Subwoofers are still an oddity to many. The average mobile Dj, band or performer just doesn't really understand why their system would need a dedicated subwoofer, and just see it as an extravagance. It's a common misconception that buying a pair of 12 inch or 15 inch active speakers should cover everything you throw at them and not grumble.
Reproduction of bass, especially subsonic frequencies, is an incredibly hard thing for the average speaker to do. This is due to several factors, some being physical mechanical restraints and some electrical.
While crossover frequencies and multi frequency signal handling play a large part, and huge current demands of the connected amplifier mixed with large heat energy waste also cause issues, it's the large driver movements and vast air pressure created that really defines a units suitability.
The typical consumer subwoofer will have a frequency range of 20 - 200 Hz, which gives a wide scope for incorporating into systems. These frequencies can be filtered or cut off for true subsonic use, where live concert subs are set to around 100Hz, and the THX reference for cinema use is 80Hz, so is literally just reproducing the low low bass and nothing more.
Human hearing is generally rated at 20 Hz up to a peak of 20 kHz being audible, with anything below 20 Hz being a physical rumble you are aware of more than an actual sound.
Hearing is at its most sensitive between 1 kHz and 4 kHz, and humans can detect a 3 kHz sound at a 0 dB level, but would require a 40 dB level to detect a low 100 Hz signal, which is an amplitude gain of 100.
Put into real world applications, this means the ability to produce sub frequencies at loud volume requires a unit that can handle vast SPL levels, much much larger than you would need for midrange, treble, and even standard bass. This in turn means the ability to deal with huge amounts of air pressure, and deal with the large current needed to move an oversized driver in such force.
Unit size and power
Many modern TV soundbars and home cinema systems come with subwoofer units of just 8” or 10” driver size, with relatively low power ratings. This is due to their being supplied a programmed bass signal from digital soundtracks, and only being used more for adding a bit of low end to effects like explosions. Plus the fact that your entire system will only ever be at moderate to low house volume.
Things change quickly though when you want a sub to shake a dance floor and be heard over your large driver sized PA cabs that are running hard. The force and pressure levels involved in making those ultra low notes audible at that volume means your 8” sub now needs to be an 18” sub, with an amplifier ten times more powerful in order for it to handle the current and heat generated. A pro level active subwoofer will often have a 1200 watt mono amplifier to drive a 15 or 18 inch driver as any less just won't get the results.
This electrical power rating is also directly partnered with cabinet design and construction, as the internal air pressure produced is huge, and is coupled with the enormous vibrations present at these frequencies, which is why you will rarely find a lightweight subwoofer for live sound use as most are built from dense 18 or 20mm ply with adequate chamber size to offset the internal pressure and aid the driver. This generally means a bloomin big lump of cabinet ! Sadly for your back, it simply has to be this way in order to be effective
The offset of having to carry such a beast around with you is what it gives you. Obviously the ability to produce loud and room filling sub bass is always great, as it gives such a mighty punch to the sound of your system and can happily handle any musical style with ease, no matter how heavy.
The longer lasting and arguably even more beneficial reason is the ability to eq your full range PA speakers for use as midrange tops rather than them having to do everything. One of the biggest reasons for a poor sounding system when used at high volume is that the main drivers become overworked trying to reproduce both bass and midrange at the same time, and eventually failing at one in favour of the other (bass usually wins), which then leads to the mids being boosted at the mixer, which just ends up clipping the amplifier into distortion. You end up with a very loud but very nasty sounding result, which resembles bass mixed with heavy static.
Feeding your main speakers through a subwoofer allows you to set the crossover to filter off the heavier bass (say anything below 120 Hz) away from the top pair allowing their main driver to deal purely with light bass and midrange, which takes a massive stress off of both the speaker and amplifier as its predominantly low frequency reproduction that causes excess thermal build up. Taking this away results in far more efficient running of the amplifier which not only results in far better sound quality and definition, but will also allow higher volume levels from the system as a whole.
For serious power output and the best sound clarity, you can use two separate subwoofers, one for each top speaker, which gives you a huge frequency spread and allows the subs to be run more efficiently as they share the work. Having a pair also allows them to be set to deal with different sub frequency ranges, giving you a full subsonic sweep in stereo. They also give you a heavy and stable base for pole mounting your top speakers, negating the need for speaker stands and creating a serious visual impact in the process.