So we are thinking abuot getting a sub-woofer for our PA. Currently we have a powered board running some passive JBLS mains. I imagine we will need to get a crossover but do we need to get a sub-woofer for each side or can you get away with just one?

I'm thinking you'd need two to get a proper spread of the low end stuff?

If you get one good sub and a good amp to drive it, you won't need a second one unless you start playing large venues. A crossover is highly recommended, but stereo is unnecessary in the LF range, because that frequency range is non-directional in nature (the human ear cannot discern direction in subharmonic frequencies).

Frankly, stereo is a waste in club venues anyway. You rob one side of the audience of half your band when you operate stereo mains, plus you waste amplifier power. Bridged mono for the mains, bridged mono for the sub, and stereo on the monitors only if you require different people to hear different mixes on stage, otherwise mono there as well.

Thanks for the reply. We run mono and not really looking to go stereo. My main concerns were the cost of having to buy two subwoofers versus one. We don't normally play very large places but we've been thinking that adding a sub-woofer and mic'ing the kick might be an overall better sound and maybe encourage more dancing.

You'll find that running a sub will not only improve your low end, but will clean up the high end as well because your other speakers won't have to work so hard. Plus, a lot of times, in order to try to compensate for the lacking bass, people boost low EQ, which ends up muddying up the sound. I'd cross it over around 80-100Hz. Some of the powered subs have built-in crossovers. Just pick yourself up a decent Active Sub and you should be good.

Already fantastic advice here.

I just want to say, Once you've added a bass bin, you will wonder how you ever got by without one.

A passive crossover works for smaller setups, but, if you can afford it, look toward picking up an active crossover in the future.

We used to run 4 of these puppies for our low end:

It does, actually. Shaking their butts in the chair leads to them wanting to shake their butts on the dance floor.

Dan is spot on about the clarity of the top end as well. It's amazing how much lower volume you can run your mains when you have sub support, because you're no longer trying to get frequencies out of the mains that (despite their ratings) they're really not that good at reproducing. Most bands find themselves turning up a lot louder (to try and get some bottom end) which really scares folks off the dance floor because of the racket.

Yes, a passive sub and a good clean PA amp plus a crossover will do wonders for your sound, and you don't need two if you buy one good one and a good amp to drive it.