[cyberbri]

An octave is halving/doubling of frequency. So an octave above 80Hz would be 160Hz, and an octave below 80Hz would be 40Hz.

But 80Hz is a good starting point for crossovers, unless the speakers can't do 80Hz. But just because speakers only go to 60Hz, you don't have to set the crossover to 120Hz. 80Hz would likely work fine in that case as well.