|Feedback: What It Is and How to Control It|
|One of the most
common complaints from people concerning their sound system is that they
can't get the sound loud enough before they hear a loud "tone," "squeal,"
"whistle," or "noise."
When Feedback Occurs
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 and some of it arrives back at the microphone and gets amplified again.
As the gain of the
amplifier is increased, the level the microphone picks up from the
loudspeaker increases. At some point, the level the microphone gets from the
talker and the level from the loudspeaker are the same. We are now at unity
gain. If we attempt to raise the system gain a little more, we will start
hearing a tone or feedback. The whole system is going into oscillation, and
it can build up so loud that the system will self-destruct if the gain is
There are several factors which affect gain. First, position of the microphone in relation to the loudspeaker is very important. (Fig. 1) The "absolute maximum gain" is dependant on the acoustical distance between the loudspeaker and the microphone. If we move the loudspeaker closer we will decrease the absolute gain. Moving it farther away will allow us to increase the system gain.
But what if you can't move the loudspeaker or the microphone? One solution is to increase the "apparent acoustic gain" by having the talker move closer to the microphone. The microphone is now receiving an increased level so the loudspeaker will put out a louder signal. Notice that we did not change the gain or volume of the electrical system. We just put a stronger signal into it and so, to the listener, the gain or volume was "apparently" increased. Another way to increase apparent gain is to get closer to the loudspeaker. It will obviously "appear" to be louder.
Other Factors and
Components Affecting Feedback
This is where an equalizer can help. An equalizer is a very elaborate set of tone controls, and we can cut or boost certain frequencies. Let's say a particular microphone has a peak in its response at 6000 hz and that frequency is where feedback is occurring when we try to get more gain. We could install an equalizer and electronically cut the response of the system at 6000 hz. We now have "flattened" the system response, and we can raise the system gain until we hear another frequency starting to ring or feed back. By careful adjustment we can often achieve considerably more gain in the sound system.
A word of caution is in order here. An equalizer can do (and often does) more harm than good if it is improperly adjusted. It takes good test equipment and judicious use to realize the optimum benefit. Also, an octave equalizer is of little use in feedback suppression because it affects so many other frequencies besides the offending ones. A third octave or parametric equalizer must be used.
There are many other factors which can affect the available gain, including loudspeaker directivity (pattern), the room acoustics, reflections, the pickup pattern of the microphone and the number of microphones that are on. We will try to address these factors in later articles.
In summary, there are several things which can be done to achieve acceptable system gain.