Suppose you’re listening to the house sound system reproducing a play or musical. Some of the actors’ voices sound “puffy” or “muffled,” as if they were covered in a blanket. Other actors might sound “spitty” or overly sibilant.
Fortunately, those problems can be fixed with the equalization knobs (EQ) in your mixing console. EQ adjusts the bass, treble, and midrange of a sound by turning up or down certain frequency ranges.
To do that, EQ operates on the spectrum of the sound source—its fundamental and harmonic frequencies. The spectrum helps to give the instrument or voice its distinctive tone quality or timbre. If some of these frequencies change in level, the tone quality changes.
An equalizer raises or lowers the level of a particular range of frequencies (a frequency band), and so controls the tone quality. That is, it alters the frequency response. For example, a boost (a level increase) in the range centered at 10 kHz makes voices sound bright and crisp. A cut at the same frequency dulls the sound.
Types Of EQ
Equalizers in a mixing console range from simple to complex. The most basic type is a bass and treble control (often labeled LF EQ and HF EQ). Its effect on frequency response is shown in Figure 1-A.
Figure 1: Click to enlarge.
Typically, this type provides up to 15 dB of boost or cut at 100 Hz (with the low-frequency EQ knob) and at 10 kHz (with the high-frequency EQ knob). You have more control over tone quality with a 3-band or 4-band equalizer: you can boost or cut several frequency bands (Figure 1-B).
Sweepable EQ is even more flexible. You can “tune in” the exact frequency range needing adjustment (Figure 2-A).
Sweepable EQ is often incorrectly called “parametric,” which also allows control of bandwidth. The parametric equalizer allows continuous adjustment of frequency, boost or cut, and bandwidth —the range of frequencies affected. Figure 2-B shows how a parametric equalizer varies the bandwidth of the boosted portion of the spectrum.
A graphic equalizer (not shown) is usually external to the mixing console. This type has a row of slide potentiometers that divide the audible spectrum into 5 to 31 bands. When the controls are adjusted, their positions graphically indicate the resulting frequency response. Normally a graphic equalizer is used to equalize the house loudspeakers, both to flatten the response and to notch out feedback frequencies.
Figure 2: Click to enlarge.
Some engineers prefer to use an external parametric EQ or an automatic feedback device for feedback control. (The use of graphic EQ is beyond the scope of this article).
So far we’ve classified equalizers according to the frequency bands they control. They also can be classified by the shape of their frequency response.
A peaking equalizer (Figure 3-A) creates a response in the shape of a hill or peak when set for a boost. With a shelving equalizer, the shape of the frequency response resembles a shelf, as in Figure 3-B.
A filter causes a roll-off at the frequency extremes. It sharply rejects (attenuates) frequencies above or below a certain frequency.
Figure 3-C shows three types of filters: low-pass, high-pass, and band-pass. A 100 Hz high-pass filter (low-cut filter) attenuates frequencies below 100 Hz. Its response is down 3 dB at 100 Hz and more below that. This removes low-pitched noises such as room rumble, microphone handling noise, and mic breath pops.
Figure 3: Click to enlarge.
A filter is named for the steepness of its roll-off: 6 dB per octave (first-order), 12 dB/octave (second-order), 18 dB/octave (third-order), and so on.
The frequency response and placement of each microphone affect tone quality as well. In fact, microphones and mic placement can be considered as equalizers.