ICEinLAVA's Domain

One of the true skills a sound person needs is to be able to identify frequencies, especially when ringing out monitors. This piece is kind of a back to basics on ringing out stage monitors, but even experienced sound techs can find this a bit of validation as there are many ways to ring out wedges. If you have tips and tricks not mentioned here, drop us some mail and share your methods.

Squeeeeeeel...

The infamous ring that Hollywood loves to add in movie soundtracks when someone approaches the podium microphone is a cliché, but it serves as an example of acoustic feedback. This comes from sound from either a stage monitor or main speaker being “fed back” into the suspect mics and thus being amplified again and again. Normally good—as in flat—speakers and good mics are the first line of defense against feedback, but most acoustic-electric transducers (speakers, mics, instrument pickups) have non-flat frequency responses. Indeed, sometimes that non-flat response is desired and engineered (i.e., the famous Shure SM58 “presence bump”).

While you may choose mains speakers for a special frequency response and coverage, or mix the mains with your favorite flavor of FOH graphic EQ curve, monitor speakers live in the land called “Flat is Good.” This is because any peak response frequencies in the speakers or mics will be the first to ring when the monitor is turned up. Added to this is that monitors and mics have ever changing responses when the two are positioned relative to each other. Then add “Tex” with his large brimmed hat, making a beautiful presence band reflector to guide even more wedge mix into his vocal mic.

When it comes to mics, we all have been through the lecture about cardioid and hypercardioid mic pickup patterns, and where the feedback notches are. The challenge is to educate ourselves, and the performers, how to position the stage monitors for each kind of mic. Cardioid mics like the ubiquitous Shure SM58 prefer the wedge front and center of the mic, with the mic normally bore-sighted from ball to cable to the wedge. I call this the “erect position.” When dealing with hyper-cardioid mics like an EV ND767A, the wedge or wedges should not be bore-sighted but placed slightly offset. This is because of that nasty response lobe that super and hyper-cardioid mics have, especially in the troublesome presence band (2 to 8kHz) that most mic manufacturers like to enhance. If you are required to center up a wedge on these kinds of vocal mics, teach your muso to position the mic horizontally (i.e. flaccid) to achieve the required maximum feedback rejection. I know the terms sound silly, but from my experience working with performers with hormones, terms like flaccid and erect get their attention, and the double entendre keeps it on their minds when handling the mic during the show.

Equalization

The whole process of ringing out is to counter the un-flatness of the contributing electrical signal processing and electro-acoustic transducers. That is why most monitor mixes employ electronic frequency equalizers between the monitor mix auxiliary output of the mixer and the monitor amp rack. Parametric or graphic equalizers are used for ringing out, with most monitor sound persons using a graphic equalizer. Note that lately many of the new equalizer products are now introducing both kinds of equalization, especially graphic equalizers with high-Q notch filters that can be swept into place for surgical feedback removal.

One item to watch out for is the kind of graphic equalizer filters used for ringing. Both constant-Q and proportional-Q equalizers are useful, but you should be aware that different types of ringing require broader or sharper filtering. For example, the ever-popular Klark Teknik DN360 uses proportional-Q filters and makes wide, sloppy dips or boosts if used for only a couple dB. Likewise, a Rane GE30 with constant Q filters maybe too brutal on the sound if severe notches are called for. Of course, feedback is not necessarily a high-Q phenomenon, and a slight low-Q notch may be the perfect fix.

Basic Method

Ringing out the monitors works best with a few prerequisites. One is that your line checks are complete and all mics are up and close to their performance gain settings. The next is that performers have clued you in to their preferred mixes and you have them roughly dialed in. Then drop all your aux masters driving mixes to very low levels and kick on the monitor processing and amps. Taking one mix at a time, slowly crank up the aux master until a ring begins to appear.

Depending on your experience and style, you may take a notch-up or notch-down tactic in ring suppression. Experienced sound persons typically do a notch-down by hearing a ring, slightly drop the aux master level, drop the suspect frequency fader by three to 6dB, and resume bringing the aux master level up to get to the next ring. This all works well if you have experienced ears and know your third-octave frequencies in your head.

Notch-up tactics take just a bit more time and work best with constant-Q graphic equalizers. The process is to hear the ring, back the aux master level down to stop the ring and selectively push a suspect fader up until a ring occurs; if the ring frequencies match, then notch down that fader, and continue on. This allows the sound person to train on the frequencies at the same time as doing the ring out.

When about two to four ring tones are suppressed, that is about when you should stop the process on that mix and call it good enough. Other clues that you are getting to end on that mix ring-out are very little aux master increase as each ring occurs, or multiple rings at the same time. One other clue that something is terribly amiss on mic/monitor placement or a suspect bad mic is when one ring frequency keeps coming up after killing it repeatedly. If you were to get really anal about ringing out, eventually your equalizer would trace out the combined frequency response curve of the wedge and vocal mic.

Trickery

One trick in learning to ring out is to employ a limiter circuit with an equalizer, either with an on-board limiter or by chaining a comp/limiter in the aux drive lines, or by placing a comp/limiter in an aux send insert jack. Normally you would hardly limit aux send signals (if at all) to above zero dBu, and likely in the +5 to +15dBu range. By cranking the limiter down to about -30dBu, you can start the ring out process, and the limiter (or 10:0 ratio compressor) would catch and hold the ring at a low volume until you can search and destroy the ring with an equalizer. I call this the “training wheels” method, as it allows you ring without the ring getting obtrusive or running away to extreme loudness, and it gives you plenty of time for the frequency search.

Of course, other more obvious methods are out there to identify ring frequencies. Peavey’s FLS (feedback locating system) is a patented display of LEDs next to equalizer faders that permits rapid ring identification and cues the operator for a fader notch maneuver. Similarly, Real-Time Analyzers (RTAs) can be attached to mics or signal lines to display rings or potential rings as they occur. In the sometimes boring part of monitor mixing, I can send a headphone PFL mix into my Neutrik Instruments Minilyzer RTA and track a mix in mid-performance. With very careful observation, I can pick out with my ears and eyes a troublemaking third-octave band long before it has a shot of blooming into a ring.

One thing to be aware of is that many feedback suppressor products have entered the market with mixed results. While most get the job done, most do take some time to activate and let a ring persist for part of a second. The nasty thing about these automatic suppressors is that the detection algorithms are not ever good enough to determine a feedback ring from a loud guitar note held for a long time. So for you Carlos Santana wannabe performers, beware that the suppressors may engage if you push the guitar amp into the mixes. Fortunately, the monitor mix sound tech is your best feedback suppressor, using all the computing power of a brain, eyes and ears.