Feedback Suppression and Room Equalization

Feedback is the interaction between the output of your Sound System (the speakers), the input of your Sound System (the microphones), and the room or physical space surrounding your Sound System.

A change in speaker location, microphone location (or orientation), or changes to the room acoustics (empty vs full of people) changes the feedback characteristics. Therefore any time a significant change is made to any of these variables feedback may occur at a different point or frequency than it did before the change was made.

There are several methods that the typical sound system operator/designer/installer uses to account for these changes: Tone controls on the mixer for each microphone itself, System equalizers to tune the speakers and correct for room resonances, Automatic Feedback killing devices. This article will attempt to explain the typical uses and benefits/pitfalls of each of these methods.

When a sound system feeds back, it happens at a certain frequency and means that the composite sound system (mics, speakers, room) is boosted at that particular frequency above all others, and an oscillation occurs at that single frequency. If we reduce that particular frequency with the aid of some form of equalization we can then increase the volume of the system until the next highest frequency starts to oscillate.

However there are practical limits to the effectiveness of the above process. If you were to achieve a perfect system where there were no frequencies boosted more than any other, feedback would still occur but instead of being at a single frequency, it would occur at all frequencies simultaneously. In practice if you start hearing several frequencies oscillating (ringing) at the same time, you are not going to achieve much more volume in your system through simply tuning with an equalizer.

This feedback problem is unfortunately not limited to the main sound system speakers but any speakers reproducing the sound of the microphones. This includes floor monitor speakers, choir speaker systems, side fills, remote speakers, and in extreme volume situations even headphones. Its important to determine for sure which speaker system is causing the feedback even though they all may reproduce it once it has started.

In large systems all these speaker systems will have their own EQ so they can all be adjusted to achieve a "flat" frequency response.

The three things that affect the amount of amplification you can get out of a sound system before feedback (gain before feedback) are :

1. The distance from your source (the singer or instrument) to the microphone

2. The distance from your microphones to your speakers up to a point.

3. The distance from your speakers to the listeners.

To get more gain before feedback you must decrease #1 or #3 , or increase #2 until the direct sound is equal to the reverberation sound at the microphone (known as critical distance)

Maximizing the above to get the most gain possible has been the reasons in recent years for the rise in headset worn microphones, and the increase in the usage of distributed speaker systems in auditoriums to cover smaller areas instead of a central cluster to cover a larger area from farther away.

The ultimate solution would be of course if we could put the microphones in one room and the audience in a different room then feedback would be eliminated because the sound from the speakers could not get back into the microphones and oscillate at all.

Rarely in live sound is the above option viable. but it is the reason we can turn prerecorded material up as loud as the system can reproduce without worry of feedback.

Main System and Room Equalization

The most common form of feedback solution is the conventional equalizer in one of two varieties: parametric or graphic. An EQ (equalizer) is inserted into the signal path somewhere between the microphones and the amplifiers. Then using a piece of test equipment or test software on a computer, known as an audio RTA (real time analyzer), and a noise generator that can produce pink noise; the system EQ is adjusted to as closely achieve the third graph shown above, a Flat response with no significant peaks or valleys. This result should be an average of several seating positions within the space so a good average is achieved.

This method does not take the frequency response of the individual microphones into account since the noise generator is plugged straight into the mixing board. Also in practice the desired RTA curve is not completely flat unless you are within approximately 10 ft of the speaker. Since most RTA measurements are done from floor level at various positions in the room usually at least 30 to 40 ft from the speakers, the desired curve is usually flat up to around 2 kHz then rolls off smoothly at a rate of approximately 3dB/octave.

This reduction in high end is a result of the absorption of the high frequencies due to air density. All this means is: if you started with your test mic right in front of the speaker and the response is flat, and then you gradually moved the microphone toward the listening position, once you reached the average auditorium seating position the curve would exhibit the roll off described, naturally. If you were to tune the system to be flat at the seating position then moved your test mic toward the speaker, once you reached the speaker the curve would not be flat anymore. The amount of high frequency roll off is a subject still in debate and is one typically left up to the person tuning as to what sounds natural. However the roll off does aid in reducing feedback above 2 kHz.

