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Thread: Placement options – Impacts of location, orientation and treatments

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    Super User JonFo's Avatar
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    Default Placement options – Impacts of location, orientation and treatments

    We all know that placement has huge effect on the sound of our speakers. Along with the general room layout and treatment, where we put the speaker is vital to the perceived sound and its quality.

    During my SL3XC build, I documented some of the effects of placement and treatment of the wall behind the speaker. So today, I decided to expand on that using the left-over SL3 from my center build.

    So here goes a series of posts on placement, with supporting measurements.

    First, the context. A pic of my SL3, the measurement mic, the VOM and the Variac (variable AC voltage device) used to update results on the panel Voltage vs frequency / SPL thread.

    The laptop is used to do Windows Remote Desktop to the Automation PC in the equipment room; it has the microphone amp, and other Audio I/O for measurements (an M-Audio FireWire 410). I use the latest R+D (essentially ETF v6, from acoustisoft.com)
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    Jonathan

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    All the following measurements were done energizing the Panel only, since for the Variac tests I did not want to have to deal with the output of the woofer as well. Also, for these placement related metrics and discussion, I wanted to separate the bass, as it is VERY placement dependent, and much has been written and discussed about how to place a speaker with a box woofer. I figure it’s more relevant to discuss placement impacts for dipole ESL’s, since that’s what we have

    First up are the Measurements of the SL3 at the usual recommended 3 feet from the wall behind it. It is exactly aligned to the wall behind, with no toe out.

    The wall in this shot is untreated, bare drywall.
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    Jonathan

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    Here is the impulse response of the 2.5’, untreated real wall corresponding to the Freq. response plot above:

    Note the massive spike at 4.2ms and its reversed echo at 16.5ms.
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    Jonathan

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    Note the massive spike at 4.2ms and its reversed echo at 16.5ms.

    These are really bad, as they cause comb filtering as seen in this Bode response of the same measurement:
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    Jonathan

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    Now we move on to a treated surface behind the speaker.

    This is the same distance, but with a treated surface behind it:

    Note the decrease in artifacts at 4ms.
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    Jonathan

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    And the accompanying Bode response. Notice how much less comb-filtering there is in this plot.
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    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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    Jonathan,

    Thanks for the data you’ve collected. It actually reflects my experience in my den with the Odyssey’s. I appreciate you taking the time applying your knowledge and tools in presenting the information so we all can gain a better understanding of sound reproduction. I must say this forum is the Best with people like your self being part of it. Peace!

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    Member Ethan Winer's Avatar
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    Hi Folks,

    DTB300 invited me to chime in with my comments, so here I am. I usually work best with specific questions but I see a few things to discuss.

    There seem to be two philosophies about the back radiation of dipole speakers. Some argue it's an important design feature, and others (like me) see it more as a by-product of the design. As JonFo clearly showed, leaving the front wall untreated (the wall you face when listening, behind the speakers) creates severe comb filtering. I can't imagine how a response so severely skewed would ever be considered desirable. So the next step is how best to treat those reflections to achieve a truer response.

    The diffusor Jon shows in Post #11 is very thin and looks to be made of styrofoam. This sort of diffusor is not very useful because it's not deep enough to work to a usably low frequency. The material is also too "squishy" (technical acoustics term) to reflect very well. Most "real" diffusors are at least 3 inches thick, and RPG's 9-inch wood model is a staple in professional recording studios. The deeper the wells, the lower in frequency it will work to. Most diffusors either pass or reflect frequencies below their working range. Stryofoam passes, but then the waves are reflected by the wall behind. Wood reflects. As Jon mentioned, the RealTraps diffusor absorbs bass, which is more useful because it reduces comb filtering to a much lower frequency. I'm not here to tout my company's products! I'm just explaining the science.

    My personal preference is to absorb front wall reflections rather than try to diffuse them. Diffusion can be great in a room! But in this case the reflections are very strong in most rooms because the wall is not far behind the speakers. Maybe in a very large room, where the speakers are 5 to 10 feet away (or more) diffusion might be useful. But good absorption costs a lot less than good diffusion. So I usually recommend absorption on the front wall when using dipole speakers.

