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Acoustic treatment for rear wall and TV

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vinylmad View Drop Down
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    Posted: 06 Jan 2021 at 10:31pm
Having read with interest previous threads here on room treatment, it's something I've toyed with the idea of but never actually put into practice.  This is mainly because the treatment supplier I approached for advice recommended so much treatment that it would have made my smallish listening space feel, well, quite uninviting.

If I've got this right, accepted wisdom indicates that, if creating a listening room from scratch, things should be tackled in the order of: regular shape, dimensional ratios, listening position, speaker positioning (with symmetry in mind), followed by acoustic treatment, initially for bass with other frequencies treated later.  Well, that's me screwed - on many fronts Ouch.

I'm shoehorned into the lounge end (3.2m x 3.5m) of a small, L-shape lounge-diner, with speakers 1.9m apart along the main wall, a tv between them (my setup is a combined 2.1/5.1.4), and with my sweet-spot listening chair 2.3m from the speakers and 0.7m from the rear wall.  To my left is a return wall and the dining space, while to my right is a bay window area.

If there is a plus side it could be that this is almost a near-field setup, so side first reflection points may not be a major concern.  However, if I turn side-on to the speakers while music is playing there seems to be an almost equal volume of sound hitting my ear facing the rear wall as my other ear pointing at the speakers.  This can't be good, even though I do perceive a wide stereo field (as with Jean-Michel Jarre's "Aero", and especially Roger Waters' "Amused To Death" 2015 re-release), identifiable instrument placement, and a stable central image.  The sound balance tends to be more natural at lower volume, where the treble on certain tracks gets a bit sharp or over-emphasised as the volume is cranked up, and I wondered if the parallel reflecting surfaces of the tv and the wall close behind me could be a major factor.

I read on here somewhere, that treating the rear wall was found to be beneficial, but I'm not sure in which context.  Assuming that I'm not using any other treatment, could it be worth me trying a couple of wideband absorber panels on the rear wall, plus another directly in front of the tv, to tame upper frequencies a tad, or would this be likely to unbalance the sound in worse ways?  One company I've seen offers a pack of three 1200mm x 600mm panels for an amount of money I'd be willing to sink.  Sadly my own fabrication facilities are rather limited.


Edited by vinylmad - 06 Jan 2021 at 11:47pm
Colin G.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jan 2021 at 10:08am
Careful, Colin!

You're bound to hear almost equal volume turning yourself side-on. Sound pressure - in free space - halves per doubling of distance, and 0.7m is nothing compared.

With speakers spaced 1.9m, your "sweet spot" ought to be a bit nearer than 2.3m. Couldn't you try 2m? You'd then be a full 1m away from your rear wall.

Room modes are where specific frequencies like to hang about, and bass concentrates in particular places. If things are too bright, then moving further into a concentrated bass area solves that. If too boomy, moving away from a concentrated bass area solves that, and so on.

To illustrate room modes to yourself, play with https://amcoustics.com/tools/amroc. Be warned that some browsers won't load it.

A flat bare rear wall will reflect, and I find clutter is a lot cheaper than acoustic treatment and tends to sound more natural, being haphazard. I have a desk with shelves above, complete with all sorts of clutter and books. Some people have "wall units," others might have a "sideboard" with a tremendous big floral arrangement on it - that sort of thing breaks up reflections.

In my experience, absorbers work best where most of the energy is, and that's where the speakers stand. Speakers tend to be positioned in corners because that's all we have spare. If one speaker is in a corner, then the other needs to be in a corner. We pull them as far away from the corners as possible, and that's because the bass frequencies depart from the baffle and use the surroundings as the baffle.

Bass-light speakers pushed back might make use of these free baffle surroundings. Sometimes, pulling the speakers out from the walls might place them in a room mode where the bass is concentrated, making them "one-note."

Corner absorbers can be a blessing or a curse. In my room, the upper corners needed wideband absorption. The careful placement of homemade BBC specification absorbers helped transform the acoustics to "control room" quality. But, you see, my room is a workshop which is no part of domestic bliss.
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote vinylmad Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan 2021 at 7:31pm
Thanks for your insights, Graham!

I've encountered that room mode calculator tool while following "Acoustics Insider" on Youtube - for casual interest only, as it focusses on acoustic treatment techniques for home studios rather than for hi-fi listening rooms, though it is presented using language that I can comprehend!  I think I have a basic understanding of room modes, but my main issue with being able to use such a tool is that my room shape is an 'L', essentially two rectangles, which doesn't lend itself to straightforward calculation.

It would be great if I had more freedom to move stuff around, for trying different speaker and listening positions, but the room is rather compact and the floor space well occupied with furniture and stuff.  I've managed to keep the speaker fronts 0.5m out from the front wall and 0.9m from the nearest corner.  With my ears 0.7m from the rear wall it helps to avoid some of the worst bass issues, and another 0.3m might help further in that regard, but the chair would then become a physical obstacle.  As would the speakers if brought further into the room.  The whole thing must be - and probably is - a nightmare for hi-fi audio, but I manage to mostly put up with it, up to a certain volume: 10 - 1 o'clock, depending on the material.

Conditions might have been more promising if the listening position had been oriented towards speakers at the bay window end, but as they also function in my HT setup it would have meant squinting at the tv with a large, bright window behind, which is a pet hate of mine even when just watching the news.  Also the bay is offset, so symmetry would still have been absent.

My idea of trying absorber panels on the rear wall - not thick ones for bass trapping but slimmer (60 - 70mm) for wideband absorption - was to avoid (ever more) items which would project into the room as a piece of furniture might.  As you helpfully imply, simple shelving for odds 'n' ends would work out cheaper, and I hadn't considered that it also might "sound" better than panels; so I appreciate the tip.

