How do you address nulls caused by phase cancellation?

Discussion in 'Subwoofers' started by MI55ION, Jul 30, 2012.

  1. MI55ION

    MI55ION
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    Hi,

    My room layout and position of speakers cause serious null between 45-80 hz region. My main speakers can run down to about 30hz with room gain and running REW sweeps indicate a serious null in that region. Adding the sub makes it even worse. I've tried adjusting distance in the processor, frequency cut-off and phase adjustment on the sub but the problem persists. The room is 4.5x3.9x2.6m with a bay window and a chimney breast. The position of speakers is set so can't really be moved elsewhere. Just want to understand how people get round this issue and if there's anything that can be done?
     
  2. AngelEyes

    AngelEyes
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    You need to shorten the room, having one dimension double the other is almost as bad as a square room unfortunately. :laugh:

    I can't see this as being a phase issue if adjusting the sub phase doesn't help at all, just a room null?

    Anyway the quickest way to find out what your options are is to measure just the speakers or sub from a few different positions and see if you can ameliorate the null. If you can then you will probably need to move something, even if it is just the seating position.

    Adam :)
     
  3. MI55ION

    MI55ION
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    I think you're correct in saying that it's a room null, I didn't think about that. I suspect it could be a combination of both phase cancellation and room null. Having done sweeps of each speaker independently, on their own their is a null (left speaker being more problematic) but it isn't so dramatic as combining both speakers and/or adding the sub. As for moving seating position that's pretty much fixed I'm afraid. Thanks.
     
  4. AngelEyes

    AngelEyes
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    If you can't move the speakers, sub or yourself you are stuffed I am afraid :(

    Edit: Unless you add another sub somewhere else of course :smashin:
     
  5. MI55ION

    MI55ION
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    Ok thanks Adam, I'll try that since I have a spare velo 800 8" sub, do you think it will work ok with the pc ultra for the target range? I'll probably need to engage the phase switch on one of the subs, is there any negative consequences as I read somewhere that phase controls are best avoided as it can degrade the sound? Thanks again.

    I'll add some screen shots later, see if you can make anything from it.
     
    Last edited: Jul 30, 2012
  6. AngelEyes

    AngelEyes
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    Don't worry about the phase yet, it really depends where the second sub is going. The smaller sub probably won't integrate that well as it is likely to run out of puff without a suitable high pass filter, it should hopefully allow you to work out if a second sub will help.
     
  7. MI55ION

    MI55ION
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    ok having spent the last few hours going insane i think I've finally managed to get a fr that at least looks like how it should. I'm very happy with it and having done some listening it certainly feels as though the sound has improved. The velo is located as shown in room plan below next to the sofa under the rear right speaker. The velo gain doesn't have to be very high so no probs in terms of distortion that i can hear. I used to get waves of bass hit my right ear before, that feeling has all but disappeared. Oh the joy! :laugh:

    The pics are in order of sequence so i hope it illustrates my findings better...:)
     

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    Last edited: Jul 30, 2012
  8. MI55ION

    MI55ION
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    Room plan:
    S = SVS
    V = Velodyne


    image-25125295.jpg
     
  9. MI55ION

    MI55ION
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    OMG, after a little more tweaking and applying Eq to both subs simultaneously just been listening to some multi channel music and hand on heart I don't think I've ever heard it like this before. The sound is just so incredibly balanced/full I'm sitting here crying out of joy. Everything just seems to come from the front, if I didn't know I would never guess theres a little sub tucked away in the corner to my right. I don't know why I never thought about trying a second sub in this corner doh! Adam, you the man! Thank you so much....
     
  10. AngelEyes

    AngelEyes
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    :blush:

    No problem, really happy it has made such an improvement for you!

    Adam :)
     
  11. MI55ION

    MI55ION
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    I think in my case the important thing was find the location for the second sub, I tried all three corners, the front right and back left corners had the same issue of bass suck out. Always had a feeling something wasn't right about the back right corner, the bass was always very strong in this corner at certain frequencies.... The second sub has incredibly balanced this issue. Eq-ing both subs together has also been a big plus along with phase adjustment and distance settings on the processor. Ultimately for the final tweak you have to play it by ear and see if it sounds right.
     
    Last edited: Aug 1, 2012
  12. MI55ION

    MI55ION
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    Lol i should mention in all my excitement the response isn't perfect but it is significantly better than before. Theres still room for improvement...
     
  13. Member 639844

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    With issues like yours, two subs is always a must. I measure a sub in every position possible, then compare the results in rew and find a complimentary pair. I'll never run a single sub ever again, well, if i ever own any av kit ever again lol
     
  14. MI55ION

    MI55ION
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    Hey Moonfly,

    Wish I had the space for another svs, would have been ideal, I had to do some back breaking work just to fit the tiny velo.

    Btw, what's with this selling up business, once your heart is in it didn't think it was that easy to give all up? I've always thought if ever I was in the situation that I had to give it all up, the svs would be the last to go although they would have to pry it from my cold dead fingers!
     
  15. Member 639844

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    Over the years i've never sold any of the kit i had no matter what, but the move i made was a big one and I couldn't have done it without selling up. One day, when i can afford a setup i'll enjoy, ill probably get back on the horse, but until then all I use is a set of cans. It was one of the hardest things I think I've ever had to do lol.
     
