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Rear wall reflection absorption

garbz

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I see plenty of discussion on front / side walls on here but I'm seeking advice on rear wall treatment.

My living room is only about ~4m across. I use curtains on the large window on to me left, and the right is completely open into the rest of the room. Rear wall... The couch is right up against it meaning my head is only about 10-20cm from the wall and it is practically impossible to move my listening position away from it.

I'm looking for treatment advice. Without any treatment high frequencies sound shrill. Indeed if someone claps in the room you can hear a slight echo, so I started listening with a pillow behind me which was a pain in the rear.

Now I'm looking for something better, I've a 70mm rockwool panel in a 90mm frame (20mm gap to rear wall so the rockwool is not flush) but now I'm struggling with it sounding dead. By that I mean now the echo is gone, but also rather than being engulfed in sound I'm quite clearly hearing the speakers (I think this is what is described as a "dead room"?)

What are the recommendations? Is rockwool going overboard? Do I use thinner rockwool or should I be using a different material? Any advice on what to specifically test for as well? (have REW + miniDSP measurement mic)
 
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garbz

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increase the depth
What would that accomplish in the context of my problem? I'm currently suspecting that the damping of frequencies of interest is already excessively high. I would suspect thinner panels or panels with a lighter material may actually be more relevant to what I'm trying to accomplish...

Also livingroom, 200mm thick panels are definitely not an option. Happy to consider that in the study where the missus doesn't have a say :)
 

Bjorn

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Broadband absorption with low density material with the most sound transparent material is you best option behind your head. Broadband=highly effective down to Schroeder.
 

abdo123

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What would that accomplish in the context of my problem?
Well right now your absorber (blue) is sucking out all the high frequencies while doing nothing to the lower frequencies. that's why you're experiencing the tonality shift you're describing. And the uncomfortableness when it comes to speech and stuff.

Getting thicker absorber should provide a much less steep slope when it comes to absorption vs frequency and as a result the problem would not be present (or not as bad as it is right now) as the entire spectrum is more or less or absorbed.

try to use the lowest density rockwool your local shop sells btw.

1648561235073.png
 
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garbz

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I don't follow. Not sure if I don't understand the principle or if I wasn't clear in my description (or both).
Tonality wise what I hear is quite fine. My speakers are overly bright (B&Ws) and the addition of acoustic absorbers have made the actually tonality quite pleasant. I actually used to shelve them down via EQ before I started working on room treatment.

What I'm finding strange is the complete lack of echo making it sound less like I'm hearing music in a room and more like I'm hearing two speakers.

Your low density comment sounds more intriguing. I didn't realise rockwoll comes in different densities, thought it was just different thicknesses. I'll see if I can find something lighter to play with.

Broadband absorption with low density material with the most sound transparent material is you best option behind your head.
Do you have a recommendation for material choice? Not sure I follow the link between broadband absorption and sound transparent, aren't they at odds with each other?
 

abdo123

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What I'm finding strange is the complete lack of echo making it sound less like I'm hearing music in a room and more like I'm hearing two speakers.

Using thin acoustic treatment only creates an artificial acoustic environment.

What you have right now is not really outdoors (no reflections across the whole frequency sepctrum) or a reverberant indoors space (plenty of reflections across the entire frequency spectrum). Your space basically has alot of reflections below 1KHz but no reflections at all above 1Khz.

This creates unpleasantness because we never experience in our daily life an environment like this. it feels foreign.

Using thicker absorption would make the experience a bit more gentler (closer to outdoors or a really good pair of headphones) as you remove reflections across the entire spectrum not just one part of it.
 

CinDyment

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A middle ground solution not as good 200mm with a 100mm gap. 140mm was chosen as it is a standard lumber width and insulation thickness. One of my friends was in a similar position to you and got creative as he didn't want to loose too much space but didn't want to move a wall either. He removed the drywall in the listening room for added depth, and doubled it on the other side for stiffness. That became the air gap for his absorber. Seemed like a lot of work to gain <100mm, but it was important to him. Personally I think he just wanted to try it and see if it would work :)

1648645060566.png
 
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garbz

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What you have right now is not really outdoors (no reflections across the whole frequency sepctrum) or a reverberant indoors space (plenty of reflections across the entire frequency spectrum).
Now I understand. Thanks.

In lieu of thick panels which are simply not an option in the living room, I'll look for something less dense which doesn't absorb as much and see if I can find some balance between reducing echo and making the sound unpleasant.
 

