When it comes to tools of the trade, the equalizer is easily the most important signal processor in any audio engineer’s arsenal, and to use EQs properly, we need to understand how all the different types of equalizers work. In this article, we’ll break down everything you need to know about filters, graphic EQs, parametric EQs, linear-phase EQs, and dynamic EQs. We will also explain the role that each type of EQ plays as part of your audio toolbox.
Filters are considered one of the basic building blocks of signal processing. Generally speaking, a filter is any device or circuit that changes the tone, or timbre of an audio signal by amplifying or attenuating a range of frequencies. Filters are commonly thought of for removing unwanted elements, but a filter is more technically any single frequency band of an equalizer. In this article, we will mainly discuss “pass” type filters, which only allow certain frequencies to come through while rejecting everything else.
High-pass filters, for instance, cut all the frequencies below a selected point, attenuating the low frequencies and allowing the high frequencies to pass through. Alternatively, low-pass filters block frequencies above a selected point, attenuating the highs and allowing the lows to pass through. High-pass filters on microphone channels can remove low-frequency rumble from air conditioning, trucks driving by the studio, or bass that bleeds through the control wall into the vocal booth.
Band-pass filters remove the content both above and below the center frequency, allowing only the audio around the center frequency to be heard. 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 the sound of a cell phone speaker, while notch filters can remove ground buzz or other resonant frequencies.
The slope of a filter describes the shape of the transition between the filtered frequencies and the passed frequencies. Most analog filters have slopes of between 6dB and 24dB per octave, with higher numbers correlating to steeper slopes. Some advanced filter designs create a resonance peak (boost) right at the cutoff point for a little emphasis at the cutoff frequency.
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. The sonic characteristics of a filter are based on the type of analog circuit or algorithm that it uses. For example, a Chebyshev type 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. Each brand and model of equalizer chooses design elements that impart a particular color or purity that suits an intended musical purpose. That’s why there are so many EQs to choose from.
Graphic equalizers (like the API 560 graphic EQ pictured below) are simply a collection of fixed-frequency peak/notch filters that can be used to cut or boost several predefined frequency bands at once. Graphic EQs come in various sizes, including 31-band, 15-band, 10-band, 5-band, and even 3-band.
31-band or “1/3 octave” graphic EQs feature 31 center frequencies spaced in 1/3 octave intervals—in other words, three adjacent bands cover the range of one musical octave. These models typically offer gain adjustments up to 12 or 15 dB. With the ability to quickly notch out multiple frequencies, 31-band graphic EQs are an excellent choice for live sound applications, where notching out resonances is a necessity.
Some graphic EQs, like the 10-band API 560, use proportional Q technology for a musical response. Small EQ adjustments create wide curves while larger gain changes create narrower boosts and cuts.
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, these graphic EQs are well suited for gentle tone shaping.
Applying multiple graphic EQ bands simultaneously can create sophisticated EQ shapes that can emulate typical shelf or bell curve responses.
With dedicated controls for frequency, gain, and bandwidth (Q), parametric EQs offer more flexibility than graphic or simpler selectable equalizers. Analog parametric EQs range from three to five 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 band; low, low-mid, mid, hi-mid, and high, allowing 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 recording and live sound consoles. The famous Neve 1081 EQ, for example, provides adjustable frequency and gain controls, but only lets you choose between narrow vs. wide Q on its two mid-band sections.
Parametric EQs often allow you to switch the high and low EQ bands 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 around 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 frequency choices and adjustable gain for each band, along with a proportional Q circuit. A proportional Q circuit automatically widens the bandwidth at gentle gain settings and narrows the bandwidth at more extreme boost/cut settings.
Traditional analog and digital EQs are referred to as minimum phase equalizers. These EQs create a certain amount of phase shift to impart their tone-shaping. “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, all-analog EQs—from your trusty Neve 1073 to your favorite SSL or API channel strip—are minimum phase EQs.
Minimum phase EQs sound great in most instances, but during mastering or when applying EQ adjustments to delicate acoustic instruments it’s often recommended to use linear phase EQ plug-ins. Linear-phase EQs maintain a more natural tonal effect as they do not alter the phase of a signal. However, linear-phase EQs suffer from greater latency and higher CPU usage, which means they can’t be used during tracking or in large, CPU-intensive mixing sessions. Because of their lack of phase shift, linear phase EQs are excellent for parallel processing duties.
Linear phase EQs introduce their own type of distortion called pre-ringing, which can be noticeable in some cases. In practice, pre-ringing is rarely audible or much of a concern, and many plugins, like FabFilter’s Pro Q3, Sonarworks’ SoundID Reference, and Izotope’s Ozone 9 EQ allow the user to audition and choose between linear phase, minimum phase, and even mixed-phase EQ modes. You should try some linear phase EQs to see how they sound compared to minimum phase equalizers.
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 as you mix. This is where dynamic equalizers come in.
Dynamic EQs work similarly to multi-band compressors, except they don’t need to separate the audio into separate frequency bands for processing. Each EQ band provides frequency, gain, and Q controls along with a threshold setting and maybe even speed controls. When a particular frequency band crosses above or below the threshold, the EQ band gain automatically adjusts to maintain a constant loudness in that frequency band.
Let’s say you want to tame a harsh-sounding hi-hat in the overhead mics by applying a high-frequency shelf to cut the highs. When the drummer switches from playing the hi-hat to the ride cymbal in the chorus, the overheads will sound dull and flat. By using a dynamic EQ, the high-frequency cut would only engage when the harsh hi-hat is playing and the EQ would remain unaffected during the chorus, leaving the overheads sounding even and balanced throughout the entire song.Dynamic EQs can be especially helpful for mixing instruments with a lot of tonal variation, like drum kits, guitars, and vocals. They can also be helpful for subtle tone shaping on instrumental and mix busses. In specialized scenarios, dynamic EQs can 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.
You may only recognize that an EQ is a passive design if the name states that information—or you’ll learn that info when you read the manual. You do read manuals, don’t you? In any case, many great-sounding EQs are passive designs, most notably the Pultec line of equalizers. Passive EQs use passive circuit elements, like inductors, to provide tonal changes so they sound different than typical console-style equalizers. Passive EQs are well-regarded for their musical bass and treble tone-shaping capabilities, but passive EQs can produce parametric or band-pass filters. Try a passive EQ to add weight to the bottom of a bass, piano, vocal, or full mix. Then try using it to add air to some acoustic sources.
Go Forth and Shape Some Sounds
You can find an equalizer type that is optimized for every tone-shaping task imaginable, so go out there and try a few different types of EQs on every sound you come across. To learn the best uses of all your EQs, you may find it useful to try one particular model on many instruments for a few songs and then switch to another brand or type and use that one for a few songs. After a short time, you will instinctively know which EQ is best for any given situation.