Crossover point for a single subwoofer

Discussion in 'Archived Threads 2001-2004' started by Jones_Rush, Mar 22, 2001.

  1. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Ok, you have convinced me that a good subwoofer (like the Shiva) can play 80hz signal with no problems at all.
    My next question is:
    If I only use one sub, which is located between the front speakers, and want to use my mains as "small", what will be the highest frequency that I can set the crossover and still enjoy a large and spacious soundstage ?
    I have read in many places, that when using the fronts as "small", in some systems the soundstage becomes much narrower. What is the frequency at which we REALLY start to not localize the sound ?.
    I made a test and played a tone signal at 80hz, one time through the left speaker, and the other through the right. I was able to easily localize the signals. This mean that 80hz signals still contain localization cues which our ears can detect, which mean that when playing movies/music we will lose these localization cues and make the soundstage narrower if sending them to a sub.
    If I want to retain large soundstage, will I have to use two subs ?.
    I know I can always set the sub's crossover at 35hz,
    but then I will suffer from huge room modes, since my main speakers are not EQ'ed for flat frequency response like the sub will be (and my room modes are between 40-70hz).
     
  2. Patrick Sun

    Patrick Sun Studio Mogul

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  3. Chris Hoppe

    Chris Hoppe Stunt Coordinator

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    Often overlooked is the slope of the crossover. A 24db/oct at 80Hz will be much less localized than a 12db.oct at 80Hz!
     
  4. ThomasW

    ThomasW Cinematographer

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    Jones
    There is no "ideal" standard XO point. The chosen point will usually depend on the placement of the sub, and the LFE capability of the mains
    The ideal placement for a single sub is centered between the mains and in line with the frontal boundry of their baffle. This allows for XO points as high as 100Hz with no smearing of the stereo image. And the lowest possible chance of phase problems.
    There are no hard and fast rules beyond that. If that centered placement isn't possible then lower XO points should be used.
    Unless it is centered a person with trained ear can always find the location of a sub even with a low XO point. It will always function as a sound "source", but the lower the XO point when the sub location is off center will decrease the potential smearing of the stereo image.
    Chris's statement regarding 24db/octave as the slope of choice is dead on!
     
  5. Richard Greene

    Richard Greene Stunt Coordinator

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    The soundstage should not be affected by a subwoofer
    with limited output above 80Hz. The stereo effect
    is almost entirely from frequencies above 80Hz.
    coming from the satellite speakers.
    Audiophiles are not able to hear a stereo effect at frequencies below 80Hz. when listening to music with the satellite speakers turned on. Bass ouput from the satellite speakers masks the location of the subwoofer.
    Your experiment using a test tone is meaningless unless you often listen to test tones rather than music.
    With sine wave test tones, 40Hz. can be localized by some listeners. Once again, that's not relevant unless you listen to test tones.
    In summary there are no localization cues below 80Hz. while listening to music, so your concern about the soundstage is a moot point.
    The ideal crossover point depends almost entirely on the
    quality of the output from your satellite speakers when playing at the highest volume you'd ever use.
    The purpose of a subwoofer is to play those frequencies that are missed by you satellite speakers, or cause an excessive amount of harmonic distortion. (Turning the volume of the satellite speakers to the highest level you'd ever use generates the maximum amount of harmonic distortion you're likely to hear from them.)
    The crossover point also depends on whether you want to take advantage of the benefits of using a room corner for your sub. Subs in corners have one disadvantage in most listening rooms: They may not integrate well with your satellite speakers unless you use a steep slope crossover at a relatively low frequency. The THX standard of 80Hz. (with 24dB per octave slope) is the highest crossover frequency I'd recommend, although some audiophiles would be satisfied with 100Hz. 24 dB/octave. My rule-of-thumb is to use a crossover frequency and slope that result in bass output at
    150Hz. down at least 24dB relative to bass output below the
    crossover frequency.
    The ideal subwoofer location to optimize bass frequency response in most listening rooms is in a room corner playing mono bass. If a second sub is desired for higher output, it should be stacked in the same corner or placed right next to the first sub, with both playing mono bass.
    A corner position will provide the highest output/lowest harmonic distortion and smoothest frequency response possible in most listening rooms. This is not a theory.
    It is a conclusion based on sophisticated measurements
    in dozens of real listening rooms. If the room is square or cubical, a corner position is not likely to be optimum for the simple reason that a corner position excites all room modes to the maximum -- if two axial room modes are at the same frequency, as with a square room, a sub position that excites both modes to the maximum is not likely to be satisfactory.
    In general, you will hear bass no matter where you place a subwoofer within a listening room, and there is no rule-of-thumb that can be applied to all listening rooms.
    The old-fashioned sub position between the main speakers
    is useful for subs with 100Hz. / 12db/octave crossovers
    that would not integrate well if placed in a room corner.
    That position rarely measures or sounds as good as a corner
    location for frequencies under 80Hz. where there is no stereo effect.
     
  6. John E Janowitz

    John E Janowitz Second Unit

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    ThomasW writes:
    "The ideal placement for a single sub is centered between the mains and in line with the frontal boundry of their baffle. This allows for XO points as high as 100Hz with no smearing of the stereo image. And the lowest possible chance of phase problems."
    This would be the best position to avoid localization of the subwoofer with higher Xover points. However, this placement will cause all kinds of problems with room modes. To get the flattest response, corner loading is the best choice. The idea is to excite all room modes equally.
    John
     
  7. John E Janowitz

    John E Janowitz Second Unit

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    FYI on corner loading:
    "At low freqeuncies the problem is a low density of room modes. Corner placement will excite as many as possible. Makes energy distribution as smooth and even as it can get .... although it still won't be perfect. " Quote from Tom Nousaine
    John
     
  8. Kyle Richardson

    Kyle Richardson Screenwriter

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    On movies I have mine up quite high (around 100 hz) but on music I turn it way down to around 40Hz. I find the higher crossover setting not as noticable on movies as on music only playbacks.
    ------------------
     

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