When it comes to tools of the trade, the equalizer is easily the most important signal processor in any audio engineer’s arsenal. But in order to use EQs properly, you need to understand how all the different types of equalizers work.
In this blog, we’ll break down everything you need to know about filters, graphic EQs, parametric EQs, linear-phase EQs and dynamic EQs. We will also talk about what each type of EQ is best used for to help you on your path to mastering the craft.
Filters are considered to be one of the basic building blocks of signal processing. Generally speaking, a filter is considered to be any device or circuit that changes the tone, or timbre of an audio signal. In the real world, most people refer to a filter as a type of EQ that specifically removes unwanted material. In this article, we will mainly discussing “pass” type filters, which only allow certain frequencies through, while rejecting others.
High-pass filters, for instance, cut frequencies below a selected point, attenuating the low frequencies and allowing the high frequencies to pass through. Alternatively, low-pass filters cut frequencies above a selected point, attenuating the highs and allowing the lows to pass through. High-pass filters on microphone channels typically remove low frequency rumbling from air conditioning noise or trucks outside your studio.
Band-pass filters cut both the high and low frequencies, allowing only the midrange to pass through. The opposite of a band-pass filter is a band-rejection filter, also known as a notch filter, which removes a narrow frequency band and leaves most of the signal unaltered. Band-pass filters can create effects like music played through a cell phone speaker.
The slope of a filter affects the shape of transition between the filtered frequencies and the passed frequencies. Most analog filters have slopes of between 12dB and 24dB per octave, with higher numbers correlating to steeper slopes. Some advanced filter designs create a peak resonance at the cutoff point for extra emphasis.
The sonic characteristics of a filter are based on the type of circuit, or algorithm that it uses to attenuate frequencies. For example, the well-behaved Chebyshev filter has a smooth slope; the easy to implement Butterworth filter may have notable phase nonlinearities; the gentle Bessel filter has minimal phase shift artifacts; and the Elliptic filter has the steepest filter slope.
Filters are used in almost every stage of audio processing. They’re built into instruments, amplifiers, and equalizers and are used by everyone from musicians to mastering engineers.
Graphic equalizers are basically a collection of fixed-frequency bell filters that can be used to cut or boost signals. Graphic EQs come in various sizes, including 31-band, 15-band and 10-band (give or take a band).
31-band or “1/3 octave” graphic EQs feature 31 bands spaced in 1/3 octave intervals—in other words, three bands cover the range of one musical octave. These models typically offer gain adjustments up to 12 or 15 dB and feature a fixed Q value of around 4. With the ability to easily notch out multiple frequencies, 31-band graphic EQs are an excellent choice for live sound applications and precise subtractive EQ.
Some graphic EQs, like the 10-band API 560, use proportional Q technology for a musical tone, while 1/3 octave EQs work well for notching out resonances, but struggle to create smooth EQ curves.
15-band, or 2/3 octave graphic EQs, are commonly seen in smaller live sound setups, built into bass and guitar amps, or mounted in studio racks. 10-band graphic EQs are available in rack mount and 500-series units for the studio, as well as guitar pedals. With broader Q values, smaller graphic EQs are well suited for gentle tone shaping.
By using multiple bands simultaneously, graphic EQs can be used to create more sophisticated EQ shapes like shelving or bell curves, similar to parametric EQs.
With dedicated controls for frequency, gain and bandwidth, sometimes called Q, parametric EQs offer more flexibility than graphic or selectable equalizers. Analog parametric EQs typically range from three to eight bands, while digital parametric EQs may offer even more bands.
Parametric EQs come in two basic variants: semi-parametric and fully-parametric.
Fully-parametric EQs include frequency, gain and Q (bandwidth) controls for each frequency range, like low, low-mid, mid, hi-mid and high, allowing for advanced tone shaping capabilities. These are commonly found in high-end mixing consoles and analog studio equipment.
Semi-parametric EQs offer frequency and gain controls, but not fully adjustable Q controls for each band, and are commonly found in studio and live sound consoles. The famous Neve 1081 EQ, for example lets you select frequency and gain for each band, but only lets you choose between narrow or wide Q on the two mid-band sections.
Parametric EQs often contain high and low EQ bands that can be switched between shelving and bell shapes. Shelf EQs boost or cut a signal above or below the specified frequency, while bell curves boost or cut the signal centered on the selected frequency.
Although they’re not technically parametric EQs and do not offer adjustable Q controls, selectable-frequency EQs like the API 550A/B offer a choice of frequencies and gain for each band, along with a proportional Q circuit. A proportional Q circuit automatically widens the bandwidth with gentle gain settings and narrows the bandwidth at more extreme boost and cut settings.
Traditional or standard analog and digital EQs are also called “minimum phase” equalizers. These EQs cause a small amount of latency when modifying frequencies, which affects the phase of a signal. “Phase smear,” as it’s sometimes called, can create audible artifacts in your signal, which may be an interesting color or an unwanted distortion depending on the desired EQ effect.
Don’t get the wrong idea, there’s nothing wrong with minimal phase EQs. In fact, most EQs—from your trusty Neve 1073 to your favorite SSL or API channel strip—are minimum phase EQs. In fact, all analog EQs are minimum phase EQs.
Minimum phase EQs sound great in most instances, but during mastering or when applying delicate EQ adjustments to acoustic instruments it’s often best to use linear phase EQ plug-ins. Linear-phase EQs do not alter the phase of a signal, creating a more natural tonal effect and may also work well for parallel EQ duties. However, linear-phase EQs suffer from greater latency and high CPU usage, which means they can’t be used during tracking or in large, CPU intensive mixing sessions.
Linear phase EQs also introduce their own type of distortion called pre-ringing, which some very picky people find offensive. In practice, pre-ringing is rarely audible or much of a concern. Many modern EQ plugins, like Fab Filter Pro Q, Sonarworks Reference and Izotope’s Ozone EQ allow the user to audition and choose between linear phase, minimum phase and even mixed phase EQ modes.
Sometimes it can be tough to dial in the right tone with a traditional equalizer. At any given moment during a performance, there’s either too much of one frequency or not enough of another. You can’t find the right frequency balance and it feels like the song is changing right in front of you and you’re just mixing in circles. This is where dynamic equalizers come in.
Dynamic EQs work similarly to multi-band compressors, except they affect the frequency response and not the signal’s dynamics. Each band features frequency, gain, and Q settings along well as a threshold setting and maybe even speed controls. The selected frequency’s gain self-adjusts when that frequency’s threshold is reached to automatically turn up or down the level of a particular frequency band.
Let’s say you tame a harsh-sounding hi-hat in the overhead mics by applying a high frequency shelf to cut the highs. Then, when the drummer switches from playing the hi-hat to the ride cymbal in the chorus, the overheads sound dull and flat. By using a dynamic EQ, the high frequency cut would only engage when the harsh hi-hat is being played and the signal would remain unaffected during the chorus, leaving the overheads sounding even and balanced throughout the entire song.
Dynamic EQs can be especially helpful when mixing dynamic instruments, like drum kits, guitars, and vocals. They can also be helpful for subtle tone shaping on instrument and mix busses. And in more specialized scenarios, they can be used to carve out space between two competing instruments, such as a kick drum and bass guitar. Dynamic EQs are especially useful for evening out the occasional boomy or strident word in an otherwise great vocal performance.
There is an equalizer optimized for every tone-shaping task you can imagine, so go out there and try some different types of EQs on every sound you come across. You may find it useful to try one particular EQ plugin on everything for a few songs and then switch to another EQ and use that one for a few songs. After a short time you will instinctively know what EQ is best for any given situation.