Phase Control on subs - Explanation please.

Discussion in 'Archived Threads 2001-2004' started by Christopher Lyn, Sep 25, 2002.

  1. Christopher Lyn

    Christopher Lyn Stunt Coordinator

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    I was just wondering what the theory/science behind the phase control setting on the sub.

    When I try to calibrate the sub using this control, the manual says to turn the phase control, starting from zero degrees, until you hear the greatest quantity of bass output. However, the setting seems very subtle.

    I have a servo-15 and borrowed a friends x-30 crossover. Why does adjusting the phase control allow the sub to increase it's bass output?

    Since the servo-15 does not have a phase control on board, is the use of this control necessary since I only heard a subtle difference? I really had to strain to hear a difference.

    I understand if the explanation is very long and complicated so links would be fine if anyone knows of any.

    Thanks.
     
  2. Bill Kane

    Bill Kane Screenwriter

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    Theory notwithstanding, here's a practical look. As the driver cones in the Mains speakers and the sub move in and out, moving air and producing sound waves, the cones may be out of PHASE.
    At 180 degrees out of phase, one set of cones is pushing and the other one in the sub is moving the oipposite, or inward. This can result in cancellation of common frequency sound waves.

    In practicality, it's not much of a concern, nor discernible, according to SVS-Ron, spokesman and partner for SVS Subwoofers.
     
  3. Gary King

    Gary King Second Unit

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    It has to do with some physical properties of waves.

    If 2 speakers are each playing a 40Hz sound, what you will hear will be the sum of those two speakers. However, since the two sounds reach your ears at slightly different times (known as the phase difference between two waves), this sum can be anything from 0 (total destructive interference), to twice (total constructive interference) what each speaker was playing.

    If in this example, the two speakers are 180 degrees out of phase at your listening position, you will hear nothing. However, by phase shifting one of the speakers 180 degrees, the phase difference will be 0, and the two speakers will be in phase so you will hear twice the amplitude of an individual speaker.

    Phase shifting subs won't make much difference if your speakers are set to small, since the sub is the only speaker generating the bottom octaves, so nothing will interfere with it*. If you have multiple subs, or speakers set to large, then phase shifting can make a difference as speakers start interfering with each other.

    * This explanation isn't quite accurate. The sub can interfere with itself due to reflections off of walls and other surfaces, and even with speakers set to small, some bass will still be passed to the main speakers (due to some physical limitations with analog filters); however, the amplitude of these signals is a fraction of the signal being generated by the sub, so wave interference effects aren't anywhere near as noticeable as if there were two subs.
     
  4. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Phase is primarily used for adjusting the bass with the mains I completely agree.
    But I found that phase also controls where the bass resonances are located in the room. So you may be adding more bass to one position but then removing it from another.
    This is due to the standing waves peak/dip location for the room's response which moves around when the phase changes. Of course I could be wrong as I didn't read this anywhere but concluded it from my own testing.
    Phase is a pretty advanced control that is more of a fine tuning tool. Yes, there's a subtle difference of about 3db or less. I actually needed a measuring device to see if my phase knob was working. [​IMG]
    So with one sub, calculate your room resonances and what you are doing is moving the nulls/peaks around. But with a sub and mains you are deciding how the direct sound interacts with eachother at the listening position. (Adjusting phase this way ignores standing wave phase tuning)
    (Gosh I still don't know if this is correct.. somebody please confirm)
    Here you can calculate room resonances if you have Excel. Also at my site are directions how to make a sound absorbant panel to help put an end to unwanted reflections.
     
  5. Arron H

    Arron H Second Unit

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    I am by now means an expert but a fairly simple test you can do with the phase is to maximize the output of the mains and sub at the crossover point. Play a crossover point test tone, such as 80Hz, through both the mains and sub and turn the phase until maximum output is reached. You will need an spl meter to measure output and a test disc like Stryke for the 80Hz tone. Make sure to turn your center and surrounds off so that the tone is played only through the mains and sub. The benefit of this test is supposed to be a more blended crossover.
     