  16. Jim2012

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    Bass traps are what you need. ;)

    Link
     
    Last edited: Aug 7, 2012
  17. MI55ION

    MI55ION
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    no thanks, they're useless
     
  18. Jim2012

    Jim2012
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    Don't mention it. You'd better tell the thousands of people with recording studios how useless bass traps are - they might be misled by how well they appear to reduce low frequency standing waves. RealTraps - Acoustics Fact & Fiction.

    Still, you obviously know best.
     
  19. MI55ION

    MI55ION
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    I don't need to tell anyone anything.

    I said the ones in your link were useless.

    I take it however that with your conviction on the subject that you're talking from actual experience.

    You got the last bit right.
     
  20. Jim2012

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    I apologise for my uncalled for fit of pique. You are undoubtely correct about the bass traps I linked to.

    My original reply was slightly tongue in cheek, as bass traps really need to be designed into the space they are used in and are not at all appropriate for domestic listening environments.

    However, they do work well and are the only solution other than rebuilding your room or moving house.
     
  21. Member 639844

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    Bass traps won't fix phase cancellations, which is what this thread is addressing. They're used to reduce too much boom and can be employed to reduce the ringing inherent to most rooms.
     
  22. Jim2012

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    Although it's impossible to completely fix bass cancellations, bass traps are intended to reduce low frequency reflections from opposite walls, thus minimising the standing waves in the room and also therefore phase cancellation.

    The boom you mention is caused by phase addition due to the same standing waves - depending on your position in the room and the frequency of the sound, you will either get too much bass or too little.
     
  23. Member 639844

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    Thats common knowledge. Phase issues can be tackled by adjusting the phase, or moving the subwoofers around the room. Modern eq systems can help, and modern freely available software like rew are invaluable tools to help identify exactly where the problems lie. Those techniques will be more useful than bass traps, which have a limited amount of usefulness for tackling certain issues. They're much more effective at reducing low end gain than targeting a specific narrow frequency band in order to fix a specific phase issue.

    My advice would always be to move the subs to the best positions in the room first, then if that route gets exhausted, employ eq, which i would also always recommend anyway. Thats generally much more practical than putting bass traps all over your room in an attempt to tackle a phase cancellation. Its not impossible to fix phase issues as long as you can tackle the cause of them. Bass traps are the only way to reduce ringing in your room though.
     
  24. Jim2012

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    See edit to my original post.

    My final act on this thread is to point out something I said earlier: -

     
  25. KelvinS1965

    KelvinS1965
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    I'm pleased to see that the OP managed to resolve this issue as it's something I suffer from too: I have a dip around 35Hz in my main seat, but due to a planned room redecoration I might be able to squeeze my old XLS200 in a position about 1.5 metres from me (my Monolith is about 3 metres away) a a second sub. I'd planned to try to limit the signal range fed to the XLS200, but as it's a sealed box there shouldn't be any danger of over driving it at the low end, plus as it's half the distance away then it shouldn't need so much gain.

    I have an 8033 so I presume that once I've connected the second sub and set levels up that I should just do a fresh setup sweep and it should sort out any peaks for me. I have an SPL meter and have used manual test tones in the past to try to resolve my issue (with a BFD which I still have somewhere in the loft) so any recommendations for what I should do first, before running the 8033?

    I'd really like to get this dip sorted as when I sit in another seat (which is off centre for the other speakers :( ) the bass there is almost too much in comparison.
     
  26. MI55ION

    MI55ION
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    My apologies too, I must have been having one of those days.

    Agreed about the domestic environments.
     
  27. MI55ION

    MI55ION
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    Perhaps it's unclear so here's the steps I took:

    Placed the second sub in different corners and run REW sweeps to find the corner that has the best bass response/avoids the main room null.

    Connected the antimode to the main sub (svs) and then daisy chained the signal out of the svs to the second sub (velodyne).

    Set gain levels and run the antimode calibration.

    Changed distance settings on processor and run REW sweeps to see the effect.

    Once happy with the above, played about with the phase settings on both subs whilst running REW sweeps to see the effect until happy with result.

    Listened to some bass heavy tunes so that both subs were well balanced (subjectively) and bass appeared to come from the front. Adjusted gain accordingly.

    Run further REW sweeps to ensure the frequency response did not deviate too much from before.

    Job done.

    Getting the phase settings spot on was most time consuming.

    The above worked for me, it may or may not work the same way for you but take your time and be prepared to use a different method if required. I have an unused BFD but I tried my best to avoid using that since the antimode does a better job of sorting out peaks and based on previous experience boosting nulls using EQ manually has serious consequences for headroom and sound quality.
     
    Last edited: Aug 10, 2012
  28. KelvinS1965

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    Thanks for the help, I'll follow through this process when the time comes. I haven't got REW, only the SPL meter and the excel spreadsheet that corrects the SPL meter's readings. Maybe I'll look at getting REW as well to speed the process up. :thumbsup:
     
  29. hoppaz

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    REW is free to download and nowadays very easy to set up and use.

    If you have any sort of auto eq avr or processor put the antimode into bypass when running the Mic set up and then you can re run the antimode setup.
     
  30. KelvinS1965

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    Thanks, no Processor EQ as it's an Arcam AV9 I'm using. I'll just manually set up using the SPL meter and REW once I get it installed. :thumbsup:
     

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