Unusual Live Sound Trick

Most music performance spaces are less than ideal acoustically. This can lead to problems with P.A. systems feeding back, and guess who gets blamed for that? The engineer! Here is a quick trick you can use when shape of the room is working against you. Caution, this requires an understanding of phase (polarity) and accurate labeling of cables. Don’t try this at home, or rather; do try this at home before you try it on a gig so you can make sure you did it correctly.

When P.A. systems are properly set up, the main speakers face forward into the room, the monitors face back toward the band members and the microphones are placed behind both the mains and the monitors. Sometimes, even with this ideal setup there is still feedback. Consider that when someone sings into a microphone, the main speaker cones move forward and then backward. The monitor speakers react in the same way, except they are facing the other direction from the main speakers. What you have in this situation is two independent sets of speakers that are working out of phase from each other. This can cause resonant frequencies in the room to be excited and viola, feedback! This feedback is often in the 100-400Hz region. This is the region that is crucial to the fullness of voices and instruments.

Before you reach for the equalizer and start pulling these frequencies out, try this little trick. Reverse the phase of your monitor speakers. What you want to achieve is all the speakers moving in the same direction at the same time so they are in phase. Most mixers and power amps do not provide for phase reversal of the monitors so you will have to modify your cabling. This can be done at any point after the monitor send from the mixer.  If you are using banana plugs on your monitor speaker cables, this is very easy to accomplish. Simply plug the banana cables into the monitors backwards. This is a 180 degree phase (polarity) reversal. If you are not using banana plugs then you will have to open up your cables and physically switch the (+) and (-) wires on one end of the cable only. Let me repeat that, on one end of the cable only! Then immediately label this cable as phase reversed. If you don’t, this cable will eventually get used at the wrong time and it will cause you problems.

So there you have it. A simple fix for feedback due to room resonances that is cheap and relatively easy. This will not fix all of your feedback problems, but in the right situation it can save you from a lot of headaches.