While the quality of both features and sound have increased dramatically in the last few years on PA speakers, especially on active systems, they are still limited to the frequency ranges that the cabinet design, driver size, and amplifier specification dictate.
A good active speaker does have a lot of bass available for sure, and for many users this will be more than adequate. If you're giving speeches, singing, playing acoustic instruments or just providing background music, then you won't ever be producing signals of a low enough frequency to even get a subwoofer moving, in which case you are obviously well set already. If however, you are in a band where the drummer, or the entire band is being sent through the desk, then you are not only at risk of blowing your bass drivers, but you are also compressing the rest of the signal as the bass reproduction takes priority due to its current pull on the amplifier.
This gets even worse if you are a Dj that's playing bass heavy music, as the speakers are being constantly hit with programmed low frequencies that require the woofer to be in constant heavy movement, causing large heat buildup in both driver and amplifier, whilst it's trying desperately to deliver you the full signal, but failing.
You then end up artificially boosting the missing midrange and treble via the desk, and hitting the speakers with a heavily gained signal. The result is not desirable. If you've ever heard the term ‘wooly’, or that it sounds like there is a blanket over your speakers, then this is the reason.
Running them this way will also quickly lead to the destruction of either your woofers, or your amplifier (internal or external).
This is where a subwoofer comes into play. It is specifically designed to handle the large current draw, air pressure build up, cabinet resonance and extreme driver movement of bass frequencies from 200hz down, and Sub-bass from 60hz down to 20hz, whilst remaining stable and controlled. They are specified with either 15 inch or 18 inch driver units and amplifiers rated at anything from 600 to 1200w as standard. These are over specced this way to allow for the electrical sag and heat loss that occurs during heavy use. The power reserves and cabinet designs allow extreme SPL / dB ratings of up to 140 on the larger units.
The use of such a unit then allows your standard speakers to become ‘mid-tops’, and they can be equaliser adjusted to only handle the bass frequencies of the mid-low range, with the sub handling everything else below that. The difference in sound when running a full system this way is just incredible, and will give your setup the performance and sound quality that running without one simply cannot achieve.
An active sub is by far the simplest way to add in a unit to your system, as its totally self contained. The speaker and cabinet have a purpose designed amplifier and active crossover system built right in, so hookup to your main speakers is simple. The amplifier will be a custom unit designed specifically for the larger current pull and heat that comes with low bass reproduction, and being mounted into the cabinet means the high current feed only has a very short length to the driver, which reduces stress and thermal loss by a huge amount when compared to a passive system with separate amplifier. It will also have been carefully matched to the units driver electrically, so impedance and sensitivity will be at their optimum.
Hookup is simple, with your output feed from your mixer going to the subwoofer, and then your speaker lines coming off the sub to supply the main PA units. The subwoofer then filters off the selected frequencies for itself and passes the rest out to the main pair. It's by far the most simple and effective system upgrade you can do and just a few minutes of use will have you wondering just how you managed so long without one.