Every speaker produces sound waves that move back and forth like a wave in the ocean. "Phase" describes where that wave is in its cycle at any given moment. If two drivers are in phase, their waves add together for full, accurate sound. If they're out of phase, the waves partially cancel each other — like two ripples colliding and flattening out.

The crossover is where one driver hands off to another. Right at that handoff frequency, both drivers play simultaneously. If their phases don't match there, you get a hollow, thin sound — sometimes a noticeable dip — right in that region.

An all-pass filter passes all frequencies at full volume — it doesn't change level at any frequency. What it does change is timing. It rotates the phase of the signal, concentrated around a specific frequency you set. Think of it like adjusting a clock hand: you're not removing anything, just shifting where the wave sits in its cycle so it lines up with the other driver.

This is different from the delay setting in your DSP, which shifts timing equally for all frequencies. An all-pass is more surgical — it concentrates the correction right at the crossover point where the problem lives.

Three parameters control the correction. The order (1st or 2nd) determines the total rotation available — a 1st-order filter rotates up to 180°, hitting 90° at the center frequency. A 2nd-order filter rotates up to 360°, hitting 180° at center. The center frequency is where the rotation is most concentrated — set it at your crossover point. The Q controls how fast the phase changes and how wide the affected bandwidth is. A higher Q narrows the band and steepens the rate of change. A lower Q spreads the correction over a wider frequency range. Most DSPs with all-pass filters expose all three parameters.

The blue trace and the red trace show the phase angle of each driver across frequency. Every crossover filter shifts phase as a side effect of filtering, so the two traces will naturally land at different positions. The purple dashed line marks your crossover frequency. What you're looking for: at the crossover frequency, the two traces should be close together — ideally within 45 degrees. When they're far apart, especially approaching 180 degrees, the drivers are fighting each other right where they need to work together.

This shows what you actually hear — the combined output of both drivers. A flat line near 0 dB means they're adding cleanly. A dip means partial cancellation. A deep notch means significant cancellation. This is audible even to untrained ears as a hollow or recessed sound in that frequency range.

Once you have a measurement mic (miniDSP UMIK-1, Dayton Audio iMM-6, or any calibrated mic) and software (REW is free; the Helix DSP PC app has its own measurement suite), you'll see the same types of traces shown here but from your actual system.