A Brief Overview of Equalization & How It Works

In the audio world, equalizers are an indispensable tool that helps solve a large set of problems we find ourselves challenged with. EQ’s help us shape the balance and tone of a track, both in the mixing and mastering stages. So what exactly is an EQ?

An Equalizer is one or more audio filters that are applied either constructively to boost the signal, or destructively to reduce it, for a specified frequency band. These filters have different shapes and types, which impact the original signal differently depending on their slope (measured in dB/oct) and Q Factor (Q) which defines the bandwidth of frequencies affect by the filter. The higher the dB/oct ratio, the steeper that filter will be, which will in turn affect

Some basic filter types you’ll see:

-          Bell – This is a filter that will attenuate frequencies around a specified center frequency. These are good for boosting or lowering specific frequency bands without impacting all the frequencies around them.

-          High-pass (or low-cut) – this is a filter that will attenuate (or reduce) any frequencies below a specified cut off frequency. These can be used to reduce or remove low-end from a signal.

-          Low-pass (or high-cut) – this is a filter that will attenuate any frequencies above a specified cut off frequency. These can be used to reduce or remove high content from a signal.

-          Band-pass – this is a filter that will allow only certain frequencies through a specified frequency. Anything above or below that frequency will get attenuated based of its slope.

-          High-shelf – This is a filter that raises or lowers all frequencies at the same rate above or below a designated frequency. These can be used to attenuate a large amount of high frequency information with a consistent level.

-          Low-shelf – This is a filter that raises or lowers all frequencies at the same rate above or below a designated frequency. These can be used to attenuate a large amount of low-end frequency information.

-          Notch (or band stop) – This is a filter that removes all frequencies at a specified frequency, and let signals pass above or below that given frequency.

As we’ve discussed, an EQ applies filters to achieve its sound attenuation. The steeper a filter is, and its proximity to another filter, can determine how effective that filter is. Imagine this – two extreme filters sit next to each other – one boosting 40dB, and the other cutting 40dB. The frequencies that overlap between them have to go up 40dB now, and then immediately back down 40dB. In the digital world, unless you’re using a Linear Phase (more on that in a moment) filter, you lose a little bit of the original frequency every time you apply a filter to it due to approximation of how a signal would be impacted by said filter. When possible it’s best to use series of filters to apply changes to regions with broad strokes (when mixing – for sound design, all rules are off!).

Beyond the filter types, in the digital world there are different methods for applying equalization. In the physical world, equalizers often differ in the tone they impart by applying these filters, where things like signal strength, heat, electricity, and other physical components impact how the final signal sounds. While digital equalizers often times replicate these physical traits through artificial saturation or subtle algorithms, we also have the power to change how our equalizers are impacted by the time domain of a signal. Equalizers take time to impart their filters upon an incoming signal, and as such phasing occurs when applying filters to a signal, as the original sound has moved forward in time by the time the equalizer completes its processing.

Here are some of the basic types of digital equalizers you’ll run into:

-          Parametric EQ – This is a type of equalizer that is most common when working in a DAW or mixing in the box. A parametric EQ offers multiple bands of filters, with (oftentimes) the ability to change the types of filters being applied at bands, along with the slope of the filter. You’ll often see these to be Zero Latency, which offers very little phasing and good tone.

-          Linear Phase EQ – This is a type of (usually Parametric) equalizer that applies its filters in perfect phase with the original, leaving no phasing artifacts. Why wouldn’t you always use these then? Because they have to slowdown the incoming signal to compensate for the perfect phase attenuation. These are best used on a single track (such as in mastering), where the only one signal is present – otherwise the track with the Linear Phase EQ on it will be delayed behind other tracks playing.

-          Graphic EQ – These are more common in the physical world for equalization, and offer fixed frequencies they attenuate at a given gain scale. They are usually broken down into multiple bands and have faders or knobs to allow raising or lowering the fixed frequency.

-          Shelving EQ – These are equalizers that are used for lowering or raising large portions of the tracks low or high frequency content. They apply shelf filters that can have varying slopes to help tonally shift a signal.

-          Dynamic EQ – These are equalizers that have thresholds, similar to a compressor, upon which they apply their filters based off the incoming volume of the specified frequency band. These are beneficially when a signal needs to be loud during certain sections, and then lowered when other elements come back in. Compared to a compressor, these offer the ability to boost as well as reduce a signal, and can be useful for situations where the balance can change throughout a mix.