Several years ago, a technique most commonly called “aux fed subs” has been developed for reducing low-frequency “muddiness” from front of house loudspeaker systems.
This technique has resulted in considerable benefits over a wide variety of sound reinforcement applications, but there is confusion as to what 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 be addressing some frequently asked questions on the topic.
In a traditional loudspeaker system where subwoofers are used, the primary mix 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 of both the subs and the midrange drivers next up in the frequency scale of the loudspeaker system. Most often the crossover point between the subs and midrange 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 the position of the subwoofers versus the position of the full-range loudspeakers. Once crossover points have been set, the relative gain levels (and etc.) must not be altered, because this will change both the crossover point and phase alignment of the system.
In an aux fed sub system, each input to be fed into the subwoofer system is sent through the house console’s “post-fader/post-EQ” aux send, from each selected channel. By routing through this aux send, any changes in channel fader position will result in a comparable change in the subwoofer level (just as would be the case in a traditional subwoofer system).
A properly configured and operated aux-fed subwoofer system will maintain the gain structure and the crossover relationships that have been derived from the system optimization process.
The significance of assigning specific inputs to subwoofers lies in removing all other channels from the subwoofer mix. These channels are not just attenuated so many dB at whatever high-pass frequency you have available, they are completely removed from the subwoofers and are also attenuated (by as much as 24 dB-per-octave) below the high-pass frequency feeding the full-range 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 up ambient sound – mostly LF rumble and/or leakage. Mics used for pickup of choir, horn sections and string sections are the most frequently encountered that fall into this category.