Which crossover frequency?

What really sets it

• How high the sub stays clean. An 18-inch driver in a vented box gets sluggish and directional above roughly 100 Hz; a 12-inch can go considerably higher. It is in the datasheet, or audible: above which frequency does it stop sounding like bass and start sounding like a note from a box?
• How low the top may go without suffering. That is the harder limit. Running a 10-inch top at 70 Hz means cone excursion instead of level: distortion rises, the midrange gets muddied, and at full tilt the driver goes. In doubt, cross higher, not lower.
• Where the room stops helping. Below the Schröder frequency, individual room modes dominate; above it, a diffuse field. If your crossover sits right in that transition, the handover behaves differently from seat to seat. That is no reason to change the frequency — but it is a reason to measure at more than one position.

The catch: crossing higher demands tighter timing

In the crossover region, sub and top play at once. Whether they add or cancel depends on arriving together — and how little offset is enough depends on frequency: half a wavelength is 2.86 metres at 60 Hz, 1.72 metres at 100 Hz, and only 1.14 metres at 150 Hz.

The higher you cross, the less offset the handover forgives. Cross at 150 Hz and place the boxes by eye, and you have a good chance of a hole exactly where bass and voice meet. So crossing higher relieves the top — and turns time alignment from fine-tuning into a requirement.

How to verify your choice

Measure the sub alone, the tops alone, and both together. In the crossover region the sum must have MORE level than either path on its own — both are playing there, so they add. If the sum loses level there, the timing or the polarity is wrong, not the frequency.

Then the same test one metre further along. If the handover stays stable, the choice fits this room. If it flips, the crossover is too high for the distance between your boxes — or you need more than one measurement position.