Clearing up the confusion as to what this approach actuallyencompasses and what it accomplishes. by Tom Young

Several years ago, a technique most commonly called “aux fed subs” has beendeveloped for reducing low-frequency “muddiness” from front-of-house loudspeaker systems. This technique has resulted in considerable benefits over awide variety of sound reinforcement applications, but there is confusion as towhat it encompasses and what it accomplishes.

Let’s clarify the details and describe the set up of an aux fed subwoofer system. We’ll also address some frequently asked questions on the topic.

In a traditional loudspeaker system where subwoofers are used, the primarymix bus from the house mixing console feeds the loudspeaker system processor. This provides the separate output that in turn feeds the subwoofer amplifier and loudspeakers.

The crossover parameters for the subs are derived from measurement ofboth the subs and the midrange drivers next up in the frequency scale of the loudspeaker system. Most often the crossover point between the subs andmidrange is chosen for the best (most equal) phase response to ensure coherence through the crossover region.

Electrical delay is likely to be required to achieve this alignment due to theposition of the subwoofers versus the position of the full-range loudspeakers.Once crossover points have been set, the relative gain levels (and etc.) mustnot be altered, because this will change both the crossover point and phasealignment of the system.

In an aux fed sub system, each input to be fed into the subwoofer system issent through the house console’s “post-fader/post-EQ” aux send, from eachselected channel. By routing through this aux send, any changes in channelfader position will result in a comparable change in the subwoofer level (justas would be the case in a traditional subwoofer system). A properly configured and operated aux-fed subwoofer system will maintain the gain structureand the crossover relationships that have been derived from the system optimization process.

What’s It Mean?

The significance of assigning specific inputs to subwoofers lies in removing allother channels from the subwoofer mix. These channels are not just attenuated so many dB at whatever high-pass frequency you have available, theyare completely removed from the subwoofers and are also attenuated (by asmuch as 24 dB-per-octave) below the high-pass frequency feeding the fullrange loudspeakers.

This is most effective with those microphones that are not closely positioned to the source and which therefore are very likely to be picking upambient sound — mostly LF rumble and/or leakage. Mics used for pickup ofchoir, horn sections and string sections are the most frequently encounteredthat fall into this category.

Other common microphone applications are also very good candidatesfor removal from the subwoofer mix, such as acoustic piano, where externallow-frequency leakage is prone to be focused (by the piano body) into themicrophones. Ditto male vocals using cardioid handheld microphones (withproximity effect).

For spoken-word applications and especially with cardioid lavalier microphones, the normally huge LF explosions that occur through subwooferswhen the person talking uses plosive consonants (B, F, P, T, etc) are reduceddramatically.

In fact, when mixing pop music on virtually any reinforcement system, therewill 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 ofenergy below (approximately) 100 Hz.

Getting It Done

Normally, the most effective and readily available tool is the high-pass filterprovided on the console’s input channels, in the form of fixed or sweepablecorner frequency with (typically) a 12 dB-per-octave filter. But high-pass filtershave restricted effectiveness near their corner frequency and there will remainat least some low-frequency (LF) energy that is still fed into the subwoofers.

And note that even with high-pass filters engaged, groups of microphonespositioned in the same area have an accumulative and substantial amount of LFenergy that is passed into the subwoofers. The result is an almost omnipresentand collective mish-mash of LF energy that serves no useful function but has anegative impact on the clarity of those instruments that we intentionally willattempt to reinforce, or enhance, in the subwoofer frequency range.

The best example I’ve seen to illustrate this point: choir microphones. Inalmost any church featuring either contemporary praise music or gospelmusic, 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-peroctave, and if a corner frequency of 100 Hz is chosen, this equates to theresponse for each mic being -3 dB at 100 Hz and -15 dB at 50 Hz (one octavelower). This looks (on paper) and sounds (when soloing) like a significantreduction in LF junk.

But think about this: these choir mics are employed in “area mic’ing” (notclose) 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 energythan they do individually, primarily at low frequencies. (Note: two or more micspick up more mid- and high-frequency energy as well, but at these higher frequencies there is both addition and cancellation at various frequencies and theresulting 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 100Hz and -9 dB at 50 Hz. Now double it again for eight mics and the combinedresponse is +6 dB at 100 Hz and -6 dB at 50 Hz.

