What does that subwoofer ‘phase’ switch do?

Phase reversal Most subwoofers have a switch on the back that’s labelled ‘Phase’, and has positions of 0 and 180 degrees. What this switch does, electrically, is the second easist thing to understand on a subwoofer’s wiring (the easiest is the ‘on/off’ switch).

Put the switch one way, and the two conductors carrying signal from the input are connected to the rest of the electronics in one way. Flick the switch and the two conductors swap position. This means that the waveform is turned upside down (which is what engineers mean when they talk about reversing phase, or switching it 180 degrees).

But what does that mean in terms of the listening experience?

The image on the right is my frequency response graph, measured last night, of a Miller and Kreisel K5 satellite speaker and a K-9 subwoofer working together. This shows the frequency performance from 55 to perhaps 350 hertz. All conditions were identical between these two measurements, except that the Phase switch was in the ‘+’ position in the top one, and the ‘-‘ position in the bottom one. M&K recommends that you set the switch to the position which gives the greatest amount of perceived mid and upper bass at the normal listening position.

As you can see from the graph, with the switch in the wrong position for the particular room setup, more than an octave from below 100 hertz to 220 hertz disappears, down in level by an average of about 15 decibels. Nothing subtle about this at all. You can clearly hear the difference.

The reason is that in the area where both the subwoofer and the satellite speakers are producing output, their respective outputs interfere destructively with the switch one way, and constructively the other way. Which way depends upon such things as the locations of the satellites and the sub, the distances between them and the listening position, and the room boundaries. So experiment, otherwise you may not be getting all the bass you’ve paid for.

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3 Responses to What does that subwoofer ‘phase’ switch do?

  1. tom0mason says:

    The switch only inverts the signal and this is NOT the same as 180° phase shift. A 180° phase shift is a timing correction not a signal invertion!
    On symetrical waveforms (like sinewaves, or 1:1 mark-space ratio squarewaves) it will looks like it has been shifted by 180° but this is only an attribute of it being a symetrical waveform. Inverting this sinewave still has the wrong timing when compared to the signal source displaced by 180°.
    On non-symetrical waveforms (musical signals with a very sharp but short time positive swing followed by a longer period lower level negative swing; like that from plucked instruments) they will look entirely incorrect as well as the timing being wrong. This is because when all the filtered parts are combine (at your ear) the inverted filter channel now has a timing and amplitude errors when compared to the other non-inverted (filtered) channels.
    If you look at the envelope of a sound (say a piano) the initial strike of the piano is not symetrical, indeed most acoustic, and electro-acoustic, instruments have this property. Thus when these are processed by the crossover with one driver’s signal being inverted most musical waveforms will now have both envelope and timing errors imposed on them because one frequency band out of the speaker system has an inverted output compared to the rest of the sound signal.

    True hi-fi is about signal intergrity, testing with sinewaves does not show these subtle timing issue in parts of the signal spectrum that has had signal invertion applied. That is because the sinewave is symetrical – – the positive and negative parts of the waveform are exactly the same shape, and the same amount of energy in them.
    A better method would be to appy a timing filter to the frequency band that requires a phase shift. To do this accurately a carefully designed digital system is required.

    How much this affects the sound and fidelity of the percieved sound is debatable. I believe it is the major issue that give very good systems there particular signature sound. I also think this is why some lesser systems sound as if the sound stage has been smeared and lacking coherance on certain voices and instruments.

    Thank-you TM.

  2. Robert says:

    So, TM, if I m listening to (A) rock with mostly guitar and drum, (B) jazz with mostly horns, or (C) classical with mostly strings and piano, and have a front firing/vented port, where in the 0 – 180 phase would best be suited to each type of music? I can estimate this based on your comments but would appreciate your articulation. Thanks.

  3. Chris says:

    Robert – I think the lack of response from tomomason is a timing issue, not a signal issue. But tomomason’s explanation was so inciteful I don’t blame you! If only your question wasn’t 4 years later. . . 🙂

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