Understanding Loudspeaker Power Ratings

Well, what can we say. Feedback is the bain of a sound engineer's life. The 'howl' or 'squeal' produced by sound systems can be extremely distracting during a performance as well as hurting your ears. However, by understanding what causes feedback and using good techniques combined with the right equipment, you can minimise the chances of feedback occurring.

How does it occur?

Feedback is merely the result of an audio-loop and can affect any system with speakers and microphones. Sound from the microphone is amplified and fed out through the speaker. Some of this output sound may be picked up by the microphone. If the level of this is high enough it will be amplified and output through the speaker, picked up by the mic again, amplified again, and so on.... This quickly builds up into the ear piercing whine associated with feedback. In effect, the output is 'fed back' into the input, amplified, and output again at a higher level.

So how do we stop it?

There are a number of factors that will influence feedback.

Microphone placement. If you place the microphone right in front of the loudspeaker then you are going to get feedback regardless (unless the gain is right down). Moving the mic out of the dispersion area in front of the loudspeaker, to the side or behind, will drastically reduce the amount of output sound picked up and 'fed back' into the loop. So, as a rule of thumb, keep the microphones as far out of the speaker's soundfield as possible.

Microphone Type. Microphones fall into different pick-up patterns; cardioid, hyper-cardioid, omni and figure-of-eight. Most microphones used for live work are cardioid which means that they will pick up mainly from the front but also, in decreasing sensitivity, from the sides. Hypercardioid mics are superior in the fact that their pickup pattern is more directional and less likely to pick up the output from the speakers. Always use the most directional microphones possible as these are less likely to pick up sound from unwanted sources. It is rare to use Omni-directional microphones for live work except in a few specific situations as they capture sound from all directions and are much more likely to pick up the sound coming from the loudspeakers.

Room Acoustics. A room full of hard surfaces will generally yield more feedback issues than a room with absorbent surfaces like carpets and curtains. If there is time and budget to add drapes to a venue to act as acoustic baffles then the sound will be much easier to manage. Too many soft surfaces, however, will deaden the sound too much so a compromise usually has to be reached.

Signal Processing. It is highly unlikely that the previous techniques will completely eliminate feedback so a little signal processing may be required. Feedback, contrary to popular belief, does not occur across the entire frequency spectrum. Instead, it will be limited to a number of frequency bands, these bands being determined by room acoustics. If you can identify these bands you can reduce the signal at these points which will hopefully allow an overall increase in gain for the system.

Now to the tools to help remove the offending frequencies. Firstly the Graphic Equaliser or EQ for short. These come in various configurations but the type that will be useful is a 1/3 Octave version which will normally have 30 or 31 bands, for example our BSS FCS966. Identifying the bands to cut is really a job to be done by ear. Try the following, known to some as "squealing the room out".

Have the room kept silent. Clear out noisy people if you have to.

Set all the microphones to the gain they will be operated at.

Start pushing up the overall gain of the system until feedback begins.

Locate the frequency that is causing the problem by cutting each frequency in turn until the feedback vanishes.

Now push the overall system gain up until more feedback occurs. Again, cut the offending freqency. It may be the same frequency or it may be a different one.

Repeat the above process until you have managed to get an acceptable amount of gain from the system

This method, although simple, is not always suitable. In a room with poor acoustics, you may find yourself cutting so many frequency bands that the sound/music will be adversely affected. One solution to this is to use an EQ with frequency bands much narrower than 1/3 Octave. Some parametric EQ's can offer this as well as some specialist units like the Behringer Feedback Destroyer. This offers EQ filters as narrow as 1/60 of an octave which hardly affect the musical content at all. The unit also offers automatic detection of feedback frequencies which can save a lot of time.