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phase reverse on Sub

Discussion in 'Subwoofers' started by alphabet, Jul 5, 2005.

  1. alphabet

    alphabet
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    Hi There

    Let me ask a silly question!

    What is the purpose of the Phase Reverse switch on a sub?

    I suppose I should switch it on and hear if it makes a difference, but I have not done it yet as I am not sure if it may cause any damage.

    Cheers
     
  2. eviljohn2

    eviljohn2
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    It won't cause any damage if you try it so I recommend that you do.

    I suspect that you won't hear any difference in a normal setup although it's very useful if you're trying to integrate multiple subs into the same room to minimise interference. :)
     
  3. micb3rd

    micb3rd
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    It is used to flip the electrical phase which it used if you have a phase problem at certain frequnecies.

    Common frequencies playing by two sources can lead to frequnecy reinforcement if in acoustically in phase or frequency cancelation if acoustically out of phase.

    Phase can alter across the frequnecy range and can be affected by many factors.

    Speakers and subwoofers can play common frequnecies even with sharp slope high pass/low pass filters, so if there is cancelation (loss) in the bass frequencys you flip the phase so the frequencies are reinforcing rather than canceling.

    In a different situation a pair of subwoofers playing common frequencies can cancel each other out to have almost no output bring them into acoustical phase and they will play togther!


    It is something you need to listen out for, when playing with the phase switch often the louder of the two settings is the best.
     
  4. alphabet

    alphabet
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    Thank for the responses.

    I did switch the phase on my Sub and it made a huge difference!

    My problem was that in stereo playback the sub was working, but not as I would have liked. I have MS908 mains which I have tested to play down to at least 30Hz with minimal dropoff and it did not matter if I switched the sub off or on. After reversing the phase, the sub came alive in these situations and reinforces the bass as I wanted it to do.
     
  5. Mr Cat

    Mr Cat
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    cool - what setting is it at, where is the sub in your room - compared to whjere you sit..? - ie at you side, facing you etc..?
     
  6. alphabet

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    I have set the phase to 180 degree reverse. The sub is facing me, below the TV.

    As Eviljohn suggested, try it out with the different phase settings and decide which suits you best.
     
  7. Brash

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    The phase switch will also bring the sub back into alignment with the main speakers if you are using two crossovers on the sub. For instance my Yamaha amp crosses over the main and sub at 100hz, placing a phase shift on the signal (probably 180deg) while the sub itself has a crossover inbuilt, placing another 180deg phase shift. The phase reversal will then bring the sub back into phase with the main speakers. This made a huge difference to the directional clarity of the bass.
     
  8. Nimby

    Nimby
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    Phase is easy to understand if you think in terms of skipping ropes.

    You stand still in the playgound just behind one of a pair of kids facing each other and swinging a skipping rope between them.

    The rope represents the long sound waves coming from one of your main speakers.

    Now another pair of kids comes over and they each stand beside one of the original pair and they too start swinging a skipping rope between them. This rope represents the long sound waves coming from your subwoofer.

    Both ropes are IN PHASE because the two ends of the waves are the same distance from you. As are all the points on the loop of rope in the air.

    Now your main speaker and subwoofer are IN PHASE.

    Now one pair of kids starts to move away from you. They are still swinging their skipping rope and staying exactly the same distance apart from each other.

    The sound wave that the second rope represents has remained exactly the same wavelength. But the ends of the wave (and all points on the loop in the air) no longer match with the original pair of kids.

    The moment the second pair of kids started moving away from you the phase between the ropes changed. They became OUT OF PHASE by a certain amount. (usually measured in degrees)

    When the two loops of rope in the air overlap exactly half way this represents 180 DEGREES OUT OF PHASE.

    Now you understand phase you can add another pair of kids with a third rope to represent your second main speaker. They stand beside the first pair of kids and swing their rope too.

    Now try moving sideways. Away from the first pair of kids and their rope and towards the third pair. What happens? Your speakers are now out of phase with each other. And only you can only guess the phase of the subwoofer if the second pair of naughty kids wander all over the playground ignoring your orders to stand still.

    In a real room with real speakers and subwoofers there are all sorts of reflections of the sound waves. Which can be in phase or out of phase at different frequencies. This is why you get peaks and troughs in your in-room frequency response.

    Nimby
     
  9. micb3rd

    micb3rd
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    NIMBY you post while give a lovley image in my head is a bit simplistic it is not quiet that simple as just distance.

    Your speakers/subwoofer crossover points, crossover type, crossover db per octave slope, subwoofer enclosure type, enclosure size, Speaker enlosure type, Speaker air space, subwoofer location location, User room, listener sitting points, can all have effects on phase.

    BRASH:

    A friend of mine wrote this recently.....

    As you can see the Industarys used "180 degrees" as the same meaning as polarity swap but it is actually they are different.
     
  10. Nimby

    Nimby
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    Sometimes less is more. :lesson:

    If you can come up with a simple memorable image that matches your long list of variations on a simple (low-tech) demonstration of waves and phase then by all means go ahead.

    You can fill the playground with kids and ropes. All you get is more complexity and the information is vastly more difficult to understand.

    My simple explanation didn't even touch on frequency. But that doesn't make the imagary worthless. At a single fixed frequency out of doors my visual demonstration holds up rather well.

    You can write the next chapter if you like.

    Every journey starts with one small step. Just make sure your feet are 180 degrees out of phase or you'll fall flat on your face. :rolleyes:

    Nimby
     
  11. micb3rd

    micb3rd
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    Please don't get me wrong, I'm honestly not trying to be overly Pedant, I'm just tring to fill in extra info where needed.

    A true life representation of your image would actually be a Slinky spring compressing toward the kids.

    Sound waves are Pressure waves, sound waves are mechanical longitudinal waves, like compressing a slinky spring horizontally.

    A very common error is often made when visualising waves.

    "Do not conclude that sound is a transverse wave which has crests and troughs"

    Have a read......

    http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/u11l1c.html

    As you can see the wavelength is commonly measured as the distance from one compression to the next adjacent compression or the distance from one rarefaction to the next adjacent rarefaction.

    So if you plot those pressure compressions aginst time it looks like a traditional transverce wave which is often what we think when talking about sound waves but it is actually not acurate.
     
  12. Nimby

    Nimby
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    You obviously missed the hidden sophistication of my simple imagary! :cool:

    My skipping ropes were forming cigar-shaped audio longwaves in the air! :devil:

    The rarefaction took place at the child and the pressure wave at maximum diameter of the rotating loop. :)

    The loops could also be thought to continue behind each child as endless mirror images of the first cigar-shaped loop just like a string of sausages.

    Less is more! :lesson:

    Regards
    Nimby ;)
     

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