What's the rule of thumb when setting your sub's crossover?

Discussion in 'Archived Threads 2001-2004' started by Keir H, Oct 22, 2001.

  1. Keir H

    Keir H Second Unit

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    My mains are listed to go pretty low (Def Tech BP30's) at 18hz. However I know they cannot go this low at decent spl in a 19x16x8 room. I'll try them setup as small since I have a "tie me over" sub the ever popular Sony SAWM-40. I'll have this one til about the holidays when I'll then choose from the SVS line or REL. So, to get the most outta the Sony, what crossover setting should I use? I have a THX processor. I heard if I set too high, I get bloated (doubled) bass in the higher frequencies. By filling the port, will I have the sound of a sealed sub i.e. the REL? thanks
     
  2. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    If you're setting your speakers to small, then you'll want to set the crossover on the Sony itself as high as it will go. This lets the receiver's crossover control bass management and prevents crossover cascading (which can cause gaps in the frequency range). Most THX receivers/amps are crossed at 80hz, but if yours lets you adjust it on the receiver you might play with it and see how it sounds with different settings.
     
  3. Kevin C Brown

    Kevin C Brown Producer

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    Keir- I have BP-30's too, and they do not go to 18 Hz.
    That's Def Tech's marketing talking.
    I have my system set to small, and the crossover set to 80 Hz, and that works pretty well.
    By the way, rule of thumb for sub crossover is, 1 octave above the low freq extension of your mains.
    I estimate that BP-30's are flat to 40 Hz (maybe -3 dB at 35 Hz).
    One octave up from 40 Hz, is ... 80 Hz.
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  4. Jay Mitchosky

    Jay Mitchosky Producer

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    Given that you're using external bass management (be it through the receiver, processor, or otherwise) than do as suggested and crank the sub's crossover level. Otherwise it has the potential to interfere and cause "cascading" where gaps result in bass response. Ideally the sub would have a complete bypass option, but cranking the threshold is the next best thing.
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  5. Vin

    Vin Supporting Actor

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    I agree with Jeremy's recommendation to set the sub's cut-off to it's maxiumum setting (or setting to bypass if it has one) and just let the receiver handle the job of routing deep bass to the sub for any speakers that are set to small.
    Also, don't forget that the LFE track can contain info up to 120Hz so if the sub's cut-off is set any lower than that, you'll be losing that part of the LFE between 120Hz and wherever the sub's cut-off is set.
    Just my $0.02 worth. [​IMG]
     
  6. Evan Mann

    Evan Mann Agent

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    Something similarly related. I've got a Denon AVR-2800. I believe they have a fixed crossover at 80hz in the receiver on the sub pre-out, is this correct? I want to make sure before I move my x-over on my Infinity sub.
     
  7. Jeremy Anderson

    Jeremy Anderson Screenwriter

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  8. John Kotches

    John Kotches Cinematographer

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    Jeremy,
    I agree with most everything you wrote. A few caveats.
    Depending on implementation, the crossovers are not symmetric. THX Standard crossover (most common) is 12dB/octave High Pass (to mains) and 24dB/octave Low Pass (to subwoofer).
    Let's talk about what's really going on here though, in terms of output SPL with the crossover engaged. Further, let's assume a tone that is 75dB being sent to one speaker that is defined as small, and that the crossover frequency is 80Hz, with a standard THX slope.
    To produce the 80Hz tone, at 75dB requires that the main speaker deliver 72dB and the subwoofer produce 72dB. The goal is that together, they produce 75dB.
    Now, let's move the tone down 1 octave below the crossover frequency. Here, the main speaker is only required to deliver 60dB, while the subwoofer delivers 74+dB -- the logarithmic basis of the dB is really being brought to light here. Pushing down another octave to 20 Hz, changes things inconsequantially, with 48dB from the main speaker, and still 74+ dB from the subwoofer.
    Moving back the other direction, and working above the crossover frequency at 160Hz. Your main speaker is required to produce 74+dB, while the subwoofer is already pushed down 24dB to 48dB. At 320Hz, the subwoofer is no responsible for 24dB, and almost all the output is in the main speaker (still 74+ dB).
    So while I understand the crux of your point, it's important to understand by looking at the speaker outputs (and the dB values) to produce a tone at a particular SPL for a managed speaker.
    Just thought I'd throw out this food for thought.
    Regards,
    ------------------
    John Kotches
    Contributing Writer
    Secrets of Home Theater and High Fidelity
     
  9. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    Thanks for the detailed explanation, John. It's always good to learn more about this issue, since it seems like a thread pops up on this topic on a regular basis. Hopefully between the two of us we haven't confused anyone beyond reason!
    P.S. -- I lay no claim to absolute HT knowledge -- I work at a law firm! I've just been reading this damn forum for almost two years! [​IMG]
     
  10. Robert Spalding

    Robert Spalding Second Unit

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    ok, dumb quetion here...I assume that means turning my crossover all the way clockwise?
     
  11. Kevin C Brown

    Kevin C Brown Producer

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  12. John Kotches

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    Kevin,
    The subwoofers I've worked with have a specific LFE / processor in, and others that are specifically designed for full range input, and crossover high pass output.
    IMO, a competently designed plate amp should have an line level inputs that are unfiltered and line level inputs that hit the crossover.
    I am working with a subwoofer at the moment that has all inputs filtered, and am working around that by setting the dial to maximum crossover frequency. In fact, the manual for the sub gives that same instruction.
    Robert Spalding,
    Giving a direction (clockwise or counterclockwise) is not universally applicable. Turn the dial to the highest possible frequency is the safest advice I can give you.
    Regards,
    ------------------
    John Kotches
    Contributing Writer
    Secrets of Home Theater and High Fidelity
     
  13. John H

    John H Second Unit

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  14. Karim Nogas

    Karim Nogas Stunt Coordinator

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    Robert,
    No dumb questions. Mine is all the way clockwise or at the highest setting (100Hz).
     
  15. Robert Spalding

    Robert Spalding Second Unit

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    ok, thanks for the help....I set mine all the way to 125. I have my receiver set to "small".
     
  16. Chris S

    Chris S Cinematographer

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    I'll admit that I'm a real novice when it comes to everything that has been talked about in this thread. Is there any tutorials, FAQs, or something to look at on the web to help get up to speed? Chris
     
  17. Clinton McClure

    Clinton McClure Casual Enthusiast
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    All my speakers are set to small on my receiver and, if I am reading the manual correctly, the LPF cutoff for the sub output is 90Hz (Yamaha RX-V995). I have my sub's x-over ramped up to 150Hz for full frequency response below 90.
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  18. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    Well, the important thing to remember here is this: Whether your speakers are set to large or small, you need to have the sub's crossover defeated or turned as high as it will go. The reason -- because regardless of speaker size, the receiver will be sending the LFE to the sub (which goes to 120hz).
    The crossover on a subwoofer is for compatibility with other kinds of setups (for instance, you might use it in a dual subwoofer setup or when using a sub just for particular channels).
     

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