Aux Fed Sub Phase relationship with mains?

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This topic contains 3 replies, has 3 voices, and was last updated by Profile photo of NicholasCatrow NicholasCatrow 7 years, 6 months ago.

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    Profile photo of dymondaudio

    Hi all

    Can anyone confirm that the aux fed sub is in perfect phase with the mains? Has anyone ever measured it?

    I normally always use aux feb sub but some speaker manufacturers have recommended me against it because of phase shifts at the crossover point etc.

    Profile photo of John-S

    There are a lot of issues that can happen at x-over points. Since the room and equipment conditions are not audiophile quality you can do this down and dirty. Filter frequencies on the mains and subs should not have overlap. That would cause huge boosts in level at the duplicated frequencies.

    Phase shift can be pretty much dialed out with delay on the forward most speaker.

    The process is not difficult. Ideally it should be done out of doors to eliminate reflections. You do not need to adjust the system at a very high level either so you shouldn’t piss off the neighbors. All you need is a variable sine wave oscillator, two matching microphones (like Shure SM57) and a two channel mixer with a phase invert switch on both the mic inputs and the sub aux feed (my digital board has all of these). Without knowing your level of experience no one would know where to start in the process. Otherwise describing the steps here would take allot of time. Do you know someone who can help you with this process?

    -Decide on x-over freq
    -Oscillator at one octave above x-over freq
    -Calibrate the mics one woofer speaker touching each other pointing like flashlights onto cone cap. with one mic inverted. Bring one input meter to 0 VU. Mix the inverted mic and tune fader for deepest null on meter. You should be able to obtain a null of at least -30 db.
    -Mixer inputs are now calibrated. Do not change inputs again.
    -place mics closest to both subwoofer and woofer cone cap you can get without touching the grill. Both mics exact same distance from respective cone caps please.
    -Set osc to middle of x-over freq.
    -Using sub gain, phase and delay (in that order), tune for deepest null. Delay is in the order of one millisecond per foot difference in mics from each other. 1 foot start with 1 millisecond. Close enough to start at subwoofer frequencies.
    -sweep osc up and down one octave from x-over freq. Confirm null us present (mostly, not perfect). If not you did something wrong.
    -You have achieved unity at x-over freq.
    -Set osc to x-over freq again.
    -Flip phase of sub. Does it get louder and softer with phase flip? Leave at loudest phase.

    You are done.

    Someone reading this may have a link to another process that is already written. I do not, sorry.


    Profile photo of SteffenR

    this is related to measurement software SATlive, but describes the process in general

    Initial question:
    the inputs have a fixed delay to all busses, every buss adds extra delay, so using only channels is best way to have time aligned signals.
    Remember DCA is not adding delay because it only controls the faders of the desired channles.

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