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Thread: Wall behind speakers amount of reflexion

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    Default Wall behind speakers amount of reflexion

    I am considering to switch from ordinary loudspeakers to dipole ones, and would need your help to clarify some questions on the matter.

    It is my understanding that a dipole speaker needs a specific amount of reflecion from the back wall behind the speaker.

    I am not sure if this back wall behind the dipole speaker should rather be absorbing or reflecting or a mix of the two?

    Does there exist a precise physical measure ( oder: dimension) that helps to determine the optimal degree of reflection?

    Currently the back wall behind my speakers consists of book shelves. Is it possible to find out if this book shelf is acting more as an absorber or more as a difusor? Some people say that bookshelfs act more as absorber some say that they act more as difusors. So I am kind of confused. Can someone please explain how this can be done?

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    Forum Administrator twich54's Avatar
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    Itill, first off... Welcome to our fold !

    Your space behind (bookshelves) represent a very good diffusor (assuming that the proper items are on the shelf !). Books and other soft objects at various depths help in this.

    As far as distance forward from your shelves for speaker placement, 1 1/2 - 2 meters would be ideal.

    As for the comment that book shelves act as an absorber ......utter nonsense, for NO hard surface could act that way. 'If' the selves were filled with acoustical absorbing material that would be the only way.
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    you do NOT want the rear wave reflecting off anything and interfering with the sound waves coming out of the front of the panel.
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    Quote Originally Posted by TomDac View Post
    you do NOT want the rear wave reflecting off anything and interfering with the sound waves coming out of the front of the panel.
    TomDac, thank you for the reply. I am not sure I understand your point. It was my understanding until now that reflecting against the back wall was exactly the thing that makes dipoles different.

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    Quote Originally Posted by twich54 View Post
    Itill, first off... Welcome to our fold !

    Your space behind (bookshelves) represent a very good diffusor (assuming that the proper items are on the shelf !). Books and other soft objects at various depths help in this.

    As far as distance forward from your shelves for speaker placement, 1 1/2 - 2 meters would be ideal.

    As for the comment that book shelves act as an absorber ......utter nonsense, for NO hard surface could act that way. 'If' the selves were filled with acoustical absorbing material that would be the only way.
    Thank you, twich54.
    Until now my understanding was that ( for monopol loudspeakers) you would need absorbers behind the speaker and diffusors behind the listener (LEDE Concept).Is my understanding of your comment correct that you recommend to have only diffusors behind the dipole speakers?

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    Forum Administrator twich54's Avatar
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    Quote Originally Posted by ltill View Post
    Thank you, twich54.
    Until now my understanding was that ( for monopol loudspeakers) you would need absorbers behind the speaker and diffusors behind the listener (LEDE Concept).Is my understanding of your comment correct that you recommend to have only diffusors behind the dipole speakers?
    here's the 'General Rule' ....... absorption is only used when the distance is too small between the speakers and the wall behind them. Given your book shelves and set up that I suggested along with the distance I recommended NO absorption will be needed.

    I've used di-pole speakers since the early eighties up until my recent change to my current Revel's and I've always made it sure that they had enough room to 'breath'. I have never seen or heard di-poles that were set up other than intended that performed to their best. Many on here will tell you of the options for less than ideal set-up can be had and I will tell you it's a compromise !
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    Thank you very much indeed. I will certainly give it a try. Happy New Year!

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    Quote Originally Posted by ltill View Post
    TomDac, thank you for the reply. I am not sure I understand your point. It was my understanding until now that reflecting against the back wall was exactly the thing that makes dipoles different.

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    The rear wave needs to be absorbed or diffused so it cannot interfere with the front wave... The rear wave is NOT part of the soundstage. it's an inherent property of dipoles that must be dealt with..

    You can experiment with this by moving your speakers close to the wall and then have a listen in the sweet spot. Now move the speakers out into the room, away from the rear wall about 2 feet. Do they sound better or worse? They should sound better.

    your ultimate goal is to make your speakers disappear when you are sitting in the sweet spot, so you can "see" the soundstage (with your ears)...
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    On that note since I had hardwood floors installed and the 65" TV sitting on a console with felt pads that sits between my speakers is now easy to slide around, I found that the sound stage was absolutely at its best when I pull the TV and console even with panels. Granted my wife DOES NOT like how it looks in our den when I do that, but the A/B on it was very noticeable. The sound stage was much sharper and the music sounded cleaner.

    I'm pretty sure the this is blocking a lot of what bounces off the front wall. I believe the same would happen if you move the speakers far forward of the front wall so that the sound bouncing off of the front wall had more time to decay. I experimented with putting sound deadeners behind the front panels and had little luck.
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    Itill, for a stereo pair of dipoles, putting them 1.5m out from your bookshelves, and ensuring the shelves have books on them, staggered in depth, will provide some good diffusion that 'spreads out' the rear wave in way that minimizes strong, correlated reflected comb-filtering of the front wave.

    The reflected (and in the above example, diffused) rear wave is often considered a nice 'effect' that produces a wide soundstage, albeit limited to a very narrow 'sweet spot' where the font wave dominates.

    Personally, I recommend and use absorption, as killing the high-frequencies results in less comb-filtering and helps with mid-bass cancellation.
    Jonathan

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    Senior Member MPS's Avatar
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    I have never liked absorption behind panels. Maybe in case having much less than 1m to front wall it might help but in that case I would look other than dipole speakers.
    However combination of absorption and diffusion can work wonderfully, only need careful set up to balance those. Everything will depend on room acoustics so every case is somewhat unique.
    Luckily absorption materials are very cheap (think about rockwool etc) and diffusion panels made from EPS are also light and easy to install.

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    Another option for reducing the back wave "smearing" effect on soundstage/imaging (without absorption or diffusion), is to place your speakers "catty corner" (i.e. straddling a corner) so the reflected rear wave is directed ~45 from the front. That's what Roger Sanders (Sanders Sound Systems) does with his demo's. It works well, but you need a very specific room configuration to do it.
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    I have found that if the speakers are placed such that the speaker panels are 1 meter or less from the rear wall, absorption is the best solution. But if they are placed 1.5 meters or more from the rear wall, diffusion is best.
    Rich

    This comment is intended solely for educational purposes and should not be construed as conveying any express or implied warranty of fitness for any other purpose. Said comment constitutes merely the humble opinion of its maker and does not reflect the views of the MLOC or of ML, Ltd. YMMV. Trust your own ears.

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    Hi, thank you all for your comments so far. Next week I will get my Martin Logan's delivered and will then try the different suggestions made. In the meantime I am happy about further comments should you have another suggestion or idea.

    Happy New Year to everybody!

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    Hi, I found GIK absorbers made my sound behind Logans lifeless. I took them off. The diffusers made the sound weird. I had light absorbers behind my listening position which worked fine. Reflecting the rear way to create a pseudo soundstage was relatively a much better alternative.

    W.r.t diffusers, one of the best rooms I have been in, with Apogee Grands, has diffusers, but the owner, who is also the leading restorer of Apogees, advises that you need to sit 5m from diffusers to reduce comb filtering. I haven’t tried, but he has a 40ft. room.

    Best is to initially keep the wall bare, and then but absorbers and diffusers in the used market, see if you like them, or sell them off, you won’t lose much

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