    I also saw comments about how significant small changes in placement can be. And this is true in all untreated rooms. But once a room is well treated to reduce or avoid the reflections, placement is then much less critical. This is true with all speakers, not just dipoles.

    As for the Golden Acoustic diffusors, I have never seen or heard one in person. But I agree with DTB's points. These have been on the market for a long time, so there's no excuse for not offering performance data. More to the point, they are indeed too thin to do what is claimed. And even more to the point, in a domestic size room bass frequencies are best absorbed, not diffused. Reverb and ambience can sound great at mid and high frequencies, but not so great at bass frequencies where the result is a lack of clarity. If you play a triad chord on a piano at different octaves, chords played up high sound great while chords a few octaves below middle C sound awful. When bass notes sustain due to room reflections, the same sort of thing happens. A walking bass line becomes like a chord with multiple nearby notes all sounding at once. This is a Bad Thing.

    I hope this helps, and I'll be glad to chime in further if the group would like to hear more.

    --Ethan
    I believe in Truth, Justice, and the Scientific Method.

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    Ethan, thanks for joining in on the conversation. I am a satisfied Real Traps customer. Currently I have HF mini traps behind my Martin Logan Summits, to great effect. I am wondering if you could comment on my idea of placing your diffusers in front of the mini traps, for some high frequency diffusion to add back some ambiance, in addition to providing extra mid and low frequency absorption. Might this help or would it tend to be detrimental to the sound, in your opinion? Just curious. Have you experimented with placing your diffusers behind dipole speakers? If so, how did you find it to sound?

    My room is about 14' wide by 19' long. The speaker panels sit about 3 feet out from the mini traps and my listening position is about 9-10 feet from the speakers. All corners are treated with Corner mondo traps and tri-corner traps, first reflections treated with RFZ panels, and a couple of micro traps are placed about as well. The sound and imaging is excellent, but it does seem just a tad too absorptive overall. I think I am missing some of the ambiance you get with dipole speakers.
    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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    Member Ethan Winer's Avatar
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    Hi Rich,

    Quote Originally Posted by Rich View Post
    I am wondering if you could comment on my idea of placing your diffusers in front of the mini traps, for some high frequency diffusion to add back some ambiance, in addition to providing extra mid and low frequency absorption.
    I've never experimented with diffusion in the front of the room, and I don't have dipoles. I've heard them of course! And I've experimented with absorption behind dipoles, but I never tried diffusion there. My gut feel remains that the absorption you already have is most likely better, because the main issue is avoiding the skewed response caused by reflections. Diffusion will scatter those reflections, which helps, but they'll still be "early" due to the close proximity of the speakers.

    My room is about 14' wide by 19' long. The speaker panels sit about 3 feet out from the mini traps and my listening position is about 9-10 feet from the speakers. All corners are treated with Corner mondo traps and tri-corner traps, first reflections treated with RFZ panels, and a couple of micro traps are placed about as well. The sound and imaging is excellent, but it does seem just a tad too absorptive overall. I think I am missing some of the ambiance you get with dipole speakers.
    I tend to prefer a room that's more on the dead side. Not totally dead, of course. But maybe more dead than many people (think they) prefer. In my experience, when a room has no early reflections at all, the perception is of more ambience and spaciousness because then you can more clearly hear the ambience that was embedded in the music by the recording engineers. Here's another way to look at this:

    In my opinion, small room ambience is always bad ambience. The vast majority of rooms have no acoustic treatment at all, so the inherent small room ambience dominates, and overshadows the deeper, richer reverb that's already in the music. I see people try in vain to get a "big" sound in a small room, trying hard not to kill the very reflections that damage the sound the most!

    In your case you have bass traps and first reflection treatment. So you're already 99 percent of the way there if not 100 percent. For how long have you had the treatment you described? I ask because sometimes a well-treated room can take getting used to, almost like an acquired taste.

    --Ethan
    I believe in Truth, Justice, and the Scientific Method.

