All About Feedback by Doran Oster, President, Sabine, Inc. www.SabineUSA.com
Ever since Lee DeForest invented the first vacuum tube, engineers have walked the tightrope between feedback and system gain. This article's purpose is to equip you to get all the gain you need without the agony of feedback.
What is acoustic feedback?
Feedback is the loud ringing sound that occurs when the sound leaving a speaker is picked up by a microphone and reamplified again and again. The cycle repeats until the feedback reaches the systems maximum loudness or until someone turns down the volume. Virtually every sound system that has a microphone and a speaker in the same room is susceptible to feedback.
Which frequencies feed back? All acoustic systems have distinct resonant frequencies. Regardless of where you thump a guitars top, it always responds with the same tone. This is the natural resonant frequency of the guitar. It is the frequency where all of the instruments components vibrate naturally as a unit. In sound systems, these resonant points are the frequencies where feedback occurs.
Each of the systems components, including and especially the room itself, has its own set of resonant frequencies. All components added together produce the total systems resonant frequencies. It is almost impossible to predict which frequencies will feed back without first "thumping" the system, but you only have to turn up the amp for them to rudely reveal themselves.
The frequency that feeds back first is the one that requires the least amount of energy to excite resonance. Removing the first culprit, the next feedback frequency will be the one requiring the second least energy, etc.
In order for feedback to occur, the amplifier has to be turned up enough so that sound from the speaker re-enters the microphone louder than the original sound. If there are no surfaces to reflect the sound back to the mic, the sound quickly loses energy, dropping to one quarter the energy every time the distance from the speakers is doubled. By the time the sound finally reaches the microphone, the sound energy is weaker than the original sound, so there is no feedback. From this example we deduce the Prime Directive of Feedback Control: keep the sound emanating from the speakers away from the microphones as much as possible.
Here are the most common tricks of the trade for controlling feedback:
Stand close to the microphone. Speak loudly and clearly so that you do not have to amplify the sound too much.
Each open microphone has a chance to feed back. Mute or turn down the gain of any microphone that is not in use. Noise gates can be helpful for this.
Mount the microphones in fixed positions. Moving the microphone around on the stage increases the chances that the microphone and the speaker will form new resonant paths.
Use cardioid or hyper-cardioid microphones, and point the mics away from the speakers. They pick up much less sound from the back side of the mic, which protects against monitor feedback. Be careful not to put your hand on or too close to the microphones screen, since this can cover the ports that enable the heart-shaped (hence cardioid) rejection pattern.
Place the speakers in front of the microphones so there is not a direct path back to the microphone.
Aim the speakers so the sound does not reflect directly off a wall back into the mic. You can estimate the speakers dispersion pattern (the area that is directly "sprayed" with sound) for the mids and high frequencies by imagining rays of light radiating out of the speakers horns. If you can see the center part of the horn, you are probably in the dispersion pattern. Lower frequency sounds tend to radiate out in all directions from all sides of the speakers.
Make the surfaces of the room as sound absorbent as possible to reduce sound reflections. Use acoustical absorbing tiles in the ceiling, put down carpeting, and hang curtains.
In the real world of most performance spaces, you cannot always follow these anti-feedback techniques. Lead singers insist on pointing the monitors directly at the mic. Worship leaders insist on the mobility of a wireless microphone, and night club owners will not likely carpet the dance floor and hang velvet curtains. Even after youve tried all these tricks, you may still not have enough gain and clarity to satisfy the audience. Do the best you can, and then go on to the next level of feedback control: equalization.
Equalizers (EQs) are sets of filters, or volume controls, for different parts of the audio spectrum.
Since the earliest days, sound engineers have used equalizers for two distinctly different purposes:
1) To improve the tone quality and balance of the sound, and
2) To control feedback for extra gain and microphone mobility. Some types of EQs are best at shaping the tone, and others control feedback better.
