The Complete Guide To Parametric Equalization/EQ

Parametric EQ is perhaps the most powerful type of EQ in audio technology, which says a lot since EQ is one of the most commonly used processes in audio mixing/production.

In this article, we'll learn all about parametric equalization and the parameters that can be adjusted while also taking a look at a few parametric EQ examples and the best applications of such EQ units (whether hardware or software).

The Basics Of EQ

Before we get to parametric EQ in more detail (skip ahead by clicking here), let's quickly go over the definition of EQ.

Equalization is an audio process that alters the relative balance between frequencies within an audio signal. EQ effectively increases or decreases the amplitude of some frequency bands compared to other frequency bands using filters. It is used in mixing, tone shaping, crossovers, feedback control and more.

EQ cutting and boosting refers to the decreasing and increasing of the relative amplitude of defined frequency bands, respectively.

Filters are typically thought of as processes that eliminate frequency content below, above or between two set frequency points. However, in the context of EQ, a filter can also be used to define the aforementioned boosting and cutting.

The range of frequencies affected by a certain filter of an EQ is typically referred to as a “band”. The frequency spectrum of audio/sound waves is continuous, and so EQ won't only affect a single specified discrete frequency. Rather, it will affect frequencies around its defined cutoff or centre frequency as well.

EQ is one of the most important tools for working with audio.

What Is Parametric Equalization?

Parametric EQ, when compared to other types of EQ, is the most configurable type of EQ.

Parametric EQ offers continuous control (within defined ranges) over most of, if not all, the important parameters of equalization. Therefore, parametric EQs are often the go-to choice for tone-shaping and surgical EQ situations.

The number of adjustable parameters and the amount at which each parameter may be adjusted may vary from band to band (largely dependent on the filter type of the band). Of course, some parametric EQs will offer greater control than others.

Parametric EQ designs lend themselves well to audio plugins, where DSP and computer programming can produce extremely accurate and flexible parametric EQs.

The more advanced EQs like linear phase EQ and dynamic EQ typically have parametric control and are most often in plugin form as well.

The parameters differ depending on the filter type (which is often selectable, especially with EQ plugins). The number of bands in a parametric EQ is typically between 3 and 7, though plugins may have many more bands than this.

Parametric EQs can be designed to processes mono, stereo, mid-side and other multi-channel audio formats.

So then, a parametric EQ is an EQ with lots of parameters. What are these parameters? We'll answer that in the next section.

Parametric EQ Parameters

The customization of the parametric EQ frequency bands can be achieved by altering some or all of the following parameters:

• Centre frequency (of mid bands)
• Gain (boost/cut)
• Quality factor (Q)
• Cutoff frequency (of low-pass and high-pass filters)
• Slope (of the low-pass and high-pass filters)
• Filter selection (per band)

Note that not all parametric EQs will have all the above options. At a certain point, when enough parameters are not adjustable, the EQ may be considered as “semi-parametric”.

With that, let's get into each of the aforementioned parametric EQ parameters:

Centre Frequency

The centre frequency parameter allows us to control the centre frequency value about which a bell/peak or notch filter will be centred.

Gain (Boost/Cut)

The gain control, given in decibels (dB), refers to the relative amplitude (boost or cut) applied to a band of a parametric EQ.

An EQ “cut” refers to negative gain (attenuation) of the band in question. Conversely, an EQ “boost” refers to positive gain (amplification) of the band in question.

The set gain value will generally happen at the aforementioned centre frequency. Adjacent frequencies will be affected by a lesser degree within the band's bandwidth/Q.

With shelving filters, the gain control will adjust the relative amplitude of frequency below a cutoff point (in a low shelf filter) or above a certain cutoff point (in a high shelf filter).

Note that low-pass, band-pass, high-pass, and band-stop/notch filters will not have gain controls as they’re designed to completely eliminate frequencies above, above and below, below, and between cutoff frequencies, respectively. Therefore, a “gain” value would technically be negative infinity (though the downward slope would technically only ever approach negative infinity).

Quality Factor (Q)

The quality factor (referred to simply as “Q”) is a dimensionless value that determines the ratio of the centre/resonant frequency to the filter's bandwidth.

Q = \frac{f_C}{BW} = \frac{\sqrt{f_H f_L}}{f_H - f_L}

where:
• f_C: centre/resonant frequency
• f_H: high cutoff frequency (where the signal drops off by 3 dB)
• f_L: low cutoff frequency (where the signal drops off by 3 dB)
• BW: bandwidth

In other words, Q refers to how narrow/steep or wide/gentle the boost or cut of the given filter/band will be. Higher Q values produce a narrower band in which a smaller range of frequencies is affected.

