October 10, 2007
So, you have the sound system hooked up correctly and everything is turned on and ready to go. You start to turn up the mains and whoa! Feedback! What do you do? What is the best way to get rid of that nasty stuff?
The following assumes that you are already using the proper mics and monitors, placed correctly, in addition to good mains speakers and adequate amplification. Having realistic expectations of your gear doesn’t hurt, either.
The drill starts out something like this: turn all of the volumes down at the outputs of the board. Next, center all of the graphic EQs – it’s best to have a 31-band EQ. You should also have a 31-band graphic EQ on each output of the mixer – Left, Right, Monitor 1 and Monitor 2. With the mixer outputs down, turn all of the EQ, crossover and amp gains to their proper settings. Then slowly turn up the volume one output at a time. Keep your hand on the volume and turn it up very slowly until one frequency starts to feedback, then try to find that frequency on the graphic EQ that corresponds to that output. This can be an art in itself, with frequency detectors being a great help and with some true diehards being able to do this by ear. Some exceptionally proficient soundguys have even been known to call out the exact frequency, as well as how many dBs it should be turned down.
Once the offending frequency has been identified, start with a 3dB reduction then raise the gain a little. If you don’t need to turn it back down, move onto the next frequency that’s giving you trouble. Keep repeating this process until you run out of frequencies or several pop up at once after you have eliminated most of the trouble makers.
Next, we need to do the same thing again for the mixer output. Here’s a little tip: if you have already done this on the left channel of the main output, you can usually use the same settings for the right (notice I said usually). Now, recheck the system and monitors to see if it will perform to the level needed. If not, EQ out a bit more of the problem frequencies. Keep in mind that this can be a noisy and irritating process and is best done without an audience around.
Deal with a reputable music store, and acutally listen to their suggestions.
Now, let’s take a look at the actual mechanics of sound that may be contributing to the feedback. The speed of sound changes with differences in temperature and humidity. But how might that be affecting the mix, you ask? Let’s say that when you set up the club is empty and the AC is on at a comfortable 65 degrees.
As the crowd begins to fill the venue, the temperature begins to increase. The first set goes great, but halfway through the second set, feedback problems start. In the interim, the temperature has climbed to 78 degrees, allowing the feedback to “scroll” up the frequency spectrum, doing its best to ruin your mix. Now is the time to be brave and to dial on the fly, which isn’t for the faint of heart. Alternately, you can turn down the volume on everything, or you can even try to get through it by turning down the monitor mixes. The band will hate you, but since you’re the soundguy, they probably do already.
Now is a good time to reiterate the fact that the majority of feedback problems occur due to loud main and monitor levels; because of bad mic and monitor placement; and from rooms with bad acoustic properties, such as low ceilings, brick walls, bad geometry and the generous use of glass. Spend some time in a good room moving your monitors to different positions to get an idea of how placement can affect sound and feedback.
Another option is trying new cables and microphones. Deal with a reputable music store and let them know your needs and expectations. Actually listen to their suggestions. Another piece of advice that can’t be stressed enough is to buy good gear – don’t skimp when it comes to EQs, amps, speakers, and mics. Also, there is a new generation of problem solvers, like feedback eliminators and intelligent system controllers. As with any new or existing piece of gear, the most important aspect is learning how to use it properly, so don’t be afraid to spend time curled up with a manual now and again.
To that end, there are many good books available about this and other sound related topics, with one of the best I have found being the Yamaha Sound Reinforcement Handbook, by Gary Davis and Ralph Jones. You can find it at your local bookstore or from amazon.com. Shure also has some good articles available in .pdf format available at shure.com.
No go out and kill that nasty feedback!