It may seem paradoxical to add filters to a sound system in order to increase the gain. But if you can use extremely narrow filters to turn down the frequencies that are feeding back, you will be able to increase the gain of all the other frequencies for a total net gain. There are essentially three categories of equalizers: graphic, parametric and adaptive parametric.
Graphic EQs are basically a set of volume controls for individual sections of the audio spectrum. The earliest music equalizers were the bass and treble tone knobs. As technology advanced, these filters were narrowed to give more precise control. Today, the industry standard is called a 1/3-octave graphic equalizer, which has 31 individual volume controls spaced 3 per octave.
There is a common misconception in the industry about 1/3-octave EQs that is important for this discussion. Many industry veterans incorrectly presume that 1/3-octave EQs use 1/3-octave wide filters. If this were the case, the EQ filters would not be wide enough to create smooth curves. Instead, they would produce a notched frequency response that would make the EQ useless for shaping the sound and useless for controlling feedback frequencies between the sliders. Actually, most manufacturers use 3/4 to 1-octave wide overlapping filters placed on 1/3-octave center points. These wider filters provide the necessary smooth frequency response. Its important to understand that the term "1/3-octave" refers to the spacing of the sliders, not the filter width.
Graphic EQs are excellent for shaping the sound, and they are fairly simple to use. However, using one-octave wide EQ filters to control feedback invariably causes an unnecessary decrease in the gain and fidelity of the program. Its easy to see that if feedback occurs somewhere between the sliders, you will have to pull one of those EQ sliders down pretty far to eliminate feedback. That pulls out plenty of your program, too. On the other hand, youll get considerably more net gain and much better sound quality if you use wide graphic EQ filters for tone control and insist on narrow filters for feedback control. Thats where parametric EQs come in.
In the quest for perfect sound, engineers developed very narrow tuned filters for controlling feedback points in auditoriums. In the early days of sound reinforcement, these filters were custom made to a specific frequency and width for a specific application. Now there are a number of commercially available parametric filter sets that allow engineers to dial in the width, center frequency and depth of the filter.
The problem with parametrics is that theyre expensive; they require much expertise and auxiliary equipment to tune properly; they need constant retuning whenever the room acoustics change, and they are far too slow and cumbersome to catch feedback occuring in the program.
Adaptive Parametric EQ:
The next step in the evolution of feedback control.
An adaptive parametric, such as the Sabine FBX Feedback Exterminator, is essentially a self-tuning parametric EQ. It constantly monitors the program, searching for tones that have the overtone signature of feedback. Once feedback occurs, the FBX automatically places a very narrow, constant-width filter directly on the feedback frequency and lowers it just deep enough to eliminate the ringing sound.
One of the most powerful features of the FBX is that it can eliminate feedback during the program. FBX filters come in two types: fixed and dynamic. Both filters are placed the same way: Feedback is detected, and the filter is placed just deep enough to eliminate it. The difference comes after the filter is placed. Fixed filters remain on the initially detected feedback tone - they do not move. These filters provide the initial maximum gain before feedback and are set automatically during setup. Dynamic filters can release and move to new feedback frequencies and are for adaptive feedback control during the performance. The result is more gain and clarity before feedback and better feedback protection during your program.
Hearing is Believing
To hear the difference for yourself, insert an adaptive parametric EQ in your sound system and bypass it. Mount the mics on stands to fix their positions. Remove as much feedback as possible using your normal method with just the graphic EQ. Next, lower the volume, bypass the graphic EQ, and activate the adaptive parametric EQ. Now slowly raise the gain of the system until at least six adaptive parametric EQ filters have kicked in.
Next, turn down the mics and play your favorite CD through the system. Alternately listen to the system with just the FBX and then just the graphic EQ. You will hear that the adaptive parametric EQ provides much clearer, brighter and louder sound.
There are many ways to help prevent feedback in your system. A good sound engineer will take a comprehensive approach to eliminating it. From mic placement to state-of-the-art adaptive parametric EQs, the goal is good volume, good clarity and good riddance to feedback!