With boosts and cuts below 3 dB, the bandwidth may be registered between the points where the boost or cut is half its maximum (at the centre frequency). In this case, we have “half-gain Q” or “Robert Bristow-Johnson’s (RBJ) Q”.

Though Q has technical definitions, different parametric EQs will give different bandwidths with the same Q values.

So then, what's important to know is that, regardless of how Q is calculated, by increasing the Q, we narrow the bandwidth (whatever it happens to be), and by decreasing the Q, we widen the bandwidth of the filter.

Cutoff Frequency

Cutoff frequencies are defined as the points at which filters cause half-power or -3 dB of attenuation.

The cutoff frequency parameters of parametric EQs allow us to control the cutoff frequencies of high-pass, low-pass, band-pass, and band-pass filters.

Here’s an illustration of a band-pass filter with both low (f_L) and high cutoff frequencies (f_H) marked to get a better idea:

Slope

Some equalizer bands will have slope controls to adjust the roll-off rate of their filters. These controls effectively alter the order of the filter in question, causing the slope (defined in dB/octave and/or dB/decade) to change. Some EQs even offer a brickwall setting.

Here is a table of filter orders (from 1st to 5th) and their respective slope here:

Filter Selection

Some (or potentially all) bands in a parametric EQ will have options when it comes to choosing the filter type.

A parametric EQ's filter options may include:

• Low-pass filter
• Low shelf filter
• Bell curve filter
• Notch/band-stop filter
• High-pass filter
• High shelf filter
• Band-Pass filter

Low-Pass Filter

What is a low-pass filter in audio? A low-pass filter (LPF) “passes” the low-frequencies below their cutoff frequency while progressively attenuating frequencies above their cutoff. In other words, low-pass filters remove high-frequency content from an audio signal above a defined cut-off point.

Low Shelf Filter

What is a low-shelf filter in audio? A low-shelf filter is a filter that either boosts (increases amplitude) or cuts (decreases amplitude) frequencies below a certain cutoff frequency. These filters generally have a well-defined transition band and a levelling-off of amplitude in the lower end.

Bell Curve Filter

What is a bell curve filter in audio? A bell curve filter can produce resonance (boost in EQ) or anti-resonance (cut in EQ) around a specified centre frequency. These filters are defined by a central frequency, Q factor (width of the boost/cut) and relative gain.

Notch/Band-Stop Filter

What is a band-reject filter in audio? A band-stop filter (aka band-reject or notch filter) works by removing frequencies in a specified band within the overall frequency spectrum. It allows frequencies below the low cutoff frequency to pass along with frequencies above the high cutoff frequency.

High-Pass Filter

What is a high-pass filter in audio? A high-pass filter (HPF) “passes” the high-frequencies above their cutoff frequency while progressively attenuating frequencies below the cut-off frequency. In other words, high-pass filters remove low-frequency content from an audio signal below a defined cut-off point.

High Shelf Filter

What is a high-shelf filter in audio? A high shelf filter is a filter that either boosts (increases amplitude) or cuts (decreases amplitude) frequencies above a certain cutoff frequency. These filters generally have a well-defined transition band and a levelling-off of amplitude in the upper end.

Band-Pass Filter

What is a band-pass filter in audio? A band-pass filter “passes” a band of frequencies (a defined range above a low cutoff and below a high cutoff) while progressively attenuating frequencies below the low cutoff and above the high cutoff.

Using A Parametric EQ

Parametric EQs are powerful audio processors and prove themselves useful in a variety of applications.

The ability to sweep the frequency of a parametric EQ band and set it exactly where we need it to be is invaluable in many contexts. We can also control the Q parameter and, of course, the amount of gain.

So then, when using a parametric, utilize the sweeping functionality to find the good and bad frequencies of an audio signal and EQ them appropriately. This is true of all filters, including the low-pass and high-pass filters.

Parametric EQ applications include, but are not limited to, the following:

Tone Shaping With Parametric EQ

shaping the tone/character of an audio signal/track by EQing it in a specific way.

Eliminating Problem Frequencies With Parametric EQ

noise, resonances and other nasty frequencies can be sought and destroyed with the continuous frequency controls of the parametric EQ.

