Those of you who are old enough to remember the music of the 1980’s when it was new might recall a band called Ficher Z and their album named “Going Deaf For A Living” (hint it featured the song “So Long”).
As theatre engineers the album name rather aptly applies to us, it’s something we do indeed risk doing…
Feedback Is Avoidable
Feedback is avoidable in nearly all instances, barring actors abusing microphones, because who can get the image out of their head, once seen, of The Who’s Roger Daltrey swinging his Shure by it’s cable like a helicopter blade? Enough to cause any sound engineer to reach for something stiff to drink.
The following is for those theatre sound cue engineers who also find themselves in the unenviable position of also being responsible for setting up and mixing “stage mic’s”.
One of the worst and embarrassing things that can happen during a performance, beyond a member of the cast or crew having an accident or injury (like knocking themselves out while imitating Roger Daltrey), is a screeching and ear shattering blast of feedback. It deafens the cast, the crew, the audience and you. Forget the “…it’s not my fault.” excuse, nobody buys it, even if it is true.
Feedback in theatre, more often than not, occurs with the monitor speakers – which may come as a surprise to some folks. This perfect storm of howling noise brilliantly deafens the cast and by it’s nature impresses the producers and directors no end. The audience is likewise amused – not.
The reason for this is that microphones, generally, are behind the main PA system which faces the audience. Although, if you try extra hard you’ll probably be able to encourage a nice feedback loop by pumping up the monitor speakers and the main PA speakers through running your desk just below the point of feedback. Some engineers seem to have turned this into an art – much to the horror of all involved.
I’m going to assume you’d prefer to be a little more “traditional” and avoid feedback, impress the director, win the heart of the leading lady and… OK – maybe just impress the director for now… 
Feedback, by definition, results from a speakers sound “feeding back” into one or more microphones that are connected to the same speaker. It’s literally a harmonic loop. As the frequencies resonate it will tend to raise in pitch, very similar in fact to harmonic tremors seen in volcanic eruptions and earthquakes. Of course in the latter case you can’t prevent them by turning down a fader, but with feedback – we generally can.
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Use Fewer Microphones On Stage.
Avoid mic’s where you can. Try, despite their protestations when dealing with musicians, not to mic bass guitars. DI (Direct Inject) them if you can. Unfortunately you can’t DI actors though, and even if you did there’s probably a law somewhere that prevents that kind of abuse.
If you can make a direct connection of some kind (DI box, amplifier output jack, etc.) to any or all of the instruments, do it. Each microphone you can eliminate from the stage is one fewer feedback source to plague you later.
Obviously you can’t DI a drum kit, unless it’s electronic like a LynDrum, nor will most electric guitarists allow this sacrilidge as their “sound” is frequently tied to the tonality (and overdrive) of their amplifiers. This is even more the case with those not using solid state amp’s, but valve amp’s. An engineer insisting on DI for these guys is probably going to find a phaser pedal inserted somewhere unpleasant!
2. Turn down any microphones that are not in use at the moment.
This is in keeping with the previous idea about minimizing the number of potential feedback-producing microphone-speaker loops. A microphone that is turned down all the way can’t feed back! For those desks that have them – mute buttons are preferable to altering fader levels on busy desks – but…
The fact is, I’m going to recommend that you don’t turn unused mikes all the way down – just halfway or so. This is so that when you forget that you are being careful about feedback and that you turned down the backup singer’s mike ten minutes ago and she suddenly starts singing, you (and the audience) will at least hear her to some degree before you manage to restore the proper level.
3. Don’t move the microphones around.
This is one of the main causes of monitor-speaker feedback. It’s funny, but everything can be fine, then the lead singer moves his mike two feet to the left and shortly thereafter it starts feeding back. The pattern of locations that do and do not produce feedback onstage is very complex. Once you find a microphone placement that works, try not to let it change much.
If your singer is a flamboyant, Roger Daltrey type who needs some elbow room, be sure you have checked the setup out in advance (at full volume) to isolate any “hot spots” that may cause feedback. (You can mark these “no-go” areas with tape.)
4. Don’t operate at the edge of feedback.
If you have the main or monitor speakers turned up so loud that the system is perpetually just about to feed back, there’s something wrong with your setup. You can tell you’re in this situation if there are tiny hints of tones or “notes” heard when someone talks through one of the mikes. Those tones are micro-bursts of feedback!
You don’t have to be that close to the edge to have problems, though. Operating anywhere near the feedback point cuts your margin of error and makes it more likely that one of the factors mentioned earlier will kick in, and then there goes your reputation. Back off on the gain levels and get the performer/actor/singer to sing louder or get closer to the mikes. Or, put up with sounding a bit quieter in the room. It’s better than feedback!
5. Watch out for instrument feedback.
I’ve been talking about microphone feedback above, but electric guitars are notorious for “string feedback.” This occurs when the amp speaker sets the guitar strings to vibrating, and then that note comes out of the amp speaker, which sets the strings to vibrating even more, which – well, round and round you go again.
If this happens while playing, the hand-on-the-strings method can be used (unless you want the feedback, Jimi). A more common problem is for the guitar player to put the guitar on a stand right in front of the amp, resulting in louder and louder string feedback. The best way to avoid this is to get in the habit of lowering the fader on the desk before or just after the guitarst puts the guitar down. Just don’t forget to turn it back up, less you become responsible for a “silent” lead break.
Scott Kane