Arguably one of the more important tools, if not, the most important tool in a sound engineer’s arsenal, is the audio equalizer. Mixing audio without the equalizer is like, say, cooking without the proper cooking utensils. So what is it about the audio equalizer that makes it so important? How does it work exactly?
Many audio professionals and music enthusiasts would agree that the equalizer is pretty much a “must-have” in any music production, whether be it live or in the studio. In this article, we will go through the different functions and aspects of this essential tool. As usual, the information presented here are meant to be beginner-friendly. So, without further ado, let us begin!
The circuit or equipment that is designed for the purpose of equalization, is known as the equalizer. To explain it further, equalizers actually affects the change in amplitude of audio signals at specific frequencies. In layman’s term, they are “frequency-specific volume knobs.”
On the other hand, equalization is the process of adjusting various frequency components which exists within an electronic audio signal, in order to achieve a better balance. Equalization is of course, universally used in audio recording and reproduction, however, it also has many applications in electronics and telecommunications. These devices have the function of “boosting” or “cutting” the energy of specific frequency bands.
In the world of audio production (especially in live sound), equalization is a process that is very commonly used to adjust the frequency response of a sound system (also known as “tweaking”) using linear filters. Simple filter designs are used in hi-fi equipment, to allow bass and treble adjustments. On the contrary, graphic and parametric equalizers (used in pro audio) allows more flexibility in adjusting the frequencies of an audio signal.
The concept – Its application
Equalization is a concept that was first applied in the correction of frequency response in telephone lines that are using passive networks. This was before electronic amplification was invented. The early use of equalization was for the compensation of uneven frequency responses, inherent in electrical systems. This is done by using a filter with an opposite frequency response, to help restore the fidelity of the transmission.
As a result, a plot of the system’s final frequency response would be flat, as its response to all frequencies would literally be “equal”. Hence the term “equalization.”
Some time later, this concept was incorporated into audio engineering, for adjusting frequency responses in studio productions and in live sound reinforcement systems.
Frequency response correction will be done to a sound system (by sound engineers) to achieve a better frequency balance of the music (played through speakers), which should be a more accurate representation of the live performance on stage.
Audio amplifiers are always designed to include filters or controls to allow frequencies to be adjusted. Most of the time, this comes in the form of variable bass and treble controls (shelving filters), or switches that will apply low-pass or high-pass filters, in order to reduce low frequency “rumble” and high frequency “hiss” respectively.
Filters – Their functions
In order to have a better understanding of how an equalizer works, you need to first know what is at the heart of its design, and that is known as the “Filter”. In the realm of signal processing, a filter is described as a process, or a device that removes unwanted components in a signal.
We will not discuss too much on filters in this article, as it is a very elaborate topic and might just confuse you even further. Instead, in order to not bore you too much (seriously though, you’re not bored yet?), we will just go straight into explaining about the two main filter designs used in equalizers:
- First order filters
- Second order filters
First order filters
This filter design is able to alter the response of frequencies that are above and below a set point. If you look at the graphical representation of this filter response, it will have a slope of up to 6 dB per octave. The bass and treble controls you would normally see on a hi-fi system are actually first order filters, where the balance of frequencies (above and below a set point) are adjusted via a single knob.
There are special cases of first order filters, and they are known as “high-pass” or “low-pass” filters, where the 6 dB per octave cut of low or high frequencies extends indefinitely. All filters with this design are arguably the easiest ones to be implemented, as they only require a capacitor and resistor.
Second order filters
This filter design is capable of “resonance” (systems that are able to pick out specific frequencies within complex vibrations) around a specific frequency. The factors that affects this filter’s response is not only its frequency, but also its “Q” – a higher “Q” reflects a sharper response (narrower bandwidth) around a particular centre frequency.
As an example, the red response curve that you see in the accompanying image, cuts frequencies around the centre frequency of 100 Hz, with a higher Q setting than the blue response curve, which boosts frequencies around the centre frequency of 1000 Hz.
