Signal Processing Fundamentals: Equalizers

Dennis Bohn, Rane Corporation


You may have heard it said that equalizers are nothing more than glorified tone controls. That's pretty accurate and helps explain their usefulness and importance. Simply put, equalizers allow you to change the tonal balance of whatever you are controlling. You can increase (boost) or decrease (cut) on a band-by-band basis just the desired frequencies. Equalizers come in all different sizes and shapes, varying greatly in design and complexity. Select from a simple single-channel unit with 10 controls on 1-octave frequency spacing (a mono 10-band octave equalizer), all the way up to a full-featured, two-channel box with 31 controls on 1/3-octave frequency spacing (a stereo 1/3-oct equalizer). There are graphic models with slide controls (sliders) that roughly "graph" the equalizer's frequency response by the shape they form, and there areparametric models where you choose the frequency, amplitude, and bandwidth desired (thefilter parameters - see diagram below) for each band provided. Far and away, the simplest and most popular are the 1/3- and 2/3-octave graphics. They offer the best combination of control, complexity and cost.

In selecting graphic equalizers, the primary features to consider are the number of input/output channels, the number of boost/cut bands, the center-frequency spacing of each, and the accuracy of the output vs. the front panel settings. Up until the recent development of true response graphics, the front panel settings only approximated the equalizer's actual response. Prior to true response graphics, adjacent band interaction caused the actual output response to deviate from the front panel settings. Described as either constant-Q or variable-Q (see diagrams), the individual filter bandwidth behavior determined the interaction. In the early '80s, Rane developed the first constant-Q designs to preserve the same shape (bandwidth) over the entire boost/cut range. In contrast, variable-Q designs have varying bandwidths (the shape changes) as a function of boost/cut amount. Rane's constant-Q design offered a big improvement in output response vs. front panel settings and became the most popular design until Rane and others developed the first true response graphic equalizers. Now true response graphics offer the best response.

Using Equalizers

Equalizers can do wonders for a sound system. Let's start with loudspeaker performance. An unfortunate truth regarding budget loudspeakers is they don't sound very good. Usually this is due to an uneven frequency response, or more correctly a non-flat power response. An ideal cabinet has a flat power response. This means that if you pick, say, 1 kHz as a reference signal, use it to drive the speaker with exactly one watt, measure the loudness, and sweep the generator over the speaker's entire frequency range, all frequencies will measure equally loud.Sadly, with all but the most expensive speaker systems, they will not. Equalizers can help these frequency deficiencies. By adding a little here and taking away a little there, pretty soon you create an acceptable power response - and a whole lot better sounding system. It's surprising how just a little equalization can change a poor sounding system into something quite decent.

The best way to deal with budget speakers -- although it costs more -- is to commit oneequalizer channel for eachoutside (no reflections off walls or ceiling) and up in the air (no reflections off the ground) you can get a very accurate picture of just the loudspeaker's response, free from room effects. This gives you the room-independent response. This is really important, becauseno matter where this box is used, it has these problems. Of course, you must make sure the cost of the budget speaker plus the equalizer adds up to substantially less than buying a really flat speaker system to begin with. Luckily (or should this be sadly) this is usually the case. Again, the truth is that most cabinets are not flat. It is only the very expensive loudspeakers that have world-class responses. (Hmmm ... maybe that's why they cost so much!) cabinet. This becomes a marriage. The equalizer is set, a security cover is bolted-on, and forever more they are inseparable. (Use additional equalizers to assist with the room problems.) And now for the hard part, but the most important part: If you do your measurements

The next thing you can do with equalizers is to improve the way each venue sounds. Every room sounds different -- fact of life -- fact of physics. Using exactly the same equipment, playing exactly the same music in exactly the same way, different rooms sound different -- guaranteed. Each enclosed space treats your sound differently.

Reflected sound causes the problems. What the audience hears is made up of the direct sound (what comes straight out of the loudspeaker directly to the listener) and reflected sound (it bounces off everything before getting to the listener). And if the room is big enough, thenreverberation comes into play, which is all the reflected sound that has traveled so far, and for such a (relatively) long time that it arrives and re-arrives at the listener delayed enough to sound like a second and third source, or even an echo if the room is really big.

It's basically a geometry problem. Each room differs in its dimensions; not only in its basic length-by-width size, but in its ceiling height, the distance from you and your equipment to the audience, what's hung (or not hung), on the walls, how many windows and doors there are, and where. Every detail about the space affects your sound. And regretfully, there is very little you can do about any of it. Most of the factors affecting your sound you cannot change. You certainly can't change the dimensions, or alter the window and door locations. But there are a few things you can do, and equalization is one of them. But before you equalize you want to optimize howand where you place your speakers. This is probably the number one item to attend to. Keep your loudspeakers out of corners whenever possible. Remove all restrictions between your speakers and your audience, including banners, stage equipment, and performers. What you want is for most of the sound your audience hears to come directly from the speakers. You want to minimize all reflected sound. If you have done a good job in selecting and equalizing your loudspeakers, then you already know your direct sound is good. So what's left is to minimize the reflected sound.

Next use equalization to help with some of the room's more troublesome features. If the room is exceptionally bright you can beef up the low end to help offset it, or roll-off some of the highs. Or if the room tends to be boomy, you can tone-down the low end to reduce the resonance. Another way EQ is quite effective is in controlling troublesome feedback tones. Feedback is that terrible squeal or scream sound systems get when the audio from the loudspeaker gets picked-up by one of the stage microphones, re-amplified and pumped out the speaker, 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. The problem is one of an out-of-control, closed-loop, positive-feedback system building up until something breaks, or the audience leaves. Use your equalizer to cut those frequencies that want to howl; you not only stop the squeal, but you allow the system to play louder. The technical phrase for this is maximizing system gain before feedback.

It's important to understand at the beginning that you cannot fix room related sound problems with equalization, but you can move the trouble spots around. You can rearrange things sonically, which helps tame excesses. You win by making it sound better. Equalization helps.

bandpass filter

Figure 5. Bandpass Filter Parameters

variable-q filter

Figure 6. Variable-Q Graphic

constant-q filter

Figure 7. Constant-Q Graphic

Equalizers are useful in augmenting your instrument or voice. With practice you will learn to use your equalizer to enhance your sound for your best personal expression: deepen the lows, fill the middle, or exaggerate the highs ... whatever you want. Just as an equalizer can improve the sound of a poor loudspeaker, it can improve the sound of a marginal microphone, or enhance any musical instrument. Equalizers give you that something extra, that edge. (We all know where "radio voices" really come from.)

Seeing Sound

To make loudspeaker and sound system measurements easy, you need a real-time analyzer (RTA). An RTA allows you to see the power response, not only for the loudspeaker, but even more importantly, for the whole system. 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. With the cost of laptop computers tumbling, 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 SPL meter. Highly recommended if the budget allows.

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Courtesy Rane . Used by permission.

David McLain | The Processor Guy! | CCI SOLUTIONS
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