Why does SUB have Phase switch?

Discussion in 'Archived Threads 2001-2004' started by JohnThomas, Dec 10, 2001.

  1. JohnThomas

    JohnThomas Auditioning

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    OK, I know there has to be a good reason but, How does it get out of phase? How do you know when it's in/out of Phase? I can see how it would affect the other speakers if you mix up the connections ( + - ) etc. The powered SUB just has one cable and it can only be connected one way. So, if it's out of phase it would seem the Receiver is sending a signal out of Phase. How would THAT happen?
     
  2. SamRoza

    SamRoza Stunt Coordinator

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    There are two types of "phase" corrections.

    One type is electrical, I.E.- you got the wires reversed. In this case, the sub will push itself out instead of sucking itself into the enclosure and moving air. Subs typically have no punch and you can tell the difference in sound. (this is mainly in car audio)

    Room based phase is the other type. This is causedby anomalies in your room causing waves to cancel themselves out(?).

    If you can't tell, one I know much about and the other I don't. If someone can tell more about room phase, it'd be most appreciated.

    Sam
     
  3. John Royster

    John Royster Screenwriter

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    I thought it had to do with the waveform being shifted 180 degrees out of phase. Has alot to do with WHEN the wave reaches you, you'd want it to reach you the same time the other sounds did. I think. Maybe. Something along those lines.
    oh heck, I don't really know. [​IMG]
     
  4. John DeSantis

    John DeSantis Stunt Coordinator

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    Sam:

    That's about what my thinking was. I understand a reverse connection. I've fliped my SUB switch and it doesn't seem to change anything. Guess we'll hope for more answers here.
     
  5. Darrin_R

    Darrin_R Stunt Coordinator

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    Speaker placement has a lot to do with it. The low end can be out of phase if your sub is not located near your main speakers. I think they alway recommend placing the sub within 15 feet of the mains. I becomes a timing issue.

    I use 2 subs sitting side by side. If i switch the phase on one of the subs, all bass is completely cancelled.
     
  6. SamRoza

    SamRoza Stunt Coordinator

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    Ever notice how sometimes you reply to a message and it's like your brain dribbled out your ear prior to responding?
    The phase switch simply affects the timing of the waveform by 180deg. So, for instance, it arrives at the peak instead of the trough of a 25Hz. note.
    Sometimes a room will make your sub act out of phase and cancel out. This might help it. I have my sub set to 0deg. and I never use the phase switch.
    Sam
    P.S.- I knew that EE training would help me out on of these days [​IMG]
     
  7. Kevin C Brown

    Kevin C Brown Producer

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    You can also put your sub out of phase with your mains if you don't have the distances (or delays) set correctly in your receiver/pre pro.
    I personally think that the best way to check for the correct phase is to use a test disc (such as Autosounds 2000 or Stryke) and a Radio Shack SPL meter.
    http://www.autosound2000.com/Products/cds/index.htm
    http://www.stryke.com/
    Simply measure the response of your mains + sub together from 1 octave (2 is better) below through the crossover freq to above. (For example, I use 80 Hz crossover, so I should measured from 20 Hz to 320 Hz. Need the Stryke disc for that. The Autosounds disc goes from 10 Hz to 98 Hz, but worked for me too.)
    Then adjust the phase switch and measure again. The setting with the *most* bass (the highest reading on the meter) is in phase.
    I tried doing it by ear 1st, listening to music as some suggest, and I was off by 120 deg (my sub has a variable phase knob).
    Doesn't really have anything to do with how close your mains are to your sub... Just the "timing" between them.
     
  8. JohnThomas

    JohnThomas Auditioning

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    Hmmmm... this is getting more complicated than I thought. How much is one octave from another in freq. Can't I just use Avia test tones and set the phase switch to the position that gives me the highest reading? Or wouldn't that insure that it's in phase?
     
  9. Kevin C Brown

    Kevin C Brown Producer

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    John- Your way will work. Just that instead of using Avia's sweeps (make sure to use the slow ones), I just used discrete tones on the 2 test discs, and plotted every point.

    Octaves: halves going down, and doubles going up. For example, 80 Hz crossover that a lot of people use. 1 octave down is 40 Hz, 1 more down is 20 Hz. 1 octave up is 160 Hz, and 1 more is 320 Hz.

    Larry- Your's echoes my experience. I 1st tried to do it by ear. There was less bass, but I thought it sounded better. Then after I used the SPL meter, *and* properly adjusted the sub level, it does sound better. Now, I have also added a Behringer EQ to mellow out some of the peaks, and that may help too, in that overall, the bass I'm getting doesn't seem as boomy.

    There is some science involved. Remember back to high school physics about deconstructive and constructive interference of waves? When you put your sub *in phase* with the rest of the speakers in your system, you making sure that you're getting constructive interference, i.e., that all the sound in the waves are adding together to give you even more sound.

    It was kind of neat when I did the individual test tone measurements, in that I first plotted the sub by itself, then the mains by themselves, then the sub plus the mains together. It was really bizarre seeing me get *less* output with the sub+mains than with the mains alone. Bingo: *de-constructive* interference.
     
  10. Marc H

    Marc H Second Unit

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    The phase switch is to compensate for the placement in your room. If the sub is the same distance (or fairly close to it) to your seating position as the main front speakers are, then use the zero position. If the sub was closer to twice that distance, then you would use the 180 degree position to time delay the bass so that all the sound from the speakers and the sub arrive at your seated position all at the same time. Basically it will sound like you have less bass if it's set incorrectly.
     
  11. JohnThomas

    JohnThomas Auditioning

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    Another thing, my sub is flat on the bottom ( no legs )

    Should there be something under it? I have hardwood floors and somtimes I think I hear it vibrate. Is it OK to maybe glue some rubber pads under the corners? It actually wobbles just a bit as the floor is not perfectly flat there.
     
  12. peter_anderson

    peter_anderson Stunt Coordinator

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    Experiment. Start with bricks (and something to protect the finish of the floor. If that makes it sound better to you, get proper spikes (or maybe not, if you have a hardwood floor).
     
  13. JohnThomas

    JohnThomas Auditioning

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    It's a down-firing SUB. does that make any difference as far as height? I could maybe just buy some short legs....
     

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