Why not symmetrical slopes for crossovers?

Discussion in 'Archived Threads 2001-2004' started by Kevin C Brown, Nov 1, 2002.

  1. Kevin C Brown

    Kevin C Brown Producer

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    Mains to a sub. I used to know this, but forgot. Usually it's different, 24 db/octave & 12 dB/octave for example.

    TIA!
     
  2. BruceD

    BruceD Screenwriter

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    My understanding is;

    the 12dB/octave slope of the prepro's electrical high-pass xover is suppose to SUM with the 12dB/octave acoustical low frequency rolloff of the main speakers. This SUM equals a 24dB/octave total high pass to counteract with the prepro's 24dB/octave electrical low-pass xover.

    In reality, only THX speakers have 12dB/octave rolloffs at 80Hz, and sealed speakers that have 12dB/octave rolloffs typically start rolling off in the 30Hz to 60Hz range.

    Everything else (like ported speakers) typically has higher slopes for low frequency rolloff, from 18dB to 48dB/octave slopes. This means the SUM of the high-pass (pre-pro + main speaker) will likely be a sharper slope than 24dB/octave, and thus rolloff the bass from the mains even quicker (which isn't all bad) if the rolloff starts close to the xover frequency selected.

    The issue focuses on where the main's low frequency rolloff starts and where the xover is set? If the prepro's xover is above where the main's rolloff starts then the mains don't contribute much acoustic rolloff to the high-pass and all you have is the 12dB/octave electrical slope of the prepro --this is particularly true for floorstanders. This is not enough to smoothly blend with the 24dB slope of the prepro's low-pass, and you get additional SPL from the mains below the xover.

    I prefer symmetrical xovers myself, and use 4th order L-R (24dB/octave) for both high-pass and low-pass with an external active xover. The benefit of symmetrical L-R (Linkwitz-Riley) is no 2-3dB peak in the SPL at the xover frequency.
     
  3. RichardH

    RichardH Supporting Actor

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    Don't forget phase!

    But yeah, Bruce tells it correctly.
     
  4. Kevin C Brown

    Kevin C Brown Producer

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    Bruce- OK, I have to let your explanation sink in a little. [​IMG]
    But I gosh dern really do remember that I thought there was some problem with symmetrical crossovers. Something with maybe exciting room nodes or something, and using asymmetrical crossovers avoids that. Or something. Drats...
     
  5. Bob Wilson

    Bob Wilson Agent

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    OK a question that I have often wondered about. If a THXspeaker has a natural 12db per octave roll off at 80 hz then presumably the pre pro has to do nothing to the signal going to the speaker. It simply lets the signal pass directly to the THX speaker and sends the low frequency information to the sub, rolled off. In the case of a full range speaker that is set to small the roll off would need to be applied to the speaker because it is flat below 80Hz. Since I have yet to see a pre pro that asks whether the speaker is actually flat or rolls off why do we not see a frequency response problem or perhaps we do and it just isn't so obvious. My main speakers are pretty flat down to about 35 or so.
     
  6. Dustin B

    Dustin B Producer

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    Bob, either you misunderstood or never read Bruce's post.

    The THX standard for bass management is the low pass that feeds the sub is 24dB/octave at 80hz. The high pass that feeds the speakers set to small are 12dB/octave at 80hz (in this standard you just don't set speakers large). Then the speakers are all also supposed to have a 12dB/octave roll off at 80hz that matches the high pass in the processor. So then when you sum the processors highpass with the natural high pass roll off of the speaker you get a 24dB/octave high pass slope that will match the 24dB/octave low pass.

    M&K THX speakers have this roll off.
     
  7. StephenL

    StephenL Second Unit

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