Feedback Control and Room Tuning

An Important Note: Equalization is the last step that should be taken in tuning a sound system. Unless a system is correctly designed and carefully set up, the equalization may not accomplish much, and could actually degrade the sound. Make sure your gain structure is correct before working through this process. There's a podcast on the subject here.

There are three basic approaches to tuning the system:

The easiest method is the use of an automated room equalizer such as the DBX DriveRackPA or the DBX DriveRack260, but you may not always have one of these devices available, nor the several hundred dollars to purchase one. (Let me note in passing that these are wonderful tools for small traveling systems, though they offer significant benefit to installed sound systems as well.)

The most accurate method is to measure the frequency response with a device known as a real-time audio spectrum analyzer, and adjust the equalization to obtain the desired EQ curve for the room. Instruction in the use of spectrum analyzers is beyond the scope of this article, and use of such devices is best left to qualified audio professionals, who will usually charge a substantial “consulting fee”, often as much or more than the automated equalizers.

The third approach is to “ring out” the system, whereby feedback is induced and the equalization is then applied to knock down individual feedback frequencies (nodes). I’ll describe how to ring out a room using a graphic equalizer. (Note that a 30 or 31 band equalizer will give you much better results than a 15 band or smaller equalizer.)

Ringing out a sound system is accomplished as follows: Turn up the overall gain (volume) of the system, while someone speaks into a microphone. Keep turning the volume up until you begin to hear feedback. It typically begins as a slight ringing, and then becomes a loud howl. Feedback will first occur at that frequency (or frequencies) where the system has a peak (it’s better at reproducing this frequency than others) or your room has a standing wave (a particular frequency that seems to resonate between the walls of the room).

Identify the feedback frequency, either by ear or using a spectrum analyzer if you have access to one and know how to use it. Locate the corresponding frequency band on the graphic equalizer and pull that slider down until it stops ringing. If howling commences again, pull the slider down a little more; if it's at a different frequency, pull that slider down.

Eventually, you’ll reach a point where many frequencies all start to howl at once, or where you’ve already adjusted most of the frequencies that begin to howl as you further raise the gain. That's when you can stop the EQ adjustments. You’ve gotten all the gain there available with this.

Finish by backing off the gain by 10 dB or so to leave some headroom (room between the nominal signal level and the peak signal level). When you’re done, you’ll likely find you've obtained from 3 to 15 dB more usable gain from the system than you had before.

It may not be necessary to try to obtain all possible gain before feedback. If you obtain sufficient gain before ringing out every available frequency, stop there. In any case, it’s important that the sound of your system after equalizing still be natural.

Feedback is dependent on the wavelengths of sounds, rather than the frequencies. This means that the frequencies at which feedback nodes occur may change with changes in temperature or humidity (For example, the warmer the air, the faster sound travels: hence a given wavelength will correspond to a higher frequency). Such changes may require that the system be rung out again, especially if very narrow band filters in your EQ are used. The process of “ringing out” should be performed separately for each speaker system – including any stage monitor channels.

There are also some amazing products that have come out in recent years that use a digital process to automatically detect feedback problems and make realtime adjustments with narrow EQ filters to control the feedback. These are best used to correct problems that may arise from time to time after a room has been tuned. They can also be very effective with portable systems, or applications where stage set ups are constantly changing.

By David McLain

David is a church sound and projection system consultant with CCI Solutions in Olympia, WA. He has been working with church AV systems since 1978.
He is also co-podcaster of the Sound Theology Podcast.
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