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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

By Tom Young March 28, 2019

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.


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About Tom

Tom Young
Tom Young

Principle Consultant, Electroacoustic Design Services
 
The late Tom Young served as principle consultant for Electroacoustic Design Services in Connecticut, and he also designed hundreds of systems for churches and similar spaces.

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South says

Wow that's a very in-depth informative post, thanks for enlightening me.

Dave Tipton says

Nice article from Tom. Glad to see he is not forgotten.

Joe Firriolo says

Greetings,

Great article and I am sorry we lost Mr. Tom Young.
At this time I would like to present a few questions, if I may...

The article states that... "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)."

Now, I understand that part, however what I do not grasp is how to keep the "selected sub-channels" from going out to the "Main Mix" as well, since the channel faders will be up and "active." Keep in mind that I am using small-format analog mixers (Soundcraft, Allen & Heath, Mackie, etc.) and these mixers do not allow for individual channels to be assigned anywhere except the "Main Mix." Also, the Main Mix outputs are feeding directly into active PA (Electro-Voice) speakers. The only thing I can think of is possibly using "Pre-Fader Aux Sends" for the "Sub-Mix" (instead of "Post-Fade) and keeping the individual "Sub-channel" faders all the way down (or muted.) Of course the "Sub-channel" instruments will then have to be mixed using the individual "Pre-Fader Aux-Send knobs of those respective channels, which will add a little work, but wouldn't be that bad. Is there another (easier) way to do this? Please advise.

Thank you for your time and attention.

Sincerely, Joe F. (Bronx, N.Y.)

Dale Piper says

Great article, been doing aux feed subs, for 20 years or more. What I find now that everyone with live stream the live stream mixes lack low end that are from FOH. I always let my kick and bass for example go to my mains also, (I know some who don’t put them in the mains) What I have done to compensate for that is lower my sub master 5db and that puts a little more kick, base, etc to my live stream. I have had listen over and over to my live stream to find a happy medium and still keep a good bump in the house. Alfcrancho.church

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