If the rate of high frequency drop off is greater than 3dB/octave, this would be a sign that a far throw horn or delay speakers might be required for the desired space to obtain a natural sound.

Tuning the Microphones

Typically the mixing board's tone controls are what is used to bring the frequency response of each microphone close to the "flat" response we desire for maximum gain before feedback. Typically each mic can be turned up until it rings slightly and then the ring can be located with the tone controls and reduce the response of the microphone at that frequency. Caution: Sometimes this method can be abused and results in an unnatural sounding microphone. If a mic requires more than a couple of marks of boost or cut the tone controls may not be sufficient to fix the mic's frequency response. Tone controls are pretty broad tuning filters, they boost or cut a fairly large range of frequencies to try an knock a mic's response into shape.

Again, in practice, with a properly tuned main system EQ and the tone controls set "flat" (no boosts or cuts) on the microphone's mixer channel, the mic should sound pretty natural. If it doesn't then you may want to consider if the mic is damaged, or was just inferior to start with. EQ's usually won't fix a cheap mic with acceptable gain before feedback results.

Occasionally there are requirements that go beyond what a console's tone controls can take care of; In these situations it is possible to put in a dedicated equalizer for a single microphone. The mixer must usually have an insert point jack to do this easily. Use of the dedicated EQ effectively will usually require the same test equipment used to tune the main system's EQ and as a result may be cost prohibitive.

Automatic Notch Filters aka. Feedback Killers

Typically if just a little more gain before feedback is desired, if you have a microphone that will be used in the speaker's coverage area, or if you simply can't get the mics close enough to the source (like a choir or a theater play) that is when an Automatic Notch Filter also known as a Feedback Killer can be effective. These devices listen to the audio that passes through them and try and detect feedback. Once they detect feedback they set a very narrow filter at the feedback frequency to try and remove it. Most devices can set about 12 filters max. Any more notches and the audio quality would start to suffer. These units have their place but they are not magic, and they have to be set up correctly to be a benefit. Setup incorrectly and they will just hinder the system's performance. Most units have a fixed filter and dynamic filter mode. I typically use the fixed filter mode for all the filters and set them before the performance. For the dynamic filters to work the feedback has to be audible above the program material, and in my book that's too late. Also some units will falsely set dynamic filters on program material. Make sure the mics and speakers are in the performance locations when setting the filters. If you place the notch filter in the microphone part of the mixer then make sure you have all the speakers on when setting the filters. If you placed the notch filter in the main system make sure you only have on the speakers its connected to. Other speakers making feedback will set notches falsely.

Once your all setup, turn up the gain on the desired microphone slowly until it starts to ring. Make sure to keep the ringing in check because way too loud can damage speaker systems. If the notch filter is setup correctly, it should detect the feedback and notch it out making the tone go away. Then turn the mic up a little more and set the next frequency. If you get multiple frequencies simultaneously turn the mic down a little and see if that helps the unit detect one frequency over the other. If the mic will be moved during the performance have an assistant walk the mic around during this filter setting to try and locate any problem areas.

Conclusions

Equalization is a lot like salt and pepper in cooking, use it properly and you can have wonderful results. However its use won't fix a badly setup system, or inferior system ingredients. Although it may help you make it through dinner. I have heard wonderful sounding systems that had no Equalization at all but those cases are rare since most sound systems are usually an afterthought in room/auditorium design.

Remember the K.I.S.S. method, Keep It Simple Stupid. The less EQing you have to do the better. Try and get the system to maximize the desired qualities of points 1, 2, and 3 above. If you don't need to hear a mic through a floor monitor, turn it off in the floor monitor. Don't feed the choir mics back to the choir, It makes them hard to get loud enough. If they get enough piano naturally, turn that off too. The fewer the number of mics you have turned on, means you can get the few you do have on louder because you have reduced the paths for feedback to travel.

Finally, buy quality equipment when you purchase it. Mic quality and speaker quality are the biggest X factors when it comes to sound systems and they aren't always related to price.

Page Created by Stephen Edmonds. article written by Stephen Edmonds.
Copyright © 2010 [Sound and Communications, Inc.] All rights reserved.
Web Comments? Send mail to webmaster@soundandcom.com
Revised: November 24, 2010 .