It takes a lot of energy to project very low frequencies. This means that in a medium-sized or larger venue, it’s easy for kick drums and low toms, bass guitars, synth basses, and other very low frequency sounds to get lost. If this happens, adding a subwoofer to your PA system is just what the audio doctor ordered. How can you tell? An easy solution is to step out front while band plays — a wireless mic or guitar or bass rig, or even a long cable will let you hear what the front-of-house sounds like. Or, play back a recording that you are intimately familiar with. Can you hear the low end clearly? Do the bass instruments punch through and support the mix? If not, it’s time to beef up your low end with a sub.
A sub reinforces the lowest frequencies, typically below 150Hz, or even below 100 or 80Hz — having a dedicated speaker and power amp to drive those power-hungry low frequencies not only increases the bottom end, it also takes a lot of strain off the other amps and drivers in your sound system, resulting in a clearer sound with more headroom.
Of course, it’s easy to over do it with a sub. Dial it in conservatively, based on your genre — dance club EDM takes a lot more low end support than acoustic jazz, for example — the effect you want to achieve, the volume you’re playing at, the size of the room, and how much you have going on in the bottom end of your music. Start conservatively, and compare your mix with and without the sub to ensure you’re not getting too much thump or boom and unbalancing the sound. You should miss the sub when you turn it off, but it shouldn’t call attention to itself when it’s on.