Equalization is a crucial part of any mixer, whether a physical mixer for live performance, or a virtual mixer in recording software. Equalization lets you find the right tonal balance for the sounds coming in various channels, compensate for room acoustics in live venues, prioritize some instruments or voices over others, and even help reduce feedback. You’ll find five different types of EQ in common use on various mixers; some might have only one or two of these EQ technologies, while some mixers will have all of them. Here’s what they are, and what they do.
But First…DeciBel Basics
How much EQ boosts or cuts a signal is measured in deciBels, which is a unit of measurement for audio levels (like an inch or meter is a unit of measurement for length). A 1 dB change is approximately the smallest audio level difference a typical human can hear. A dB spec can also have a – or + sign. For example, cutting response in an equalizer by -12 dB creates more of a cut than a setting of -6 dB; a setting of +2 dB would create a slight boost, while a setting of +10 dB would create a major boost.
A change of 6 dB doubles the level (-6 dB halves it), which is quite a bit. Changes of 1 or 2 dB are common when mixing; even changes of less than one dB can make an audible difference. However, some situations do call for more extreme amounts.
Each stage of EQ, regardless of the technology used, is called a band. For example, a mixer might have five bands of EQ, with some of them covering specific functions, and the rest more general purpose. The following EQ types are all examples you might find in a band of EQ.
Highpass Filter (also Called Low Cut Filter)
This reduces low frequencies below a certain frequency, called the cutoff frequency. The lower the frequency, the great the amount of attenuation. The main use of highpass filters it to reduce hum, trim unneeded frequencies (e.g., vocals usually don’t go that low, but excessive low-frequency response can lead to feedback), tighten up sounds that have an overly prominent low-frequency response, remove subsonic sounds, and reduce “p-pops” from vocalists.
There’s no boost or cut control, because the highpass filter can only attenuate below a certain frequency. Note that the response doesn’t just stop at the cutoff frequency, but rolls off in a way that’s likened to rolling down a slope. You may find a choice of slopes at which the response rolls off, specified in dB/octave. For example, a 12 dB/octave slope means that at half the cutoff frequency (i.e., one octave lower), the response will be down 12 dB. With a 36 dB/octave slope, the response will be down 36 dB at half the cutoff frequency.
Note that some microphones also have highpass filters than can supplement, or complement, the ones in mixers (Fig. 1).
This is similar to the treble and boost tone controls found in consumer electronics. A shelf response starts boosting or cutting at the selected frequency, called the corner frequency, but this boost or cut extends outward toward the extremes of the audio spectrum. Past a certain point, the response hits a “shelf” equal to the maximum amount of boost or cut and stays at that amount of boost or cut.
A frequency control sets the corner where the shelf starts boosting or cutting, with the amount determined by a gain or boost/cut control. Smaller mixers without the room for a frequency knob may have fixed shelf frequencies. There will sometimes be a control to adjust the shelf’s slope from flat response to the maximum boost or cut, and more rarely, a resonance control (also called Q or width). This can make the slope leading up to the shelf a bit steeper, and/or add a slight boost around the shelf frequency (Fig. 2).
Shelving EQ’s main function is general tone-shaping. For example, if a sound lacks brightness, a shelf response boost starting around 2 kHz can add the needed amount of treble. Conversely, if the sound is boomy and muddy because there’s too much energy in the bass range, a shelving cut below 100 to 200 Hz can help. Shelving can be used on individual instruments, or on an entire mix.
Parametric and Quasi-Parametric EQ
The parametric equalizer is a highly versatile way to adjust frequency response, because it can make broad or narrow ranges of the frequency spectrum more or less prominent. There are three main parameters.
Frequency determines where any boosting or cutting takes place. For example, if the upper midrange frequencies need boosting, you would dial in upper midrange frequency, then use the controls described next to edit how the EQ affects these frequencies.
The Gain control chooses the amount of boost or cut. Note that if the cut function is very deep and covers a narrow range of frequencies, it’s often called a “notch” filter. One example of a notch filter is to reduce hum by creating a very steep cut at precisely 60 Hz or 50 Hz, depending on the AC power frequency used in a particular country.
Width (also called Resonance or Q) sets the range of frequencies affected by the boost or cut, from broad (smaller numbers) to narrow (higher numbers). Broader settings are gentler and preferred for general tone shaping. Narrow settings generally help solve specific response problem.
A quasi-parametric EQ is simply a parametric EQ that has limited controls. For example, it may not have a Q control, or a frequency might be fixed instead of sweepable over a certain range. These types of EQs are common with compact analog mixers (Fig. 3) that don’t have the panel space for a full complement of controls, or may need to meet a specific price point.
Digital mixers often include more sophisticated equalization, but unlike analog mixers, almost never have one knob per function. For example, there may be switches to select different EQ bands (Figure 4), with the knob values shown on a readout or other display.
Because parametric EQs can create very specific, detailed responses, they’re used mainly to solve problems. For example, suppose in a miked drum set, a certain tom is louder than the rest. You can find the tom’s frequency, and apply a narrow cut to reduce the level of only that one tom, while leaving the rest of the drums unaffected. Acoustic guitars sometimes have response peaks in the lower midrange, and this can lead to feedback. Reducing the level of this peak allows for a higher overall guitar level. Or, consider a vocalist who’s not that intelligible. A boost in the 3.5 kHz range will bring out the sibilants and consonants that makes a voice more prominent.
A graphic equalizer divides the frequency spectrum into a number of fixed bands—typically 5 to 31—each with a boost/cut slider. It gets its name because the position of the sliders gives a rough idea of the frequency response (Fig. 5).
Graphic equalizers are common for live performance, but rarely found in the studio. Because you can’t dial in specific frequencies to boost or cut, graphic equalizers are designed more for general frequency-response shaping. Its popularity in live performance is due to the speed with which you can compensate for peaks and dips in sound systems, or accommodate different room acoustics.
Lowpass (also called High Cut) Filter
This is the mirror image of the highpass response that started our journey into filter-land. It progressively reduces response above the cutoff frequency; the higher the frequency above the cutoff, the greater the reduction. A lowpass filter is helpful in removing high frequency sounds like hiss or excessive brightness. When used subtly, it can also make sounds seem “warmer.”
That’s our roundup of mixer EQs. Just remember the reason why there are often multiple options is because it’s best to use the right tool for the right job. Also remember the analog vs. digital difference: analog is all about fast, hands-on control, but almost always has far more limited functionality than digital models. In any event, you can make the most out of any mix—and make your audience, congregation, or corporation happy—by applying EQ properly.