Feedback occurs when the output of a device is fed to its own input. An example that most of us have heard is guitar feedback. Let’s say an electric guitar player turns his amp up loud and faces his guitar toward the amplifier. Sound from the speakers excites the strings, causing them to vibrate. The pickups change the string motion into an electrical signal, and send the signal back to the guitar amp. The guitar amp magnifies this sound and sends it to the speakers — so the process repeats.
P.A. Feedback
Microphone feedback goes something like this: a person speaks into a mic. The microphone changes sound energy into electrical energy. This electrical signal is sent to a mixer, which amplifies it, then to a power amp which amplifies it again, and then to a loudspeaker. The loudspeaker converts electrical energy into acoustic energy. When sound from that loudspeaker is directed back into the microphone, creates a “loop,” resulting in feedback. This is usually in the form of a high-frequency squeal, a mid-frequency howl, or a low-frequency growl (as you can tell, these are all very technical terms).
There are three reasons we don’t want feedback in a P.A. system:
1. It’s usually unpleasant.
2. It often happens fast ‘n’ furious in an uncontrollable manner that can damage a person’s hearing and
3. See #2 and substitute “equipment” for “hearing.”
Avoiding Feedback
Feedback in a P.A. system occurs when sound from the house speakers or monitors reaches the microphone(s). As vocal mics tend to be loudest in the mix, they are usually the main offenders. So how do you avoid feedback? First let’s look at the house PA.
There aren’t many rules in audio life, but here’s one to always follow:
The house speakers must be placed in front of a performer or public speaker (see Fig. 1). This means that setting up the main P.A. behind the performer is an absolute no-no. It’s begging for feedback, because the speakers have a direct path into the microphones. You generally won’t have to worry much about this in larger venues (especially outdoors), but it’s a concern in small clubs, theaters and houses of worship. Heed this rule even when you are setting up on the floor in a club that doesn’t have a proper stage. It will enhance your quality of life. I promise.
Okay, so the main P.A. has to be in front of the performer. How far? The distance depends upon the volume level of the performance, the pickup pattern of the vocal microphone(s), the dispersion pattern of the loudspeakers, and the general reflectivity of the room. We generally want directional vocal microphones in a sound reinforcement situation. Cardioid, hypercardioid and supercardioid mics all reduce pickup of sound from the house (speakers and reflections) and monitors. Omnidirectional mics (and bidirectional mics to a large extent) do not discriminate against unwanted sound, so they are best avoided for vocal use in a public performance.
There’s no rule on the distance between the “front line” of microphones and the main P.A. system, but figure four to five feet as a general starting point. In a small club or coffee house where you have a singer-songwriter performing at a low volume level, you can cheat this back to two or three feet. In a theater (especially one that features a lot of acoustic reflections) I’d hope for eight to ten feet. It also helps to move the speakers wide of the vocal positions, if you have the real estate. While you have your tape measure out, observe how far the thrust protrudes from the main stage, and determine whether or not your performer will use it. Of course, you can’t move the P.A. speakers when they are permanently installed in a club or theater, but you can move the front line upstage. You’ll have to strike a careful balance, because some performers want to be as close to the audience as possible.
Given the opportunity prior to sound check when the room is relatively quiet, crank the gain on a vocal mic and observe. Have an assistant speak into the mic and see how loud you can get it before feedback. Some engineers do this from the mix position using their own voice, but I don’t find that technique particularly useful, because the P.A. leaks into the microphone.
Seek and Destroy
Use a sweep EQ on the vocal channel to seek and destroy. I’ll bump the gain up 8 to 10 dB on the low-mid band of a parametric on the vocal channel, then slowly sweep the frequency knob. If I hear feedback, I’ll either pull the gain down a few dBs or — when the problem persists at the same frequency across all the vocal mics — use the house graphic EQ to pull out the problem frequency. Low-mids can be problematic, because some P.A. hangs produce significant backwave energy in the region between 300 Hz and 400 Hz. If you start to hear feedback when you bring the vocal up to even moderate levels, it may be time to move the front line.
Ask the house engineer if a significant change occurs once the room is full, because sometimes bodies absorb reflections from areas in front of the stage. While the band is performing a song, crank up the P.A. to performance level and make sure the vocals can be mixed louder than the band. Once in a while, you’ll run into a room that is just too live to support high volume levels without feedback, so you may have to get the band to turn it down. Good luck with that.
What if your performer is a moving target, or ventures into the audience during the performance? You’ll need to sniff out the problems ahead of time by getting an assistant to walk the venue with the vocal mic while you listen; in some such instances an electronic feedback eliminator can be helpful. During the performance you may need to ride the EQ as the performer wanders.
Bad Vibrations
Feedback can also be caused by vibration transmitted through the stage to the mics stands and into the microphones. This tends to be more of an issue when the subwoofers are on the stage, and/or the stage is hollow. A high-pass filter on the kick drum and floor tom mics helps dial out the offending frequencies. You may have to sacrifice a bit of the lowest octave to avoid feedback.
Acoustic guitars present a particular challenge because they’re designed to resonate. Miking them invites feedback, and pickups often don’t produce the best sound. Yikes. As much as I like the sound of a high-quality condenser mic on an acoustic guitar, I’d try to avoid using a microphone for acoustic guitar on stage unless I was doing a singer-songwriter in a low-volume situation. Acoustic guitar is just a beast at producing feedback in the monitors and house — even when using a pickup.
If you can get the performer to use in-ears, then you can avoid feedback in the monitors (ditto for our entire discussion), but where there are wedges, there is difficulty. The body of the guitar is excited by the wedges, the strings vibrate, and we’re off to the races. A parametric EQ, a notch filter or a graphic EQ on the monitor mix can be a huge help. You’ll have to sweep the frequency control to tune the EQ to the feedback frequency (try the problematic range between 250 Hz and 400 Hz) and keep in mind that you’ll probably need to cut some of the low-end below 100 Hz. Inverting the phase of the guitar (or any signal) in the monitors is a trick that sometimes alleviates feedback, and it’s easy enough to try.
Although they change the tone of the guitar, rubber soundhole covers are very effective at reducing feedback. For a variation on the theme, check out the Fishman Neo-Buster, which combines a soundhole pickup with a feedback suppressor. EQ pedals specifically designed for acoustic guitar often include a notch filter than can be helpful in pulling out feedback frequencies.
Wedge placement and microphone pickup patterns are critical to avoiding feedback between vocal mics and stage monitors. (For a complete discussion of the topic, see “Theory and Practice” in the April 2012 FRONT of HOUSE.) Drum monitors can present problems, especially when there’s a drum sub involved. Many drummers like the monitors directly behind them which — depending upon the height of the monitor — can blow right into the drum mics. A small tweak that can work wonders is placing one monitor (or a pair) on the sides of the drum riser toward the rear, pointed at the drummers’ ears. Sound from the monitors will still bleed into the drum mics, but given the typical placement of the mics, the monitors will blow into the back (rejection point) of the floor tom mics, and perhaps the back of the high hat and snare mics. And it helps reduce the amount of sound firing toward the kick drum mic (which is almost certainly pointing at the rear of the drum riser).
All of these techniques are helpful, but the one that really trumps everything is keeping the volume down to manageable levels. At some point, no matter what techniques you are trying, it just can’t get any louder without feedback. And heeding that tenet preserves everyone’s hearing as well as making for a pleasant performance.
