Listening seat against rear-wall - How bad is it? How to reduce inherent problems? - diyAudio
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Old 13th September 2012, 11:25 AM   #1
keyser is offline keyser  Netherlands
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Default Listening seat against rear-wall - How bad is it? How to reduce inherent problems?

Under ideal circumstances most kinds of speakers are placed well into the room, free from reflecting objects. The same applies to the listening position. However, in the real world couches are often placed right against the wall. If the listening position is so close to the rear-wall, you'll get a very strong and very early reflection. For low frequencies being seated so close to a wall actually has some advantages (fewer and less pronounced nulls, boundary gain), but for mid and high frequencies I only see problems.

How bad is this reflection? Might it in some way be of advantage?

If you wanted to get rid of it, how would you do it? Because of the almost perpendicular incidence, a damping panel would have to be very thick to damp reflections to below the Schroeder-frequency.
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Old 13th September 2012, 11:37 AM   #2
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Panel mounted few inches off the wall can be 1" thick and have significant effect, acting like much thicker material.
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Old 13th September 2012, 12:04 PM   #3
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I used to listen like that keyser, with the speakers 1.5 to 2m in front, and i agree with your observations. Hanging a rug behind me helped with the imaging problems, whilst the bass remained smooth. Moving forward just 50cm brought in the room modes and lumpy bass.
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Old 13th September 2012, 12:15 PM   #4
keyser is offline keyser  Netherlands
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Buzzforb,
that is true, but if you wanted to damp down to the Schroeder-frequency (say 200 hz in a typical room), then you'd need a total thickness of (343/(200*4) = ) about 43 cm .
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Old 13th September 2012, 12:41 PM   #5
keyser is offline keyser  Netherlands
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Recently I moved and in the new living room a setup with the listening seat against the wall is the only logical option. First I tried my old 10 cm thick damping panels behind the sofa, but although things improved, I wasn't totally happy with it. Since a couple of weeks I've got a big damping panel behind me, measuring 2m40 by 1m20 and 20 cm thick. I wish I could've made it even thicker, but this is as far as the girlfriend allowed me to go .

Measurements indicate that above about 500 hz it works almost as a black hole. Below that, its effectiveness diminishes. All in all I think it's a big improvement. Imaging is now much sharper and I experience less listening fatigue. However I do experience an effect in the low midrange I don't know how other to describe than 'phasyness'. It increases the perception of envelopment (which I think is generally good), but on some material it sounds a bit peculiar. It is not audible all the time and to be honest it's just a minor quibble, but still.

The bass might be the best I've ever had. As before (in my dedicated audio room) I'm using multiple subwoofers and EQ, but it was now a lot less difficult to get the bass very flat. Even without EQ I managed to get the bass within +/- 5 dB between 20 hz and 100 hz - so no significant dips and just one peak worth a mention.

So I actually think my 'problem' is pretty much solved. My solution however doesn't seem to be one for everyone. The positioning of speakers is discussed a lot on these forums and also in user manuals of speakers and such, but it seems a lot less discussion is dedicated to the listening position. Many (especially lay-) people have a couch against the wall. What might be a more room friendly solution to fit their needs?
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Old 13th September 2012, 01:33 PM   #6
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Quote:
Originally Posted by keyser View Post
However I do experience an effect in the low midrange I don't know how other to describe than 'phasyness'. It increases the perception of envelopment (which I think is generally good), but on some material it sounds a bit peculiar. It is not audible all the time and to be honest it's just a minor quibble, but still.
Do you think it's a room resonance?
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Old 13th September 2012, 01:50 PM   #7
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i just tried my memory foam mattress out! I get the phasey comment, which is probably an artifact in stereo mic seperation being large in the recording room, and large crossover between channels at the recording stage.

With rock pop band type recordings i didnt notice the same, except on some vocal FX.
I would say that its there, but this rear reflection masks it. Removing the reflection reveals the phasey-ness.

Id guess you could be hearing lateral modes and relections, and their interèrance to direct radiation.

I would like to try what i used to hear recommended for inside vented boxes. An absorber on one wall of each plane. Got carpet? Thats one down. Rear absorber? Yep! I just need a lateral panel at the far end of the room and then to listen. Perhaps using another panel laterally would help?
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Old 13th September 2012, 02:29 PM   #8
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FWIW,
Putting free space behind you old 10cm panel may make a bigger difference than you think. Great place to put big piece of art
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Old 14th September 2012, 02:11 AM   #9
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it is easy to treat higher frequencies with 2"-4" of owens corning 703 type insulation placed, preferably a couple of inches away from your back wall. this will not help with your lower frequencies though. dealing with low frequencies needs space, money and expertise.

my suggestion is, 'get lucky'. i sit against the back wall in my tv room, and it sounds just fine, no special treatments

Last edited by audiothings; 14th September 2012 at 02:18 AM.
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Old 14th September 2012, 05:44 AM   #10
breez is offline breez  Finland
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Absorber works, just make it thick enough and choose the right material. I recently moved to a new place and had to place sofa close to the rear wall. I constructed a ~100 mm thick absorber using rockwool (mineral fibers) and modeling shows it is effective down to low hundreds. There is an excel sheet available somewhere online and also this online calculator at Porous Absorber Calculator

Here's a some modeled results for 100 mm thick absorber placed at a wall using different flow resistivities. My panels use material with 12650 rayls. Around 10000 seems to work well for a broadband absorber of this thickness.

Porous Absorber Calculator - Results
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