Feedback

One of the most common complaints we hear from people concerning their sound system is, “I can’t get enough volume before I hear a loud ‘tone,’ ‘squeal,’ ‘whistle’ or ‘noise.” What they describe is feedback—in sound system jargon.

When Feedback Occurs

Feedback can occur when you have a microphone, a loudspeaker and a path between the two. A person talks into the microphone and the sound is converted into a tiny electrical signal. It is magnified many times by the amplifier and then sent to the loudspeaker. The large electrical signal is converted back into sound that is louder than the original. The amount of amplification is called the gain of the system. As the gain of the system is increased, the loudspeaker will put more and more acoustic energy into the room. This energy bounces around the room. Some is reflected back into the microphone and is amplified again. As the gain of the amplifier is increased, the level the microphone picks up from the loudspeaker is increased. At some point, the level the microphone receives from the talker and the loudspeaker will be the same. This is unity gain. If we increase the gain any more, we will hear a tone- feedback. The system is going into oscillation, and it can build up so loud that the system will self-destruct if the gain is not decreased.

Figure 1
Positioning greatly affects gain before feedback.

More Gain and Less Feedback

Now you may ask, “How can I get more gain before feedback?”

The distance between the loudspeaker and the microphone determines the maximum absolute acoustic gain, Figure 1. Decrease this distance and you will decrease the absolute gain. Increasing this distance means the sound from the loudspeaker must travel a greater distance. That means an increase in the absolute acoustic gain.

But what if you can’t move the loudspeaker or the microphone? It is possible to increase the apparent acoustic gain without changing the absolute gain.

Have the talker move closer to the microphone. The microphone now receives a louder signal; in turn, the loudspeaker will reproduce a louder signal without increasing the gain of the amplifier.

Move the listener closer to the loudspeaker. The sound will appear to the audience to be louder.
Other Factors and Components Affecting Feedback

There are other factors that ensure your system is capable of achieving its maximum absolute acoustic gain. First, you must have high quality components. For instance, the microphone must have a flat frequency response. In other words, it should pick up all the different pitches or frequencies identically. If the microphone is more sensitive at a particular frequency, then the system will tend to feedback at that frequency. The same is true for loudspeakers. They must be able to reproduce each frequency identically or feedback will occur at those frequencies which are higher in level.

An equalizer can help by cutting certain frequencies. Let’s say a particular microphone has a peak in its response at 6000 Hz and feedback is occurring at that frequency. An equalizer can electronically cut the response of the loudspeaker system at 6000 Hz. The total system response curve has been flattened and the gain can be increased until we hear feedback at another frequency. Careful adjustment can often achieve considerably more gain in a sound system.


Figure 2
Digital automatic feedback controller.

A relatively new type of equalizer is the digital automatic feedback controller, Figure 2. These units sense feedback and insert a very narrow filter on the offending frequency. It is really a parametric equalizer that is set automatically. Some units are now available that have both a computer based multi-channel parametric or 1/3 octave band equalizer in addition to the automatic feedback filters. These are powerful tools when used correctly.

A word of caution:
An equalizer can do (and often does) more harm than good if it is improperly adjusted. It takes testing equipment and prudent use to realize the maximum benefit from an equalizer. Also, please note, an octave equalizer is ineffective to reduce feedback because it affects so many frequencies at once. A 1/3 octave band or parametric equalizer must be used.

There are other factors that also affect the available gain. For example, loudspeaker directivity (pattern), the room acoustics, reflections, the pickup pattern of the microphone, and the number of microphones that are on will all influence the system’s available gain.

Summary

Briefly, to increase your system’s gain: One, separate the loudspeaker and microphone as much as is practical (but no more than 45 feet). Two, use high quality components. Be sure to have them adjusted and tuned properly. Three, only the microphone(s) being used should be turned on. Remember, less is usually better. And four, be sure the talker is close enough to the microphone to assure a feedback-free system.

ron huisinga