The Bottom Line

To summarize: despite the use of the high-pass filters, there’s still significantbass energy that is unintentionally sent into the subwoofers. This LF energyis not only unnecessary from a musical standpoint, but it also robs powerfrom the subwoofer system, it competes with the instruments that we areattempting to enhance with the subwoofers, and it also adds to the quagmireof destructive LF energy that is projected out into the house.

In our example, we are describing what one encounters when doing mostlarger-scale productions. There are many other commonly encountered scenarios that represent similar opportunities for an aux-fed subwoofer system.

Classical symphonic music reinforcement, ethnic music, jazz big band, musical theatre, outdoor shows with wind noise and others present their ownunique mic applications that will benefit from this technique. Even smallerevents (such as basic R&B/folk/blues) can be audibly improved by getting thevocal mics, drum overheads and horn mics completely out of the subwoofers.

As is always the case in professional sound reinforcement, one must notmisapply this technique. There are events that should not be treated in this manner, such as acoustic jazz ensembles, where piano and drums mics areused “full bandwidth” because the overall LF energy is less dense and the subbass content from these instruments can be put to good use when enhanced through the subwoofers.

For such events, all that is required is to set the aux controls on each inputat the “unity gain” position and this results in exactly the same signal contentas in a traditional system.

FAQS ABOUT AUX-FED SUBWOOFERS

Doesn’t an aux-fed subwoofer system alter the gain structure inthe crossover/processor, and therefore, corrupt the crossoverpoints for the FOH loudspeaker system?

‍No. An aux-fed subwoofer system as described in this article is set up exactlylike a traditional system and those channels that are assigned to the subwoofers are operated at a set (“calibrated”) level, resulting in no change to therelative level that the subwoofers are fed. Aux-fed subwoofers should not beused by novice operators who may change the aux send settings without realization of the consequences.

Some describe an aux-fed system as an “effects” system. Is this correct?

Although there are a few who employ aux-fed subwoofers as an “effect” —by altering the aux send level for specific channels here and there during the performance — the intent for the majority of aux-fed subwoofer systems isto exactly mimic that of a traditional subwoofer system and to not vary thegain of each channel’s aux send. Any variation in the aux send levels for those channels assigned to the subwoofers will result in degradation of the balancebetween the subwoofers and the rest of the loudspeaker system. This willnegatively impact the crossover and phase response of the system.

What happens when I do a fade-out from the master faders?

Doesn’t this leave the subwoofers on?If you do fade out the masters on the console, you must also fade out themaster aux send for the subs. Consoles with VCAs obviously provide a much better means for this.

Will I need an additional crossover processor for an aux-fed subwoofer system?

Perhaps. In an aux-fed subwoofer system you will need a separate crossoverinput and output (plus filters). Many modern digital “loudspeaker management” systems have additional inputs and outputs beyond those used forstereo bi- or tri-amping. If you don’t have this input/output available you willneed an additional stand-alone crossover.

What are the restrictions as far as crossover frequency between subwoofers and full-range loudspeakers?

‍Full-range loudspeaker systems with aux-fed subwoofers must be able to function well down to the 100 Hz range. One can go as high as 12 0Hz before thereis clearly an absence of reinforcement in the fundamental frequency range ofmost inputs. But this is not that different than in traditional subwoofer systems.

What are the channels that usually get assigned to an aux-fedsubwoofer system?

‍Bass guitar, upright acoustic bass, kick drum, floor tom(s), low mic on a grandpiano, low mic on a Leslie speaker, electronic keyboards and tape/CD playback.

Doesn’t this technique remove part of the natural frequencyresponse of quite a few of the sources that are not assigned to the aux-fed subwoofers?

‍In theory this may be the case with some sources. But sound reinforcement has always been — and remains — a skill based on compromise. In the example case given in this article, the below 100 Hz response of the choir microphones provides no useful musical information and typically has a destructive effect if sent to the subwoofers. One of the cool things about an aux-fed subwoofer system is that any input channel can be assigned to the subwoofers at the whim of the sound mixers. So experimentation can be conducted for those sources you may have reservations about. Again, as long as the calibrated level on the aux send is maintained, there will be no adverse effects on the crossover alignment of the system.

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The late TOM YOUNG served as a top AV consultant, system designer, and mix engineer over along, diverse career in professional audio.