  6. Richard Greene

    Richard Greene Stunt Coordinator

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    A polarity control or a phase control only affects bass frequencies that come from both the subwoofer and the main speakers -- that usually means frequencies at and near the crossover frequency.

    Most subwoofers have a polarity control, not a phase control. A polarity control does exactly what switching the red and black wires going to the subwoofer driver would do (subwoofer driver connected in-phase with main speakers or out of phase with main speakers are your only choices). This works fine if the subwoofer driver and the bass drivers in the main speakers are the same distance from your ears, or close.

    A real phase control allows a continuum of adjustments between in-phase and 180 degrees out-of-phase with the main speakers. This type of control is needed when the subwoofer driver is a different distance from your ears than the bass drivers in the main speakers.

    Chris wrote:
    "But I found that phase also controls where the bass resonances are located in the room. So you may be adding more bass to one position but then removing it from another."
    RG replies:
    The nodes and antinodes (peaks and troughs) caused by standing waves are determined by room dimensions.
    They can't move -- that's why they are called standing waves.
    The location of the subwoofer determines how much the room modes are excited.
    The location of your ears determines how well you will hear the room modes.
    A phase control or polarity control can be used to tame a room mode at or near the crossover frequency.
    Let's say you have an 80Hz. crossover frequency and an 80Hz. room mode that causes a bass frequency response peak at your listening position when the subwoofer is in phase with the main speakers.
    You can significantly reduce output at 80Hz. by using a polarity control to make the subwoofer out of phase with the main speakers at 80Hz. But then you might have a trough at 80Hz. rather than a peak -- usually a good
    trade-off because bass peaks are much easier to hear than bass troughs.
    But if you have a real phase control, you may be able to find a setting where the subwoofer is somewhat out of phase with the main speakers (not 180 degress out of phase) so that there is no peak OR trough at 80Hz. at your listening position (this won't work everywhere in the room)-- so you've eliminated a bass frequency response peak heard at your listening position without buying an equalizer.



    Chris wrote:
    "This is due to the standing waves peak/dip location for the room's response which moves around when the phase changes. Of course I could be wrong as I didn't read this anywhere but concluded it from my own testing."

    RG replies:
    The standing waves stand -- the location of the peaks and troughs do not change. But if you deliberately set the subwoofer 180 degress out of phase with a polarity control(or less than 180 degress using a phase control) you will affect any room modes at or near the crossover frequency.
    If the crossover is at 80Hz., for example, a phase control will have little, if any, effect on a 40Hz. room mode.


    Chris wrote:
    Here you can calculate room resonances if you have Excel.
    RG replies.
    Sine wave tones are more exact as the calculations assume a rectangular room with perfect reflectors (very stiff walls) while real walls are somewhat flexible, with outside walls often stiffer than inside walls.

    Chris wrote:
    Also at my site are directions how to make a sound absorbant panel to help put an end to unwanted reflections.
    RG replies:
    That's the answer to many bass problems.
     
  7. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Thanks for clearing it up for me Richard, I'm still learning a lot about this hobby and I knew I was probably wrong.

    It makes a lot more sense when I think about it. So, when I heard the bass change, it was actually the interaction with the mains. NOT the standing wave troughs moving. :b

    Well, my absorbant panels aren't really my solution to bass problems since they're designed to absorb the upper bass notes to high frequencies. So, what I plan on doing is assembling some bass traps/helm holtz resonators to handle the real bass tuning. In the end I plan on using diffusor panels to fine tune the listening room.

    But first, I'm gonna play with the positioning of my 2 subs for mode cancellation and see if that works. (though in the past you've notioned against it) At least I still have the room treatments to rely on as well as the possibility of adding an EQ which would be my last resort.

    BTW, where did you learn all this. Is there some book I should read?
     
  8. Christopher Lyn

    Christopher Lyn Stunt Coordinator

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    Great, thanks guys, I understand more clearly now...I was not sure about some of the theory...but I get the idea.

    I'll wait until the x-30 goes down a bit more or maybe get a used one. Like I said I heard a subtle difference, and my sub, due to room constrictions and furniture, is in the best position it can possibly be.
     

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