Examples of filters that are built based off this design include, “Parametric equalizers” and “Graphic equalizers” (which utilizes filter banks).
Filter types – Equalization functions
Since we have already understood that the range of equalization functions are governed by filters, we should now look into learning more about these functions. Take note that the flexibility of these functions are dependent on the topology of the circuitry and parameters (controls) that are available to the user.
Now let us look at the various equalization functions and types:
- High Pass and Low Pass filters
- Shelving Filter
- Graphic equalizer
- Parametric equalizer
High Pass and Low Pass filters
Very simply, a high-pass filter is a filter that allows higher frequencies to pass through while attenuating lower frequencies. On the contrary, a low-pass filter allows low-frequencies of signals to pass while attenuating higher frequencies.
You would notice that in professional audio applications or programs, these are normally called “low cut” and “high cut” respectively, in order to emphasize their effect on the original signal.
For example, some audio equipment (usually graphic equalizers) will include a switch that is often labelled “high cut” or described as a “hiss filter” (hiss being high-frequency noise). Just a fun fact, during the olden times of phonographs, many stereos would be designed with a high-pass (low cut) filter, also known as “rumble filter”, to eliminate infrasonic frequencies.
While high and low pass filters have the useful function of the removal of signals (usually unwanted) that are above or below a set frequency, shelving filters on the other hand, are often used to reduce or increase signals above or below a set frequency. You can see shelving filters being used in consumer audio equipment as common tone controls (bass and treble). These filters implement the “first order filter” response and allows frequencies above or below a certain point to be boosted or cut.
A graphic equalizer is basically made up of a bank of filters. Signal from the input is sent to these bank of filters and each filter will pass the portion of the signal present, that is in its own frequency range or “band”. The amplitude of the signal passed by each filter can then be adjusted with a slide control.
This will allow you to boost or cut frequencies passed by that particular filter. Hence, the vertical position of each slider represents the gain applied (either increase or decrease) at that frequency band, which will then resemble (as a whole) a “Graph” of the equalizer’s response plotted versus frequency.
The number of frequency channels (corresponds to the number of sliders) on a graphic equalizer affects its production costs and is also intended to meet the requirements of a specific application. For example, a car audio equalizer might only include one set of controls (which applies equal gain to both stereo channels) for convenience, with a total of five to ten frequency bands.
On the contrary, an equalizer designed for professional audio (usually live sound) will have 25 to 31 bands, allowing the fine tuning of frequencies to help solve feedback problems and equalization of room acoustics. Such equalizers are known as “1/3-octave equalizer” (pronounced as “third-octave EQ”) because the centre frequency of its filters are spaced one third of an octave apart, three filters to an octave.
For audio applications where less precise control is required, equalizers with half as many filters per octave (as compared to “third-octave EQ”) are often used and they are generally referred to as “2/3-octave” equalizers.
These equalizers are essentially “multi-band variable equalizers” which provides you with control over three primary parameters (amplitude, centre frequency and bandwidth). You can tweak the amplitude of each band, shift the centre frequency, and also widen or narrow down the bandwidth (which is inversely related to “Q”).
The benefit of having a parametric equalizer is that you will be able to make much more precise adjustments to sound as compared to other equalizers. These equalizers are also commonly used in studio recordings and live sound reinforcement. Furthermore, they are available for sale as standalone outboard gear units.
Another well-known version of the parametric equalizer is called the “semi-parametric equalizer”, also referred to as the “sweep-able filter”. It still allows control over the amplitude and frequency, but with a pre-set bandwidth of the centre frequency (the “Q”). You might be able to find some semi-parametric equalizers that allows selection between a wide and a narrow pre-set bandwidth.
Okay, let’s end it here for now. I hope this article has given clarity into how the different equalizers actually work. Hopefully you can now use this information and start incorporating equalization into your own productions!
Once again, thanks for reading and do leave comments or questions!