, right side is mostly a heavy glass door to balcony but there is still room for a 45cm x 45cm corner absorber on the front-right corner and 30cm x3 0cm+ absorber on the rear right corner. Left is is completely open to dining area and kitchen (with nearest walls between 2m to 4m away from the 4.5m wide listening area).

To reduce rear wall reflections I moved the sofa about 80cm from rear wall (head ~1m from wall), tall sofa about 1m high 2.2m wide (3-seater).

What frequencies can I expect improved and by how much when placing a 45x45cm absorber in the front right corner? Just me sitting on the floor near the corner increased by 92Hz dip by 1-2db... Same for 30cm x 30cm (the width can be more than 3, depth limited to 30 due to glass door to balcony).

What can I expect by using something like a 2m x 2m x 40-50cm absorber on the rear wall? What is the optimal depth of that absorber considering the listening position is ~1m from the rear wall, to absorb reflections but also not sit too close to the reflections... Should I use a larger thickness below head level and thinner absorber above head level?

I also consider instead of using a cabinet for the receiver/streamer/router, to just place them under the TV on top of a 30-35cm tall 40-45cm deep absorber with a solid top. If I place 2nd subwoofer at rear rather than front (still need to measure and check), this absorber can go pretty wide (2m+).

Lastly, how useful is absorption behind speakers? If the back of the speaker is about 30-35cm from the wall, how thick and wide should it be? The speakers are about 2.5m away from each other and there is a 83" TV (~183-185cm wide) in between.

Note seems like my main dips, when placing speaker woofers 70cm from wall, are in the 90-120Hz area, with some problems 80-150Hz. Seems like my front right corner subwoofer is actually pretty smooth up to ~70Hz when placed in corner (maybe due to room being open on 2 sides and some of the walls are clay walls), and 2nd subwoofer seems to complement well for smooth response up to 80Hz if I place it next to the left speaker or next to the hallway at the left-back corner of the listening area/sofa.

The idea is to contact a local contractor to recommend on treatment and pass on the design to a contractor that actually builds everything, but I want to know I'm not being bullshitted and have an idea of what I actually need and also if prices will end up too high, which would be the first things to compromise. Also an advantage to any absorbers that will be easier to move and have highchance to be used when I move to a new apartment, as this is a rental.

Thanks!

 

[MRC01]

A tube trap having 45 cm diameter (about 18 inches) should be effective mainly around 250 Hz with some (though much less) measurable effect down to around 60 Hz. How much of an effect depends on its height, how many of them you have, and where they are placed in the room. Generally, you want them in all corners of the room going floor to ceiling. And the biggest diameter you can fit. If you use only 1 corner then the effect might not be noticeable or measurable.

You mention dips and issues around 100 Hz. Before you dive into room treatment I suggest determining whether these are caused by SBIR (speaker positioning), LBIR (listener positioning), or room modes. That will determine the best initial approach. You can tackle SBIR and LBIR with repositioning. You can tackle room modes with room treatment. EQ can be effective for some kinds of issues (peaks) but not others (dips).

 

[GalZohar]

A tube trap having 45 cm diameter (about 18 inches) should be effective mainly around 250 Hz with some (though much less) measurable effect down to around 60 Hz. How much of an effect depends on its height, how many of them you have, and where they are placed in the room. Generally, you want them in all corners of the room going floor to ceiling. And the biggest diameter you can fit. If you use only 1 corner then the effect might not be noticeable or measurable.

You mention dips and issues around 100 Hz. Before you dive into room treatment I suggest determining whether these are caused by SBIR (speaker positioning), LBIR (listener positioning), or room modes. That will determine the best initial approach. You can tackle SBIR and LBIR with repositioning. You can tackle room modes with room treatment. EQ can be effective for some kinds of issues (peaks) but not others (dips).

The only way to reduce the issues with repositioning is moving the speakers closer to the wall but that causes other issues in higher frequencies. For the listening position any movement within reason doesn't help, the problematic area is too wide around the listening position. Moving so far as to avoid some of those nulls would cause other bigger issues like sitting too close to back wall and too far from the tv, or way too close to the tv, that I didn't even try measure those to see what other nulls appear. Current position is the best I could figure out given the other limitations/frequencies.

As I said, just sitting in the corner raises 92Hz by 1-2db. So shouldn't a 45cm x 45cm do much more than that? Wouldn't a square do more than a tube (more mass/volume)?
Left side is open, so no corners. Right back corner only has 30cm of right wall before the glass door.

 

[ssashton]

With absorbers behind the speakers you may be able to place them closer to the wall without issues in the mid and high frequencies.

It could give you an easy way to test just by buying a couple rolls of loft insulation and putting them in the corners or hanging some behind your sofa. It is ugly and messy but a cheap way to do a listening test.

 

[GalZohar]

With absorbers behind the speakers you may be able to place them closer to the wall without issues in the mid and high frequencies.

It could give you an easy way to test just by buying a couple rolls of loft insulation and putting them in the corners or hanging some behind your sofa. It is ugly and messy but a cheap way to do a listening test.

That is an interesting idea, however to get a noticeable improvement at 90hz I have only 10-12cm between speaker and wall. Also it will increase my distance from speakers - won't that have additional negative effects with reflected sound to direct sound ratio becoming higher? It also brings my right speaker closer to the un treatable glass door and the left speaker closer to the open area, possibly making the speakers sound more different. Won't some modes at that frequency range still cause problems?
Thanks again for the ideas!

 

[ssashton]

I think the situation is too complex to exactly predict. You need experimentation to find what works well. Even with simulation software we are generally looking at modal patterns in the LF and over-all RT60 for the mid and high range.

Trying to predict the subjective result of complex specular and diffuse reflections is not practical.

So spend $100 on some rockwool slabs or insulation roll and experiment. You can always sell it after on the used market. If you worry about dust, at least for LF treatment you can get 'space blanket' roll. Behind your sofa maybe you can hang a king duvet on some poles or washing hanger etc.. You will hear the difference immediately.

After you know the locations for treatment and amount needed you can get custom panels made. This is one I will make soon to go on my ceiling and behind my sofa. It will be 4' x 8'.

Below is another design I like.

P.S. My approach is to use porous broadband absorbers as large as I can fit. But in the low bass range such as 90hz room EQ / DSP can be most effective with minimal drawback. Large wavelengths are the easiest to solve with DSP correction but the most difficult with physical treatment. Use both for the best result.

 

[ssashton]

P.P.S. When considering physical treatment, the focus should be more on decay rate and early reflection arrival, and less on amplitude. You won’t see a big change in dB at 90 Hz even with treatment, but the decay rate can improve significantly—especially in the upper bass—making the sound tighter and less boomy.

 

[GalZohar]

I'm wondering though, I am probably not as good of an absorber as a properly built one, and I see an amplitude change just by physically sitting in the corner (mic at listening position measures 1-2db extra). Wouldn't a more serious absorber placed there show a more noticeable improvement?

 

[ssashton]

Maybe, it's difficult to say without testing your personal absorption coefficient. Can you donate a 30mm diameter sample for the impedance tube test? I wonder how many drunken acoustics students found a 'sample' to fit the impedance tube...

An alternative physical solution - switch to subwoofera around 80-100hz and you can place them in a better position for that frequency range while the main speakers stay at the best mid-high position.