The EM Poll

Location Is Everything

Mar 13, 2008 3:06 PM, By Mike Sokol

           

FOR SUCCESSFUL LIVE SOUND, SPEAKER PLACEMENT IS CRITICAL.

Get Thee Behind Me, Feedback!

You must place speakers in front of the band to eliminate the speaker-to-mic path, the primary cause of feedback. All stage microphones have a directional pickup pattern, typically called unidirectional or cardioid, designed to pick up sounds right in front of the mic while rejecting the sounds at the sides and rear.

A microphone's pickup pattern acts like a flashlight beam: any sound present where the beam points is going to be picked up and amplified. Of course, some mics — like floodlights — have a wide pattern, and others — like penlights — have a very narrow pattern. A narrower pattern curbs feedback.

If you place the P.A. speakers right behind the band, the microphones point directly at the speakers. This means the mics will pick up speaker sounds and feed them back into the amplifier; the sounds come back out through the speakers and into the mics, completing the feedback loop. Each time the sound cycles through the loop, it gets amplified, resulting in a screech that forces all the amplifiers to clip and overdrives the speakers. Feedback is not just hard on the ears — it's hard on your gear, and it can destroy speakers and amplifiers.

Figure 3: Place speakers so that they are facing away from microphone pickup patterns.

To combat feedback, always place your P.A. speakers as far downstage as possible and point your microphones in the opposite direction. Simply put, you should place the speakers between you and the audience, and the mics between the back of the P.A. cabinets and the band. This puts more distance between the mic and speaker, and aligns the cancellation action of the mic pattern with the back of the speaker pattern, where you'll find the fewest midrange and high frequencies (see Fig. 3).

Monitors Are Speakers, Too

Monitor wedges are like little P.A. speakers, so you can't ignore monitor placement in relation to microphones, or feedback will likely result. Since you must aim a monitor at a player, the most effective placement is directly in front of the mic stand. Most wedges are constructed so that you can place them at two different angles: straight up directly in front of the mic, or at a shallower angle that covers the back of the stage. It is important to point the monitor's horn right at the ears of the target musician while keeping it behind the microphone's pickup pattern; this cancels out the speaker's sound as much as possible.

Figure 4: A typical monitor setup using supercardioid microphones. Note that the mics are placed 15 degrees off-axis to best utilize their rear-rejection characteristics.

Purists will note that supercardioid mics like the venerable Shure Beta 58A have a very narrow front-pickup pattern, but they achieve this by sacrificing some of the rejection pattern directly behind the mic. Supercardioid mics are designed to work well with either a pair of floor wedges split 15 degrees off the center rear of the mic, or a single floor wedge slightly offset from the rear of the mic. If you point the back of a supercardioid mic directly at a monitor speaker, it will feed back more easily than if you offset it by 15 degrees or so (see Fig. 4).

Dead Sources Tell No Tales

Switch off extra sound sources when you don't need them. Leaving your acoustic guitar “hot” while setting it on a stand next to a monitor speaker can lead to disaster. The whole body of the instrument will vibrate passively like a drumhead, channeling stage noise into the pickup and producing howling feedback. Even if the unattended guitar doesn't feed back, just having the extra sound from the pickup mixed into the P.A. can cause echo and phase-related problems. Savvy acoustic guitar players use a kill switch on each guitar line, muting the instrument whenever it is out of their hands. Also, always zero your main and monitor outputs before powering up your system to avoid jarring (and unprofessional) feedback from an unexpected open source.

Rules to Live By

You simply can't control a lot of things at gigs — obnoxious musicians who refuse to turn down, unforgiving rooms with hard surfaces and massive slap-back echo, or just plain bad equipment. It's a jungle out there!

By applying these basic speaker and microphone placement techniques, you can maximize P.A. output, get better room coverage, reduce feedback, and achieve a cleaner mix. Fewer complaints, more compliments, and additional work will soon follow — and that is the bottom line.

Mike Sokol is a live-sound and recording engineer, musician, and computer integrator with some 30 years' experience on both sides of the microphone. He lives in western Maryland with his wife, four boys, and three cats.