There is a beast that stalks all live events, wherever they may be. It lies quietly and patiently as amplifiers are connected to loudspeakers, it sits up alert and attentive when microphones are plugged in, and it crouches, poised and ready to pounce when faders are raised.
When it strikes, it announces its presence with a deathly howl of dissonant damage which dissects the performance, wrenching both the audience and the performers out of the moment.
In that instant, everyone turns to the sound engineers because they’re the only ones who have a chance of confronting the beast and keeping it at bay. That beast is feedback and this is the story of how I came to conquer it.
I first encountered the beast in band rehearsals as a teenager. At the time I didn’t think much about it. It seemed like an integral part of the chaos of those early days of trying to learn how to be rock stars. However, as I obtained bigger amplifiers and more loudspeakers I realized that my decision to pursue a career where the music meets the technology would lock me into regular battle with the beast.
I vividly remember the first time I encountered feedback at a live gig. In that terrifying instant, I didn’t know if it was something I’d done or if someone else had invited it into the venue, but now it was here and I was the only person who could do anything about it. My stomach churned and a chill ran down my spine.
I grabbed frantically at faders trying to find a way to push back the beast without destroying the mix I had taken so much care to build. Thankfully something I did beat the beast back and for the moment I was safe, but I was on tenterhooks for the rest of the gig, my attention less on the intricacies of the mix and more on the lookout for the snarling beast.
In those early days of sonic discovery, the beast rarely left me alone for an entire evening. It was equal parts frustrating and frightening; I was clearly capable of building the kind of mixes that kept the musicians happy and enveloped the audience, but if the beast made even the briefest of appearances then both parties were more likely to go home remembering that one brief moment when my guard slipped. Something needed to be done.
I noticed that the beast was more likely to appear when I was struggling to get the vocals audible above the instruments. Reducing the level of the vocal microphones would invariably push back the beast, but it also meant the words couldn’t be heard, so I tried reducing the levels of the instruments.
I soon discovered that turning the instruments down in the PA didn’t make much difference, particularly in small rooms where drums and guitars are loud to start with, so I approached the musicians. Very few drummers come with volume controls, and when you ask them nicely to play quietly they soon complain that they can’t connect to the performance unless they hit the drums at a certain level.
Thankfully guitar and bass amps do come with handy volume controls but most guitarists seem to take any suggestion that they reduce their level as some kind of affront, and those that do comply often turn them back up again when you’re not looking. It’s almost as if their ego is more important than the overall performance.
At this time, I started working in the same venue on a regular basis and resolved to find a way to defeat the beast, or at the very least arm myself with enough weaponry to be able to minimize its impact. I noticed that the sound system had graphic equalizers connected to the outputs, so I experimented.
I started coming in early. I set up mics on stage and pushed up the faders while hiding behind the mixing desk in the hope that the beast would make an appearance. When it did, I tried to isolate the specific frequency of its howls and then reduced the level of that frequency in the PA system with the filters of the graphic EQ.
Initially, this seemed to work but as I continued to increase the fader the howls would return at different frequencies so I repeated the whole procedure until I could achieve a reasonable level without the beast rearing its ugly head.
Later, when I had a band on stage, I was easily able to get the vocals to a suitable level, and when the beast did show up the disruption was brief as I was able to beat it back with further equalization.
This was a revelation! I finally had a proactive tool for the battle and was no longer on the defensive the entire time. I could meet the beast on its own terms and walk away triumphant more often than not.
I would go on to proudly tell other sound engineers of this unique method I’d discovered only to find out that they had been doing it for years. They called it “ringing out the PA system.” Part of me wished someone had bothered to tell me about it, but another part of me was glad that I had gone through the process of discovering it for myself.
Now that I had tasted victory, however fleetingly, I was keen for total domination. I wanted to banish the beast forever so I started to do some research.
The feedback beast was born at the beginning of the 20th century as a result of man’s first attempts to harness electricity to capture and amplify sound. One of the earliest recorded instances of feedback happened in 1910 when Edwin Jensen and Peter Pridham were experimenting with sound reproduction in their laboratory in Napa, California. They connected a carbon microphone to their rudimentary sound system and were greeted by a howl.
