Other common microphone applications are also very good candidates for removal from the subwoofer mix, such as acoustic piano, where external low-frequency leakage is prone to be focused (by the piano body) into the microphones. Ditto male vocals using cardioid handheld microphones (with proximity effect).
For spoken-word applications and especially with cardioid lavalier microphones, the normally huge LF explosions that occur through subwoofers when the person talking uses plosive consonants (B, F, P, T, etc) are reduced dramatically.
In fact, when mixing pop music on virtually any reinforcement system, there will very likely be numerous microphone channels that have no useful information that could be enhanced by the subwoofers. For these instruments/sources it is also likely that efforts will be made to reduce the pick up of energy below (approximately) 100 Hz.
Normally, the most effective and readily available tool is the high-pass filter provided on the console’s input channels, in the form of fixed or sweepable corner frequency with (typically) a 12 dB-per-octave filter. But high-pass filters have restricted effectiveness near their corner frequency and there will remain at least some low-frequency (LF) energy that is still fed into the subwoofers.
And note that even with high-pass filters engaged, groups of microphones positioned in the same area have an accumulative and substantial amount of LF energy that is passed into the subwoofers.
The result is an almost omnipresent and collective mish-mash of LF energy that serves no useful function but has a negative impact on the clarity of those instruments that we intentionally will attempt to reinforce, or enhance, in the subwoofer frequency range.
The best example I have seen to illustrate this point: choir microphones. In almost any church featuring either contemporary praise music or gospel music, several (or more) suspended choir mics are (most) likely used.
For purposes of this discussion, let’s say we’ve got eight condenser cardioid choir mics arrayed above, and forward, of the target choir members (or sections). For the majority of experienced sound mixers it is a “no-brainer” to apply channel high-pass filters on each of these microphones to reduce the LF “rumble” that can be heard when soloing (or otherwise monitoring) these mics.
Again, the most common slope for these high pass filters is 12 dB-per-octave, and if a corner frequency of 100 Hz is chosen, this equates to the response for each mic being -3 dB at 100 Hz and –15 dB at 50 Hz (one octave lower). This looks (on paper) and sounds (when soloing) like a significant reduction in LF junk.
But think about this: these choir mics are employed in “area mic’ing” (not close) positions, suspended at least several feet from their target sources. Further, they are combined into the mix bus(s) and will combine acoustically, and to some degree even if panned through a stereo loudspeaker system.
Two choir mics, when combined in the mixer, provide +3 dB more energy than they do individually, primarily at low frequencies. (Note: two or more mics pick up more mid- and high-frequency energy as well, but at these higher frequencies there is both addition and cancellation at various frequencies and the resulting increase is likely to be less than 3 dB per doubling of microphones).
So the high-passed response of two microphones is now flat at 100 Hz and –12 dB at 50 Hz. Double this for four mics and you then have +3 dB at 100 Hz and –9 dB at 50 Hz. Now double it again for eight mics and the combined response is +6 dB at 100 Hz and -6 dB at 50 Hz.
To summarize: despite the use of the high-pass filters, there is still significant bass energy that is unintentionally sent into the subwoofers. This LF energy is not only unnecessary from a musical standpoint, but it also robs power from the subwoofer system, it competes with the instruments that we are attempting to enhance with the subwoofers, and it also adds to the quagmire of destructive LF energy that is projected out into the house.