Another way that EQ can be quite effective is in controlling troublesome feedback tones.
Our old friend feedback is that terrible squeal or scream that sound systems make when the audio from the loudspeaker gets picked-up by one of the stage microphones, re-amplified and pumped out the loudspeaker, only to be picked-up again by the microphone, and re-amplified, and so on.
Most often, this happens when the system is playing loud. Which makes sense, because for softer sounds, the signal either isn’t big enough to make it to the microphone, or if it does, it is too small to build-up. This is because there is more level at the microphones from the speakers than from the sources being amplified, and is known as exceeding unity gain.
The problem is one of an out-of-control, closed-loop, positive-feedback system building up until something breaks, or the audience leaves. Use EQ to cut those frequencies that want to howl and squeal, and as a side benefit, the system can play louder. The technical phrase for this is maximizing system gain before feedback.
It’s important to understand from that outset that equalization can’t fix a room’s related sound problems, but the trouble spots can be moved around. By sonically rearranging things, excesses can be tamed. You win by making it sound better. Equalization helps.
Bandpass filter parameters. (click to enlarge)
We’ve been focusing on loudspeaker and system EQ, but let’s move on to creative/source EQ, done either on the input strip of the mixer or via an equalizer inserted for a particular source. EQ is useful in augmenting instruments and voices.
With practice, a sound mixer/engineer can learn to use EQ to enhance sound for best personal expression: deepen the lows, fill the middle, or exaggerate the highs… Whatever is desired. Just as EQ can improve the sound of a poor loudspeaker, it can improve the sound of a marginal microphone, or enhance any musical instrument.
Equalizers offer that something extra, that edge. (We all know where “radio voices” really come from.)
To make loudspeaker and sound system measurements easy, a real-time analyzer (RTA) is required. An RTA provides a visual of the frequency/pressure response, not only for the loudspeaker, but even more importantly, for the whole system, including the room.
Stand-alone RTAs use an LED or LCD matrix to display the response. A built-in pink noise generator (a special kind of shaped noise containing all audible frequencies, optimized for measuring sound systems) is used as the test signal.
A measuring microphone is included for sampling the response. The display is arranged to show amplitude verses frequency. Depending upon cost, the number of frequency columns varies from 10 on 1-octave centers, up to 31 on 1/3-octave centers (agreeing with graphic equalizers). Amplitude range and precision varies with price.
The latest form of RTA involves an accessory box and software that works with your computer. These are particularly nice, and loaded with special memory, calculations and multipurpose functions like also being an elaborate sound pressure level (SPL) meter. Highly recommended if the budget allows.
Dennis Bohn is a principal partner and vice president of research & development at Rane Corporation. He holds BSEE and MSEE degrees from the University of California at Berkeley. Prior to Rane, he worked as engineering manager for Phase Linear Corporation and as audio application engineer at National Semiconductor Corporation. Bohn is a Fellow of the AES, holds two U.S. patents, is listed in Who’s Who In America and authored the entry on “Equalizers” for the McGraw-Hill Encyclopedia of Science & Technology, 7th edition.