There’s nothing that makes us cringe more than being at a club, concert venue or House-Of-Worship and experiencing earsplitting feedback. Or maybe it was that low-end rumble that shakes the ground and can even make some folks nauseous. It might have been over in a second or two, or maybe it lasted long enough to interrupt the show.
On large concert tours, extensive feedback can send monitor engineer packing for home. In a nightclub, feedback can destroy the performance. Regardless of your sound engineering skill level, there is a relatively easy way to determine what frequencies are feeding back.
Sound is physical
We feel low-end rumble when it shakes our bottoms and vibrates windows. We feel high-end frequencies around our head. Do remember that Memorex commercial wirth Ella Fitzgerald holding the high note and shattering the glass? Think back to last time you were at a show and got hit with high-end feedback. Where did you hear it—or, for that matter, where did you feel it?
The Graphic Equalizer (EQ)
On the face of any graphic equalizer you’ll see that the faders are labelled with the frequencies they effect. Behind that faceplate are the various filters that match the frequencies. With these filters you can cut or boost the frequencies in the audio spectrum. Graphic equalizers are often used to “tune” front-of-house sound systems. They are also used with monitor systems to tune wedges to eliminate feedback.
A graphic equalizer can be divided into three sections, or octaves. This makes it a bit easier to start defining the sound of a single frequency, which means you’ll have an easier time identifying feedback during a performance.
- 20 Hz to 200 Hz
- 200 Hz to 2 kHz
- 2 kHz to 20 kHz
The Low-end is 20 Hz to 200 Hz, midrange is 200 Hz to 2 kHz, and 2 kHz to 20 kHz is high end.
Here’s how to identify feedback frequencies. Set up a mixing console and insert the following:
- A 31-band graphic equalizer, such as the Klark Teknik Square One
- A speaker (preferably a floor wedge)
- A microphone (a quality microphone like a Sure SM 58 or an Electro-Voice N/D767)
Next, place the microphone on a straight mic stand in front of the floor wedge. Begin increasing the level of the vocal microphone into the floor wedge until it’s just below the feedback threshold. Now, beginning at the far left of the graphic equalizer, increase one fader at a time. The EQ on the mixing console should flat. As you increase each frequency, you’ll begin to hear feed back in the floor monitor. Listen to the tone of the offending frequency and think to yourself, “Where can I feel that frequency? Is it at my feet, my stomach, my chest or in my head?” Work through all 31 bands of the equalizer. As you move up to the higher frequencies, the parts of your body where you feel and hear the sound will change.
The point of this exercise is to help you relate frequencies to sections of your body. 50 Hz to 150 Hz is felt at your feet up to your waist. 200 Hz to 2 kHz is felt from your waist to the top of your chest. Finally, 2 kHz to 20 kHz is experienced from the top of your chest to the top of your head.
The next time you’re at a show and you hear a frequency on the edge of feedback, before you reach for a fader on the EQ, try to identify where the feedback was felt on your body. This will help you reach for the correct frequency to cut, and help save the gain and sound of your mix.
For more advice on sound equipment, or for info on purchasing the best gear for your needs, contact the Production Monster.