A guide to Eliminating Feedback - Drew Mollison
We all know that familiar howl, when the amplification reaches that critical stage where the microphone (or pick-up) “hears” the amplified sound from the loudspeakers, and a particular frequency or frequencies ‘take off’… Mmmmm not so nice!
What can be done to remedy this?
Well, all too often sound engineers will dive into the available technology to fix it. First they will ‘EQ’ the offending input channel a bit, then if that doesn’t fix it, deploy a graphic equalizer to find the frequency of the ‘howl’ and pull that down, along with at least a third of an octave’s-worth of useful frequencies too.
What should be done? The first step is to identify which of the amplified signals is the primary cause of the feedback. (Often multiple sources will compound the effect). Then carefully consider the placement of the main PA speakers: Are they far enough in front of the open mics? Are they horn-loaded speakers with a controlled high frequency dispersion pattern? (Often poorer quality speakers with a significantly less-than-flat frequency response will create more problems). If the feedback is caused by a mic ‘hearing’ monitor speakers, again look at the position of the monitor speakers: Can they be positioned in a way to improve the situation? Perhaps you can get the monitors closer to the people that need to hear them, and then they could be turned down a bit.
Once you are satisfied that the speakers are positioned as best they can be, time to consider the type and pick-up pattern of the microphones in use. Are they omnidirectional? (i.e do they accept sound equally from all directions) or unidirectional (cardioid means a heart-shaped pick-up pattern). If they are cardioid, super-cardioid or hyper-cardioid are they pointing away from both the main PA and monitor speakers? Are they dynamic or condenser mics? Dynamic mics are generally much less-sensitive to sounds that are further away, which is often very useful when combatting acoustic feedback. As we already discussed, we can maximise the efficiency of monitors by putting them closer to the listener. (The most extreme example of this is the use of in-ear monitors.) In the same way we should see if the microphone is as close to the source as it can be. If the vicar’s tie-clip omnidirectional microphone is half way to his or her waist, then it is going to need to be turned up to a ‘dangerous’ level in order just to be heard. Time to try lifting the mic to get it closer to the preacher’s mouth, or even try a cardioid or ear / head-worn microphone which will mean the mic’s (sensitivity) level won’t need to be set so high.
Finally, the room. Is it an echoey space? Does it have parallel hard surface walls that bounce the sound around? Is acoustic treatment an option? If high frequency energy is the main problem, draping even one wall with thick curtains can help; a broader range of frequencies can be tamed using proprietary acoustic tiles or other materials.
OK, once you’ve examined all the options, tried better quality speakers and a few different mics, then and only then, as a last resort should we consider a Feedback Suppressor. I won’t bore you with exactly how modern feedback suppressors work, (you can read more here if you want: https://en.wikipedia.org/wiki/Feedback_suppressor )but suffice to say the technology has improved enormously and they no longer utterly ruin the overall sound; the best ones use very narrow filters and don’t even start to act until the ‘danger level’ is reached.
Yamaha’s EMX5016CF offers an excellent feedback suppressor, as do the Stagepas400i and 600i. Yamaha also offer feedback suppression in a number of their Commercial Audio Installed products, but if you have a customer for whom feedback is a major problem we have some devices specifically designed to remedy it.
Behringer also offer solutions; The two channel FBQ2496 which can be inserted over main L & R or two monitor sends:
RRP $539 and in stock now.
And the FBQ100 (a single channel unit)
RRP $249 and also in stock now!
The FBQ100 actually offers a whole lot more than just feedback suppression though, it has a mic preamp, noise-gate, compressor and even a digital delay line for timing satellite speakers in larger rooms back to the main PA! See: https://www.music-group.com/Categories/Behringer/Signal-Processors/Feedback-Suppressors/FBQ100/p/P0A3Q for more details.
And the video is here: https://youtu.be/WaE_--6GOjw
