Some folks that have a no-sub music system that they are, so far, very satisfied with, will add a sub as a supplement for the extreme lows and to give that ‘large space’ feeling when playing back a live event or similar content recording. By a supplement, I mean that no bass management is applied to the mains, and the top end of the crossover for the sub, either in the sub itself or in the processor, is set rather low, maybe 60Hz or 40Hz depending on the mains.

I prefer to apply bass management to both the mains and the sub outputs. I’ve never had a medium or large room for my audio gear, so my mains speakers aren’t all that big. As such, moving the crossover point up to 80Hz or 100Hz means that the woofers in the mains are free to more accurately handle frequencies above that point because the subs now carry all the low end – I say more accurately because the mains woofers aren’t trying to make the large excursions they would otherwise be trying to do when attempting large amplitude low frequencies from a small radiator.

Which brings me to some moving diaphragm theory for a moment. Reproducing high and low frequencies with the same diaphragm means that the high frequencies’ waveforms are ‘going for a ride’ on the lower frequency movements of the driver. This produces a mild modulated Doppler effect for the higher frequencies. Having two-way mains plus a sub is in essence dividing the reproduced frequencies into 3 bands. If those bands are theoretically 20-80 (two octaves), 80-1280 (four octaves), and 1280 to 20K (four octaves), then largest excursions required are made by the driver with the smallest ratio of low-to-high frequencies. This reduces that Doppler effect, a part of what is called inter-modulation distortion. If the mains are three-way and a sub is added, then ratio of the lowest-to-highest frequencies reproduced by one driver can be reduced even further.

As in all things audio, there are trade-offs. Most agree, however, that unless the mains speakers are truly exceptional, adding a subwoofer is a positive step.