Here's hoping I can afford to move home in a few years, to somewhere with more potential for optimising a listening room!


Edited by vinylmad - 08 Jan 2021 at 7:35pm
Colin G.

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jan 2021 at 5:55pm
This is an interesting topic as much as important. Audiophile HIFI are auditioned at dedicated showroom and my domestic environment has 4 walls every where. I have used '' foot stools and pouff foot rest to good effect and curtails and even bamboo blinds on side walls and in process to redesign ceiling decoration with different size plaster board and thin pine strip covering and pine to shape the ceiling corners. 

The 'Albert Hall'' in Kensington London and the refurbishment of the the South Bank two music theatres provided much inspiration. It seems that book shelves absorb the energy well and corners '' cats eyes'' needs some work with acoustic foam and textile but after spending fortune for HIFI equipment i will not allow poor acoustic environment spoil the enjoyment , wife permission of course.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kgilroy Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jan 2021 at 4:22pm
Originally posted by vinylmad vinylmad wrote:

Thanks for your insights, Graham!

I've encountered that room mode calculator tool while following "Acoustics Insider" on Youtube - for casual interest only, as it focusses on acoustic treatment techniques for home studios rather than for hi-fi listening rooms, though it is presented using language that I can comprehend!  I think I have a basic understanding of room modes, but my main issue with being able to use such a tool is that my room shape is an 'L', essentially two rectangles, which doesn't lend itself to straightforward calculation.

It would be great if I had more freedom to move stuff around, for trying different speaker and listening positions, but the room is rather compact and the floor space well occupied with furniture and stuff.  I've managed to keep the speaker fronts 0.5m out from the front wall and 0.9m from the nearest corner.  With my ears 0.7m from the rear wall it helps to avoid some of the worst bass issues, and another 0.3m might help further in that regard, but the chair would then become a physical obstacle.  As would the speakers if brought further into the room.  The whole thing must be - and probably is - a nightmare for hi-fi audio, but I manage to mostly put up with it, up to a certain volume: 10 - 1 o'clock, depending on the material.

Conditions might have been more promising if the listening position had been oriented towards speakers at the bay window end, but as they also function in my HT setup it would have meant squinting at the tv with a large, bright window behind, which is a pet hate of mine even when just watching the news.  Also the bay is offset, so symmetry would still have been absent.

My idea of trying absorber panels on the rear wall - not thick ones for bass trapping but slimmer (60 - 70mm) for wideband absorption - was to avoid (ever more) items which would project into the room as a piece of furniture might.  As you helpfully imply, simple shelving for odds 'n' ends would work out cheaper, and I hadn't considered that it also might "sound" better than panels; so I appreciate the tip.

Here's hoping I can afford to move home in a few years, to somewhere with more potential for optimising a listening room!

This is a useful website that discusses acoustic treatment in addition to selling it.  They will actually give you free advice specific to your room.  I have done that before even though I ended up buying a competitors products:  


The challenge with acoustic treatments is that you tend to need a lot of it to get where you want to be but you can certainly start with a little and add over time.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jan 2021 at 10:11pm
Hi Colin, I also found GIK really helpful. FWIW I did use some diffusion as well as wide band absorption/bass traps. I have some of GIK's quadratic diffusers in key locations including on the rear wall behind my listening position. It's to stop slapback or a central rear image forming, for bass frequencies enough distance from the rear wall is important as already noted above.

Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote vinylmad Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 1:30am
Jon, did you arrive at the locations of your quadratic diffusers by trial and error, and how far behind the listening position are they?  Is the effect without them noticeable?

Thanks for the suggestions to try GIK.  I did reach out to them a few years ago, sending them a 3D model of my room (created in SketchUp) with measurements.  Sadly I'd woefully underestimated the amount of treatment they would suggest as being necessary to give any real benefit in my small room.  Their "solution" would have been impractical given the amount of existing furniture and other contents, and also the visual impact would have been overwhelming, so it changed my opinion about this being a worthwhile project.  I also came away with the impression that it wouldn't be worth doing less than a full treatment; I may have been wrong, but then they are also into sales!

Now I do realise that any room after-treatment is going to be conspicuous, but having less space to begin with means it gets gobbled up more rapidly by bulky treatment.  It would be great to be able to establish at the outset the appropriate types and quantities of treatment sufficient to give an overall improvement, but it seems to be an inexact science for a regular shape room let alone an 'L' shape, once other factors such as the method of room construction, windows, and the room's existing contents have each had an effect.

In revisiting the subject, I'd been wondering if panels not primarily aimed at treating bass frequencies would alleviate some reflection of midrange and treble from the otherwise plain real wall, the tv screen (temporary placement in front of), and maybe also the front wall first reflection points behind the speakers, which might go some way to taming a perceived treble glare above a certain volume level.  This would have to be called a limited experiment and not a precursor to adding further treatment over time - I just don't have the extra space.  If it transpired that more treatment was then needed, e.g. bass trapping, to counter an imbalance introduced by the initial treatment, then I'd fall back and just be grateful for having carpet and furniture.


Edited by vinylmad - Yesterday at 1:31am
Colin G.

Rega RP8/Ania Pro/Aria/Elicit-R, Neat Motive SX2, REL S2, Chord Epic RCA/Speaker; Innuos Zenith Mk.3, Oppo UDP-205, Chord Shawline USB; Solo ULDE (loan), HD600; EtherREGEN; Puritan PSM156..
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