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Subwoofer phase

Discussion in 'Subwoofers' started by Kaleb500, May 10, 2004.

  1. Kaleb500

    Kaleb500
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    I got a question about the phase adjustment on the sub. Before that however I will explain what I believe is meant by phase, since I am not 100% sure, and therefore you can correct me if I am wrong.
    As far as I know a speaker and a sub that are in phase mean the cones move in synchronization.

    i.e. sub phase set to 0

    .............sub.....................speaker
    .......... _______..................._______
    ........../.......\................./.......\
    ..............|.........................|
    ..............|.........................|
    ..............\/........................\/

    A sub that operates out of phase means the movement of the cones is reversed so that as one is moving out the other is moving in.

    i.e. sub phase set to 180

    .............sub.....................speaker
    .......... _______..................._______
    ........../.......\................./.......\
    ............../\........................|
    ..............|.........................|
    ............. |.........................\/

    Now it is the general opinion that when the sub is facing the other direction with the phase setting on 0 you are in effect reversing the phase by making the cone face the other way.

    i.e. sub phase set to 0

    .............sub....................speaker
    ....................................._______
    ............../\..................../.......\
    ..............|.........................|
    ..............|.........................|
    ..........\_______/.....................\/

    And therefore have to re-phase it by setting the switch to 180.

    i.e. sub phase set to 180

    .............sub......................speaker
    ....................................._______
    ..............|...................../.......\
    ..............|.........................|
    ..............\/........................|
    ..........\_______/.....................\/

    When we consider how fast the cones are moving we can see that this is a very small adjustment.
    My question is, that if the fronts are set to small then the freqencies will be different between the fronts and the sub, so they will not be synchronised anyway. Therefore is the phase setting more important when the fronts are set too large?
     
  2. eviljohn2

    eviljohn2
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    I think you're right, although I'm not very good at deciphering ASCII diagrams! :laugh:

    The phase control only effects the subwoofer. If the subwoofer receives an "out" signal from the amplifier and the phase is set to 0deg, then the sub driver will also go "out" and vice versa. This is what happens in a normal speaker if it's wired correctly.

    If the phase is set to 180deg and an "out" signal is received, the speaker driver will go "in" and vice versa.

    A continuously variable control can be varied continuously between these two extremes. ;) If you think about, you don't need to do any more than this because you'll be repeating yourself (remember a repeating sinusoidal wave from school?).

    I gather from your diagrams that you're concerned with the actual orientation of the subwoofer in relation to the speakers. This is not how it works, the phase control is used to help eliminate interference between all of the sound waves that are flying around the room when you play some music. This is entirely room dependant with differenct resonant frequencies etc and is quite complicated to work out.

    In my experience, the phase control on a subwoofer doesn't make a lot of difference and is probably the final setting to tweak. Basically, adjust the phase gradually until you get the highest total sound pressure level (SPL) in the room, ideally at your listening position.

    That probably doesn't make a lot of sense (it's difficult to explain in text form but easy to describe in person!) so please read through it again and I or somebody else can help explain the sticking points :smashin:
     
  3. Nimby

    Nimby
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    The phase can only be correct at one particular frequency compared to the main speaker's phase.

    All frequencies have different wavelengths.

    All frequencies reflect repeatedly within the listening room.

    The phase knob times the moment the pressure waves leave the subwoofer cone at all frequencies (within its capacity).

    Think of it as a delay knob.

    It delays the instant of the pressure wave leaving the subwoofer.

    The phase control should be used to flatten the frequency response when testing with an SPL meter.

    Since it can only have an effect at a particular narrow band of frequencies it can be used to fill in a trough or flatten a hump at a particular frequency.

    This is why most people find it doesn't seem to affect anything. They expect to hear a real difference in the sound.

    But the phase knob is only affecting the timing of the pressure wave leaving the sub. If no out-of-phase effects are happening at that particular moment there is nothing to hear.

    Phase is also position dependant.

    So standing near the sub and turning the phase knob will not have the same effect as doing it from the listening position with a long stick. (Or using a helpful assistant) Since the relative distance between the sub and the speakers back to the listening position affects the phase.

    Moving the sub OR the speakers in the room is the same as turning the phase knob. (but only as far as phase is concerned)

    The pressure waves catch up with each other (in phase) or cancel each other when crest meets trough. (out of phase)

    So unless you have a particular problem to solve during frequency response testing: (trough or boom) You may as well leave the phase knob alone.

    Nimby
     
  4. Kaleb500

    Kaleb500
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    Cool thanks for the answers, i think i understand better. I can certainly see the logic behind the fact that the phase can only be perfected within a narrow frequency band. I must get myself a SPL meter :)
     
  5. ancientgeek

    ancientgeek
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    It doesn't make much difference which way the sub is pointing; sound travels at about 1000ft per second, so it will get round a 2 ft square sub in about 3 msec. whereas a 50Hz bass wave is about 20 ft long. So the phase setting is only much use for correcting swapped connections to the sub.

    With an AV amp, setting the distance to the listening position is more important. But what you hear also comes from reflections off the walls. Imagine walls floor and ceiling were mirrors. The reflected sounds appear to come from the mirror subs you see all around you. They are all at different distances. In a small room they are going to cancel out low frequencies. In a larger room there are still going to be peaks and troughs of bass response where the "mirror subs" boost or cancel the direct sound at particular frequencies. This problem is the reason why it's simplest to set all your speakers to "small" when setting up surround sound. You'll more easily get well defined bass across the listening area if you position your sub suitably.

    If you have an AV amp that auto-calibrates the timing of the speakers, then it will do its best to average the timing from the sub and its reflections.
     
  6. Nimby

    Nimby
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    ancientgeek

    Not wishing to be argumentative and just to further this polite discussion: :)

    I believe the purpose of placing the subwoofer close to a corner is to avoid cancellation reflections and achieve maximum room gain with the flattest response.

    If loudspeakers are really self-cancelling in the bass then not many large floorstanders would be sold for stereo.

    The addition of a subwoofer is becoming ever more popular for stereo listening thanks to availability of low distortion, near subsonic output. Which can underpin the main speaker's natural, in-room bass roll-off. Without messing up the higher frequencies with added distortion or artificial coloration.

    It is generally considered good practice to place the subwoofer near a corner AND near one of the main speakers to achieve the best possible response from the listening position.

    Though your point about using a "small" setting is probably true when the sub is placed anywhere but in a corner and/or close to a main speaker.

    Would you agree with the above? :)

    Nimby
     
  7. ancientgeek

    ancientgeek
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    I agree.

    I am not an expert, bu I do understand it a bit.

    Loudspeakers aren't exactly self cancelling for bass, but with more than one source in the room, they do self cancel for certain frequencies at certain points in the room, making it hard to achieve a consistent setup, especially with listeners not all in the same place. This is why speaker position and wall treatment are so important, even for a single listener in front of a stereo pair of speakers.

    Restricting the bass to a single source (the sub) by setting speakers to "small" helps to keep it simple.

    Placing the sub in the corner puts the nearest "mirror subs" right next to the real sub, and the other mirrors far away. So nearly all the sound arrives at any listener in phase, and again it's easier to set up.

    If you can afford an AX10, the equalisation curves it shows for each speaker are very revealing on this subject. Identical speakers at different points in the room will have very different curves.

    On the subject of large floorstanders, their advantages remain whether you've got sound reflection/delay problems or not.
     

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