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A Detailed Explanation Of The Aux-Fed Subwoofer Technique

Clearing up the confusion as to what this approach actually encompasses and what it accomplishes

In our example, we are describing a fairly typical modern church production event or what one encounters when doing most larger-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 own unique mic applications that will benefit from this technique. Even smaller events (such as basic R&B/folk/blues) can be audibly improved by getting the vocal mics, drum overheads and horn mics completely out of the subwoofers.

As is always the case in professional sound reinforcement, one must not misapply this technique. There are events that should not be treated in this manner, such as acoustic jazz ensembles, where piano and drums mics are used “full bandwidth” because the overall LF energy is less dense and the sub-bass 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 input at the “unity gain” position and this results in exactly the same signal content as in a traditional system.

FAQs About Aux-Fed Subwoofers

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

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

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

Answer: 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 is to exactly mimic that of a traditional subwoofer system and to not vary the gain 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 balance between the subwoofers and the rest of the loudspeaker system. This will negatively impact the crossover and phase response of the system.

Question: What happens when I do a fade-out from the master faders ? Doesn’t this leave the subwoofers on?

Answer: If you do fade out the masters on the console, you must also fade out the master aux send for the subs. Consoles with VCAs obviously provide a much better means for this.

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

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

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

Answer: A full-range loudspeaker system 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 there is clearly an absence of reinforcement in the fundamental frequency range of most inputs. But this is not that different than in traditional subwoofer systems.

Question: What are the channels that usually get assigned to an aux-fed subwoofer system?

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

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

Answer: 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.

This article originally appeared on PSW in 2015.

The late Tom Young served as a top consultant, system designer, and mix engineer over a long, diverse career in professional audio. He also helped innumerable sound techs as the moderator of the ProSoundWeb Church Sound Forum here on PSW.

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