Crossover

When you set the speaker crossover to 80hz the frequencies below 80hz are sent to your subwoofer, or to the other speakers that have a lower crossover or are set to large without crossover

You are talking about the crossover in the AVR (same principles apply to a crossover in a speaker between a woofer and a tweeter)...

- the crossover contains two components, a high-pass filter and a low-pass filter; what gets through the high-pass filter goes to your speakers, what gets through the low-pass filter goes to your sub

- these filters will have a slope that define how the sound falls-off, typically 12db or 24db per octave (more knowledgeable folks will talk about 2nd order and 4th order slopes, way over my head)

- so if you st the crossover in your AVR to 80 Hz that means the two filters are set to 80Hz

- the point about the slopes is that the filters are not brick walls; so not only above 80Hz goes to the speakers, below it also goes, but the further below 80Hz the weaker it will be; if you had a 24db/octave slope then the signal at 40Hz would be reduced by 24db

- so at an 80Hz crossover, your sub will get pretty much everything below 40Hz, most between 40 and 80Hz, some between 80 and 120 and little above; your speakers will get pretty much everything above 120Hz, most between 80 and 120, some between 40 and 80, and pretty much nothing below


Clear as mud

here is a generic visual representation of what the crossover setting is doing.

A proper sub should deliver frequencies below 80hz better then the speakers. Also, by crossing over to a sub wether 80 or whatever frequency you happend to be crossed, help relief the speaker so they dont have to work so hard. But some peolple like to run their speaker full range and cross lower, around 50hz.