Feedback Control

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 may actually degrade the sound.

There are two basic approaches to tuning the system. One approach is to “ring out” the system, whereby feedback is induced and the equalization is then applied to knock down individual feedback frequencies (nodes). This procedure is described below. Another approach 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.

Ringing out a sound system is accomplished as follows: Turn up the overall gain (volume) of the system, while someone speaks into a microphone. Feedback will first occur at that frequency (or frequencies) where the system has a peak. It typically begins as a slight ringing, and then becomes a loud howl. 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 band 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 is. Then back 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 may find you've obtained from 3 to 15 dB more usable gain from the system.

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 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 systems.

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. Be sure to ask one of our sales consultants about the equipment choices and methods that would be best for your system.