What Is Stereo Equalization/EQ In Audio & How Does It Work?

Equalization is one of the most commonly used processes/effects in audio mixing/production and one of the most popular formats for audio/sound reproduction. It makes perfect sense that stereo EQ would come about in audio technology.

In this article, we'll describe stereo equalization with a focus on stereophonic audio and general EQ. We'll consider a few stereo EQ unit examples, along with the situations and applications that call for stereo equalization.

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What Is Stereo Audio?

To understand stereo EQ, we must understand stereo audio.

Stereo audio produces stereophonic sound that offers a multi-directional perspective and the ability for sound to be localized in reference to the listener's “position”. This is unlike mono audio, which produces monophonic sound that does not offer a multi-directional perspective and only offers depth/distance in the mix.

Stereo audio is made of two channels: a left channel and a right channel. Note that mono is made of one channel, and surround sound (and other non-stereo multi-channel formats) has more than two channels.

Stereo audio offers left and right channels, which resembles the way we hear naturally (with a left and right ear).

With headphones, the left and right channels are heard, in theory, exclusively by the left and right ear, respectively.

Stereo monitors/speakers fill up the acoustic space and can be heard by both ears. That being said, when positioned correctly between the speaker(s) producing the left channel and the speaker(s) producing the right channel, the listener will be able to hear the stereo mix.

It goes without saying, then, that if we listen properly to played back stereo audio, we'd be able to hear sounds around the panorama and distinguish sounds as being in the centre or, alternatively, some degree left or right of centre (defined largely by panning).

Today, stereo audio makes up a large percentage of what we listen to in music and broadcast, though mono and surround sound are also common.

The key takeaway from this section is that stereo audio has two channels (left and right). The differences in these channels are intended to give the illusion of a multi-directional perspective across the left-to-right panorama.

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What Is Audio Equalization?

We know what stereo audio is, now what about equalization?

Equalization is the process by which the relative balance of frequencies is adjusted within an audio signal. EQ increases or decreases the relative amplitudes of some frequency bands compared to other frequency bands using filters, boosts and cuts. It is used in mixing, tone shaping, crossovers, feedback control and more.

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

Filters are generally thought of as parameters that eliminate frequency content below, above or between two set frequency points. However, the term “filter” can also be used to define the boosting and cutting mentioned above.

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 doesn’t only affect a single specified discrete frequency.

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

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What Is Stereo Equalization?

Let's put it all together now. Stereo equalization, then, is EQ that acts upon stereo signals. It's often paramount, in mixing, that we pass a stereo signal through processing. A stereo EQ allows us to do just that without having to reach for two mono EQs (and a splitter to feed the left and right channels in each mono EQ).

Note that the majority of EQ plugins will automatically adjust to become stereo when inserted on a stereo track. However, this isn't the case in the analog world, so stereo equalizers become important when dealing with stereo signals.

This means that the EQ would have, at the very least, a stereo input and/or two independent mono inputs. It would also mean that the EQ unit or plugin would have a stereo output. Therefore, the EQ can act upon stereo (2-channel) signals.

Now, some stereo equalizers will EQ the left and right channels of the stereo signal with the same parameters. That's okay; they're still considered stereo EQs.

Other stereo equalizers will offer independent EQ controls for the left and right channels, allowing users to EQ each of the stereo channels separately.

Note that, in the cases where the stereo channels can be affected independently, there may be a stereo link function that would effectively link the two-channel together so that they're controlled via the same parameters (typically the left channel controls).

In addition to independent stereo and stereo linking, there are some stereo EQs that offer mid-side processing. Mid-side processing requires the encoding of left and right stereo channels into mid and side channels.

The mid-channel holds the information in the centre of the stereo image (where the left and right channels are the same), and the side channel holds the information to the sides of the stereo image (where the left and right channels are not the same). Note that decoding back to stereo before the output is generally necessary for these EQ designs.

So then, stereo EQs can be used to affect each channel of a stereo signal with the same equalization or, alternatively, they may offer independent EQ controls over each channel and even mid-side control in some cases.

As an aside, the term “stereo EQ” could refer to the basic EQ in a stereo system (for home stereo, car stereo, etc.). These EQs are generally pretty simple with shelving filters and perhaps a mid-frequency bell filter control and are technically part of the amplifier and/or receiver that drives the stereo system.

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Examples Of Stereo Equalizers

Before we wrap things up, it’s always a great idea to consider some examples. Let’s have a look at 3 different stereo equalizers to help solidify our understanding of this EQ type.

In this section, we’ll discuss:

• 500 Series stereo EQ unit: Elysia XFilter
• 19″ rack mount stereo EQ unit: Pyle-Pro PPEQ150
• Stereo EQ plugin: Waves GEQ

Elysia XFilter

The Elysia XFilter is a 500 Series true linked stereo EQ, meaning it processes both of its channels with just one set of controllers.

This “stereo EQ” technically affects each of the stereo channels independently, though it uses the same controls to apply the same EQ to each channel.

This 4-band EQ uses class-A topology in its low shelf, low mid, high mid and high shelf frequency bands. The shelving filters can be switched into high-pass and low-pass filters (12 dB/octave) with resonance controls.

Each band can be cut or boost ± 16 dB, and the corner or centre frequency can be selected via a 41-step potentiometer knob for each band. Wide (0.5) and narrow (1.0) Q options are selectable for the two mid-peak filters.

In addition to its four flexible active bands, the XFilter 500 features a switchable fixed passive LC (inductor-capacitor) filter for polishing the high-frequency range.

The controls of the XFilter make it a semi-parametric EQ.

Pyle-Pro PPEQ150

The Pyle-Pro PPEQ150 is a graphic EQ with stereo processing. This dual 10 band EQ, as the name would suggest, offers 10 graphic bands per channel.

Each of the stereo channels can be boosted/cut by ±12 dB and have bands with the following centre frequencies: 32, 64, 125, 250, 500, 1k, 2k, 4k, 8k, 16k Hertz. This unit also offers a VU meter for each channel for metering levels.

Waves GEQ

The Waves GEQ Graphic Equalizer is a stereo 30-band graphic EQ plugin.

This EQ plugin offers three graphic bands per octave for a total of 30 bands per channel. The left and right channels each have an independent high-pass and low-pass filter along with a single parametric bell-curve filter and gain control.

Each of the stereo channels can be EQed completely independently of the other, or, alternatively, the two channels could be linked via the stereo link option.

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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

Should you EQ or compress first? There is great debate as to whether an equalizer or a compressor should come first in the audio signal chain. There's no rule stating that either should come first. However, in general, you'll likely get the most out of the EQ and compressor if you follow these standards:

• For tonal shaping, it's often best to compress beforehand to avoid having to alter the compressor settings.
• For audio signals that require significant filtering, it's best to EQ first so as to not feed the compressor with unwanted frequency content.