Time-aligning subs and tops

What happens in the crossover region

Around the crossover frequency, sub and top play the same notes — both at once, with decreasing share. That is exactly where their contributions add up, and how they add up depends on whether they arrive together at the ear.

If they arrive half a cycle apart, they cancel instead of adding. At 100 Hz half a cycle is 5 milliseconds — or 1.70 metres. That little is enough to turn a 6 dB gain into a hole.

Why the tape measure is not enough

The obvious approach is to measure distances: sub 4 metres, top 5 metres, so the sub gets 2.9 milliseconds of delay. That is half the calculation — and usually the smaller half.

Because a loudspeaker does not emit sound the instant the signal reaches its input. An active box with its own DSP needs time for that: converters, filters, protection. Sub and top are different devices with different internal latency, and the difference is often several milliseconds — more than the path difference amounts to. Datasheets rarely state it, and when they do, it is for a setting you may not be using.

What counts instead

• Arrival time, not distance. Measure from the mix position when the sound actually arrives — once with the sub only, once with the tops only. The difference contains path AND internal latency, so everything that matters.
• Polarity, in the same breath. If one of the two paths is inverted, no amount of delay helps: shifted by half a cycle, the crossover is right at one frequency and wrong at all others. Polarity first, delay second.
• One point, not the whole room. Time alignment holds for one place. Step ten metres sideways and the distances differ — so does the crossover. Align for the area where the audience stands, not the last row and not the mix position against the wall.
• Verify, do not trust. After aligning, the crossover region must have more level than before, not less. If it gets quieter there, the direction was wrong or the polarity is off. That is the whole test, and it takes a minute.