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    Quote Originally Posted by Ethan Winer View Post
    Hi Rich,


    I've never experimented with diffusion in the front of the room, and I don't have dipoles. I've heard them of course! And I've experimented with absorption behind dipoles, but I never tried diffusion there. My gut feel remains that the absorption you already have is most likely better, because the main issue is avoiding the skewed response caused by reflections. Diffusion will scatter those reflections, which helps, but they'll still be "early" due to the close proximity of the speakers.

    ...

    --Ethan
    Hi Ethan, thanks for joining our discussion on this topic. It's great to have a truly knowledgeable person chime in.

    I agree that diffusion is tricky behind diploes, as the ‘early reflections’ are generally in the sub-10ms range. So while it helps preserve energy, it still imparts a time smear (impulse is spread out across a few ms) and comb filtering results.

    The questions might be: Can the dipole rear-wave be angled and delayed enough that it adds the ‘ambiance’ people seek from this class of speaker?
    That is, if the travel time of the rear wave is long enough (Speaker -> wall -> side wall -> Listener) and attenuated to the right level, would that impart any benefit to the sound?
    And by long enough, I mean >15ms, and by attenuated I mean >12db down.

    I hear what you say about not trying to let the room impart a signature on the sound, and depend on the recordings capture of it. I find that quite logical and attractive, but I still wonder if a correctly managed rear wave has positive attributes that can be leveraged. That’s one of the things I’m exploring in my metrics.

    I might wind up at the ‘dampen the rear wave as much as possible’ point, but wanted to travel the road of exploring how the rear wave might be correctly managed and get some benefit.

    I also acknowledge this is akin to engaging a full-time DSP soundfield mode on a processor to add ‘ambiance’, but then again, I run all my 2ch music in ‘TriField’ mode on my Meridian (a 7.1 mode)

    For rear channels, I’ve found that there is definite (at least in my setup) benefit to diffusion. And I’m really looking forward to my RealTraps difusors to improve on what I currently have.

    One reason the diffusion helps in the rear channels is that it really does cause more perceived sound to come from the side wall reflections of the rear wave. This broadens the soundstage (in the rear) substantially. Which for both movies and DVD-A sourced music is quite pleasant. Better than without it at least.
    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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    Jon,

    diffusion ... helps preserve energy, it still imparts a time smear (impulse is spread out across a few ms) and comb filtering results.
    Exactly.

    Can the dipole rear-wave be angled and delayed enough that it adds the ‘ambiance’ people seek from this class of speaker? ... I hear what you say about not trying to let the room impart a signature on the sound, and depend on the recordings capture of it.
    This is why I'd rather see the rear waves absorbed and removed from the equation completely. But that's me, and others may prefer to keep some of the room tone. One thing that makes it so difficult to speak in absolutes is that all rooms are different. My advice for someone with a 10x12 room is very different than with an open room that's 35x40 or similar.

    The good news is you don't need to nail it on the first try. I've added to the treatment in my living room at least three times in three years, and during that time I also moved things and upgraded speakers twice. I think I finally have it now though!

    --Ethan
    I believe in Truth, Justice, and the Scientific Method.

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    Default A few specific questions for Ethan:

    Quote Originally Posted by Ethan Winer View Post
    Hi Folks,

    As Jon mentioned, the RealTraps diffusor absorbs bass, which is more useful because it reduces comb filtering to a much lower frequency. --Ethan
    Ethan,

    Thank you very much for joining the conversation. Can you please explain this statement? What exactly is comb filtering? Why would absorbing a long bass wave reduct it? What happens to the shorter waves and would why would they not contribute to comb filtering?

    Also, my general understanding of applying room treatments is to treat all corners with your mini or mondo trap products. Then to listen the room with less muddied bass and place additional products based on experience. Is this correct?

    Martin Logan speakers have a very small sweet spot. It is very difficult to dance and to enjoy the music at the same time. How realistically can this sweet spot be increased with effective room treatments or should this limitation be accepted?

    What is your take on Charlie Hansen's comment that most products do not absorb below 100 Hz, which seems to be the reason he lost faith in them.