Feedback Elimination With Parametric EQ

similar to problem frequencies. Can be used effectively in live sound situations.

Tuning Monitors With Parametric EQ

monitors and speakers are imperfect electro-acoustic transducers that may benefit from adjustable parametric EQ.

Examples Of Parametric Equalizers

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 5 different parametric equalizers to help solidify our understanding of this EQ type.

In this section, we’ll discuss:

• 500 series parametric EQ unit: IGS iQ505
• 19″ rack mount parametric EQ unit: Manley Massive Passive
• Parametric EQ effect pedal: Empress Effects ParaEQ
• Parametric EQ Eurorack module: Cwejman VCEQ-3
• Parametric EQ plugin: FabFilter Pro-Q 3

IGS iQ505

The IGS iQ505 is a fully parametric analog equalizer designed into a 500 Series module. It has 5 parametric overlapping bands, each with ±12 dB cut/boost.

The IGS iQ505 offers a notably wide EQ range spanning from 10 Hz (infrasound) all the way up to 24 kHz (ultrasound). The lowest and highest bands each have the option of either a bell curve filter or a shelving filter. Each band has a Q knob which is responsible for the quality factor of each filter.

Manley Massive Passive

The Manley Massive Passive is a passive stereo parametric tube EQ. Each of its two stereo channels offers 4 bands of parametric EQ along with a low-pass and high-pass filter.

This incredibly impressive rack-mounted EQ has a lot to offer. As mentioned, both channels have 4 bands plus an HPF and LPF. These bands offer notable overlap, and each band has a significant adjustable frequency range. The bands can be set to boost or cut by up to 20 dB and can offer either bell curve filters or shelving filters. The Q factor (for bell curve filters) or resonance/slope (for shelving filters) is also fully adjustable.

Each channel also comes with its own gain control from -6 to +4 dB.

Empress Effects ParaEQ

The Empress Effects ParaEQ is a superb parametric EQ in the form factor of a stompbox.

The ParaEQ offers three bands, each with ±15 dB of cut/boost, continuously adjustable centre frequency, and 3 Q factor options (medium, narrow, wide).

This EQ pedal offers tons of headroom. We won’t even have to worry about clipping it. On top of the 3-way pad switch (offering 0, –12 and –4 dB), the ParaEQ can also be switched to run off 9, 12 or 18V power supplies (the higher the supply voltage, the higher the headroom).

In addition to the 3-band parametric EQ, this pedal offers up to 30 dB of clean signal boost and an input pad with 0, -6 and -12 dB options.

Cwejman VCEQ-3

The Cwejman VCEQ-3 is a 3-band parametric EQ designed for Eurorack modular synthesizers.

This unit is a bit different than the others we've described. First, each of the bands is identical to start with, meaning the possible frequency range, Q factor, and relative gain are all the same (most other parametric EQs will have their bands within more defined ranges such as “low, low-mid, high, etc.).

Another interesting part about the VCEQ-3 is that the centre frequency, Q and relative gain of each band can be controlled via control voltages (or via the knobs of the unit).

FabFilter Pro-Q 3

The FabFilter Pro-Q 3 is a superb equalization plugin that offers pretty much all the functions an EQ could be programmed to have, including dynamic, linear phase and, of course, parametric EQ functionality.

First of all, the Pro-Q 3 can have up to 24 bands that can easily be added or removed from the plugin.

Each of these bands can be programmed as pretty much any filter type imaginable (including brickwall). The cutoff or centre frequency can be set at any point within the audible range. Gain values (cut/boost) are made possible anywhere between -30 and +30 dB, and Q factor values range from 0.025 all the way to 40. The slope can also be altered in addition to the Q.

Each band can be set to affect the left or right channels or both in the stereo link. Mid-side processing is also available so that a band can act solely on the mid or side information.

There's a lot more to know about this powerful plugin, and I invite you to check it out for yourself!

Related Questions

What are the different types of EQ? When it comes to audio equalization, there are several types of EQ to be aware of. They are as follows:

• Graphic EQ
• Parametric EQ
• Semi-Parametric EQ
• Dynamic EQ
• Linear Phase EQ
• Passive EQ
• Shelving EQ
• Stereo EQ
• Mid-Side EQ

What does the Q parameter stand for in EQ? Q stands for the quality factor, which is the ratio of the centre/resonant frequency to the bandwidth of the EQ's filter. The higher the Q factor, the narrower the boost or cut (the steeper the transition period) of the specified EQ filter.