When an acoustic guitar is used onstage, it is usually connected via a direct box that splits the signal to the onstage amplifier and the PA system. The PA then will split the signal again to drive wedge monitors and the main house sound system. When all of these loudspeakers are blasting at the same time, they interact. In fact, they mostly interact in the bass region where the longer, low frequency sound waves meet to either reinforce each other or cancel each other out. This effect is known as modal distortion. Recording studios commonly employ bass traps to reduce hot spots known as room modes. These are exaggerated depending on the room geometry or the room’s natural resonant frequency. And guess what … room modes, like gravity, exist everywhere including on a live sound stage.
Here’s what happens: You play a chord on the guitar and, depending on where you are standing, the sound waves from the wedge monitor and the PA system will either amplify each other if they are in phase or cancel each other out if they are out of phase. When they are in phase, the resulting amplitude at that particular frequency will increase or even double depending on where you are standing. If you find that a certain frequency is feeding back when you stand in front of your monitor, in all likelihood, you are experiencing two or more waves that are combining, causing a resonant feedback problem. There is absolutely no point trying to figure it all out by calculating the phenomena as this will occur based on a host of variables such as the PA system, the monitors, the size of the room, the room acoustics, and so on. But you can try reducing feedback by following this simple procedure.
First, start by eliminating unneeded bass frequencies by rolling off the low end below 100 Hz. This is the one fix that you should absolutely consider before doing anything, as low frequencies are the primary problem with resonant feedback. Bass below 300 Hz is considered to be omni-directional, meaning that it will be everywhere. By eliminating excessive low end, you make the task of controlling feedback easier. There is also another benefit – ever notice that it is way easier to get feedback from an electric guitar when the sound is distorted? Guess what. Like gravity and modal distortion, the same laws of physics apply everywhere. So, if your acoustic guitar is distorted, you will get more feedback. To eliminate distortion, make sure you use a high-quality direct box that is able to handle transients without choking. Since most of the sound energy is contained in the bass, when you roll off the low end, you are actually making it easier for the buffer or amplifier inside the DI box to work. Less distortion = less feedback.
Now that you have rolled off the bass, you are now ready to turn up your PA system and monitors. Start playing chords and let the guitar ring. Turn your system up until it begins to resonate. Now, take a step away from in front of your wedge monitor to see what happens. Now move sideways.
As you move around, the feedback character will change. This is because you are in the middle of a multitude of room modes. If the feedback is most active near the monitor, try moving the monitor electronically by reversing the electrical phase. Most professional DI boxes have a 180-degree polarity reverse switch to enable you to do this. What you are doing basically is causing the modal distortion to change. This can often move a phase-adding mode from where you are standing which can help reduce feedback.
Another possible fix is to “imply move” the wedge monitor away from where it is so that the physical relationship changes. If you have an instrument amp on stage, moving it back a few inches can also help. This will cause different frequencies to either amplify each other or cancel each other out depending on where you stand. Point being, we have yet to EQ the sound, but are dramatically shifting the way the natural sound will interact so that we minimize feedback naturally. Once you have maximized the output, you can then fine-tune your system using the EQ.
*Peter Janis is the President of Radial Engineering, the PortCoquitlam, BC-based manufacturer of music and audio equipment. Visit www.radialeng.com for more information. *