    What is your take on Cardas ratio of speaker placement, taking room treatments into account?
    The distance from the center of the woofer face to the side walls is:

    Room Width times .276 (RW x .276)
    The distance from the center of the woofer face to the wall behind the speaker is:

    Room Width times .447 (RW x .447)



    Thanks much!!!

  14. #14
    Member Ethan Winer's Avatar
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    David,

    Quote Originally Posted by David Matz View Post
    What exactly is comb filtering?
    Comb filtering is a specific type of frequency response error that's characterized by a series of many peaks and deep nulls. It's almost always caused by acoustic reflections, but it can also be created electronically using a delay. This graph shows comb filtering:



    The graph above is from this article on our web site:

    http://www.realtraps.com/art_spaces.htm

    There's also a video on our site that let's you hear what it sounds like here:

    http://www.realtraps.com/videos.htm

    Why would absorbing a long bass wave reduct it? What happens to the shorter waves and would why would they not contribute to comb filtering?
    All waves can create comb filtering. Comb filtering occurs whenever you combine a source signal with a delayed version of the same signal. Depending on the delay time, some frequencies are combined in phase (peaks) and some are out of phase (nulls).

    Also, my general understanding of applying room treatments is to treat all corners with your mini or mondo trap products. Then to listen the room with less muddied bass and place additional products based on experience. Is this correct?
    It's even simpler than that. My approach is as much bass trapping as possible, plus absorption at the first reflection points. If the wall behind you is very close by, additional absorption on that wall is needed to avoid comb filtering. If the wall is 3 to 5 feet or more behind you, then diffusion is a good alternative to absorption. But what you choose behind you is about the only "season to taste" aspect as I see it.

    Martin Logan speakers have a very small sweet spot ... How realistically can this sweet spot be increased with effective room treatments or should this limitation be accepted?
    Adding first reflection absorption will absolutely increase the physical size of the area where the response is more or less flat. Which is how I'd define a sweet spot.

    What is your take on Charlie Hansen's comment that most products do not absorb below 100 Hz, which seems to be the reason he lost faith in them.
    As I read his post his experiments were a long time ago. Our bass traps are effective to much lower frequencies than 100 Hz, and with enough of them (8 or more) I've measured an improvement to as low as 30 Hz. Yes, I've seen plenty of really expensive "acoustically treated" rooms that sounded terrible. The problem is not the presence of acoustic treatment, but rather an improper use of it. For example, a lot of people use too-thin foam or fiberglass panels all over their room. So the room is too dead, yet boomy at the same time. Sadly, and maybe not surprisingly, how much something costs seems to have little relation to how good it is.

    What is your take on Cardas ratio of speaker placement, taking room treatments into account?
    There are a lot of opinions on this stuff, and I admit I have not tried them all. But I think it's too simplistic to expect any "one size fits all" philosophy to work in all rooms. My approach is described in this Room Setup article on our site:

    http://www.realtraps.com/art_room-setup.htm

    This method identifies the ideal listener placement first, then uses actual measurement (when possible) to find the best places for the speakers. Understand that measuring the low frequency response at high resolution is the only way to know which placements are best. There are too many variables to be able to predict what happens in a small room. For example, if the walls are thick sheet rock, or thin sheet rock, or the space inside the wall is insulated or not insulated, and the proximity of furniture, and what the furniture is made from - all of these have a direct impact on the bass response you'll actually get.

    --Ethan
    I believe in Truth, Justice, and the Scientific Method.

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    Quote Originally Posted by Ethan Winer View Post
    There's also a video on our site that let's you hear what it sounds like here:

    http://www.realtraps.com/videos.htm
    Everyone...

    There are many great videos to view on Ethan's site to give you an idea about room treatments, the sound improvements you get, and how to go about doing it. There are also many great articles and information on the site to get you started on the right path.

    In the video, when they are adding Bass Traps to the room, Ethan and Doug mention the change in the sound of the room and you can actually hear the difference in the video as they add the traps compared to the start of the video.

    Make sure to stop by Ethan's site and start to learn how to improve your room. As you see from JonFo, Rich, myself, and others, this is a great way to get the most out of your ML's and your setup.

    Dan
    Last edited by DTB300; 12-14-2007 at 07:18 AM.
    .............

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