I wonder if they knew what this strange beast was or how they had brought it into existence? It must have been quite a shock. Little did they know how it would go on to blight the lives of sound engineers forever more.
Feedback is also known as The Larsen Effect, named after Søren Absalon Larsen, the Danish physicist who first documented the principle, and the science behind it is quite simple.
When a fraction of the output signal of a sound system leaks back into the input it gets amplified and is output where it once more leaks back into the input, creating a building loop which quickly manifests as an audible howl.
The frequency of the howl is determined by resonances present in the microphone, the amplifier, the loudspeaker and the room, as well as the physical positioning of the sound system components.
Traditionally the first line of defense is to utilize graphic equalizers on the outputs of the system to “ring out” the system in the way I described earlier. When employing this process we’re using the EQ to attenuate each of the successive resonant frequencies in the system until we’re able to achieve a reasonable signal level before feedback occurs.
We can also utilize feedback suppressors that detect those telltale howls and automatically apply a narrow filter at the appropriate frequency, but they can be tricky to set up precisely. If they’re too sensitive they can attack perfectly valid sounds which briefly resemble feedback and if they’re not sensitive enough they can miss unwanted howls.
In the context of a busy mix it can be quite difficult to tell the difference between the noises we want and those we don’t – spotting feedback can be a highly subjective task and many engineers (myself included) prefer to trust our ears over a machine. An experienced engineer can proactively eliminate problems before they occur, unlike a feedback suppressor, which is reactive.
However, it’s important when employing graphic equalization to be aware that not all EQs are equal. Each slider on the EQ applies an individual filter centered on the nominated frequency. This means that it doesn’t just attenuate or boost the specific frequency but also those adjacent to it (to a lesser degree).
The width of the filter (known as the bandwidth or Q, meaning quality) can differ greatly from unit to unit. Some have wide filters, some have narrow ones, and some are variable, starting out wide and getting narrower with the more cut or boost that is applied. Most engineers prefer wide filters for general equalization of a system to make it sound more musical, with narrow filters applied for surgical feedback attenuation.
Then To Now
Like most of the tools in live sound, graphic equalizers evolved in the good old analog days of the 1960s and 70s. Various designs emerged and eventually settled down into the 31-band standard, which has been ubiquitous ever since.
The reason this format triumphed is because each filter represents a third of an octave, covering the full range of human hearing (i.e., from 20 Hz to 20 kHz), which provides a comprehensive way to address the entire frequency spectrum in a musical manner.
At the time, there was a belief that 1/3-octave filters corresponded well to the width of the ear’s Critical Bands, although current research indicates that 1/6-octave would be a better fit. This format has proven so effective that it persists to this day, even in the software facsimiles present in the virtual racks of digital consoles.
Speaking of digital consoles, the prodigious signal processing power now available means that most manufacturers now include a set of parametric equalizers on all the outputs, which has enabled us to develop hybrid methods for ringing out the system.
Nowadays I use up to four parametric EQs with extremely narrow filters (i.e., high Q) to attenuate the first four resonant frequencies and then a graphic EQ to deal with any further resonances present in the system.
Being able to deploy equalization in real time to keep the beast at bay is a vital skill to develop, but there are other ways to improve your chances in the battle.
A basic awareness of the directional characteristics of loudspeakers and the reflective qualities of nearby surfaces can greatly aid in loudspeaker placement. There’s a reason why we typically place loudspeakers at the front of the stage pointing towards the audience with all the microphones behind pointing in the opposite direction.
Likewise with mics. While it’s common to use omnidirectional microphones in the studio environment for their open sound and lack of proximity effect, it’s no coincidence that most of the mics we use in live sound are directional in some way. (The exception is lavalier designs, which are often omni to avoid coloration as the individual turns his or her head.)
Understanding the difference between the various polar patterns of stage microphones can greatly aid in ensuring the most sensitive area of the microphone is pointing towards what you want to capture and the least sensitive area is towards the things you don’t (such as loudspeakers).
At the end of the day, we can never truly banish the feedback beast. A live show, by its very nature, is a dynamic event. Things change and mics move, but the more we tussle with the beast the easier it gets to anticipate its behavior and swiftly banish it before it can ruin everyone’s evening.