Bjorn

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RPGs Broadsorbor absorps lower in frequency with less thickness. However, it's a bit more reflective in the highs.
BAD Arc is also a product that can work well with fairly close proximities, but it would need to be minimum 6" (15 cm) thick for a good result.
 

abdo123

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Yeah, I happen to sell them.

Do you mind sending me (over a private conversation) the options you have available (thickness, size, colors or finishes) and the prices?

Also do you personally find this graph to be really truthful?

1648749158024.png
 

Bjorn

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Sure.
The graph is based on NRC, which is an average of speech where a frequency area has been left out. The graph is also only valid for rooms with reverberation, something that doesn't exist at all in small rooms. So it's quite misleasing for your usage, which is also the case with most such data.

Below is an experiment with two Broadsorbors with dimension 150x80x10.4 cm in a small room. Placed on each side wall behind the speakers.

Before placement (1/24 oct smoothing):
Frekvensrespons før og uten sidetiltak.jpg


After placement of two Broadsorbors:
Frekvensrespons med to stk Broads 150x80x10cm (1).jpg


Waterfall before:
Vannfall før og uten sidetiltak (1).jpg


Waterfall after:
Vannfall med to stk Broads 150x80x10cm (1).jpg


You'll notice that the dip around 120 Hz was reduced considerably. So there at least in the experiment, great effect down to 120 Hz area with two Broadsorbor panels with dimensions 150x80x10.4 cm was achieved.

As previously mentioned, these are more reflective in the highs. So with placement right behind the head, you'll end up with some high gain specular reflections above a certain frequency. With BAD Arc, that's actually something you'll avoid but on the hand the frequencies above 800 hz will be partially diffused and the result will be different.
 

abdo123

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As previously mentioned, these are more reflective in the highs. So with placement right behind the head, you'll end up with some high gain specular reflections above a certain frequency.

Would you consider that a deal breaker? because the way you previously mentioned it it sounded like a good thing.

Also i can also see some improvement in the 300-350Hz region. What percentage of the wall did the two panels cover?
 

DVDdoug

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By that I mean now the echo is gone, but also rather than being engulfed in sound I'm quite clearly hearing the speakers (I think this is what is described as a "dead room"?)
Maybe just a smaller panel? Or maybe try something else like a blanket or a rug?
 

Bjorn

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Would you consider that a deal breaker? because the way you previously mentioned it it sounded like a good thing.

Also i can also see some improvement in the 300-350Hz region. What percentage of the wall did the two panels cover?
Not a deal breaker, but not ideal if accuracy is a goal. Personally I would use something less reflective of highs behind the head and change the other treatment to something more broadband and perhaps some with diffusion. While broadband treatment is important, only using absorption will in itself cause a fairly dry result with no spaciousness and little envelopment. For a trully great result and avoid bandlimited treatment one should always start with low freq. treatment, but that requires treating more surface area.

Definetly very low percentage, I don't know the number but the percentage wouldn't be transferable to another room though.
 

Thomas_A

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I see plenty of discussion on front / side walls on here but I'm seeking advice on rear wall treatment.

My living room is only about ~4m across. I use curtains on the large window on to me left, and the right is completely open into the rest of the room. Rear wall... The couch is right up against it meaning my head is only about 10-20cm from the wall and it is practically impossible to move my listening position away from it.

I'm looking for treatment advice. Without any treatment high frequencies sound shrill. Indeed if someone claps in the room you can hear a slight echo, so I started listening with a pillow behind me which was a pain in the rear.

Now I'm looking for something better, I've a 70mm rockwool panel in a 90mm frame (20mm gap to rear wall so the rockwool is not flush) but now I'm struggling with it sounding dead. By that I mean now the echo is gone, but also rather than being engulfed in sound I'm quite clearly hearing the speakers (I think this is what is described as a "dead room"?)

What are the recommendations? Is rockwool going overboard? Do I use thinner rockwool or should I be using a different material? Any advice on what to specifically test for as well? (have REW + miniDSP measurement mic)
Where are the panels located and what are their dimensions?

To get more broadband, you should cover the rock-wool filled frames with perforated masonry, and of top of that 60 mm wedged foam. Place them at the wall behind the speakers, leave the middle as is (or skip the foam in the middle).

Leave the wall behind the sofa as is but get som soft stuffed head/neck-rests.
 
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garbz

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Where are the panels located and what are their dimensions?
Just rear wall at this stage. Standard Rockwool 100x60x7 with a ~2-3cm gap behind the head. The rest of the room is actually somewhat okay. I've experimented with front wall absorption but that doesn't resolve the stranger echos.

Soft stuffed headrests again sounds like I should just get a slightly less absorbing material than rockwool.
 
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