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What is compression in music? It is a specific type of signal processing that reduces the dynamic range in an audio recording. Dynamic range can be described as how much difference there is between the loudest and softest parts of a track.
Audio compression has been around since the very first days of recorded sound, but how does it work? Read on!
What is audio compression?
Audio compression is the process of squashing sounds into a narrower dynamic range, thus ensuring that quieter parts of songs are not drowned out by louder ones.
The use of compression allows for music to have more consistent volume levels, which makes it easier for listeners to hear what’s being played without having to turn up their speakers or adjust how loudly they are listening.
How does audio compression work?
Audio compression works by attenuating the gain of louder sections of music, so that the overall volume stays consistent. The continuous change in how much quieter and how much louder a song is can be seen as “peaks” or “lows,” which compression works to flatten out into more even lines.
Compression reduces how big these peaks are, giving listeners less variation between how loud they have to turn up their speakers when listening on different parts of the track, resulting in a more consistent volume across the board.
Compressor vs Limiter
The two types of devices are often confused, but they’re actually quite different.
A compressor works by reducing the dynamic range of an audio signal so that loud passages are brought down to match quieter passages. This can be used as a creative effect or for practical reasons such as to avoid clipping on playback when recording vocals or guitars.
On the other hand, limiting is designed specifically to prevent signals from exceeding certain limits in amplitude (volume). This is why limiters are more commonly used for mastering purposes (to prevent clipping) rather than mixing.
What is compression in music used for?
Compression is one of the most important tools in a music producer’s arsenal. It can be used to control dynamics, reduce distortion and noise, increase sustain (and therefore volume), make instruments sound more “punchy,” boost solos, even out vocals. The list goes on and on.
But compression isn’t just for producers – it’s an essential tool that musicians need to understand as well. Musicians often use compressors while recording (or live) to help their performance come across stronger and more dynamic in the final mix.
Besides being used in the recording process, some other common uses for compression include:
- mastering
- live concerts
- radio broadcasting
- podcasting
- video production
- voice-overs
Audio compression parameters
There are 6 main parameters used to define and control compression:
- Threshold
- Ratio
- Knee
- Attack time
- Release time
- Output gain/makeup
Threshold
The threshold is the level at which audio will be compressed.
Anything below the threshold is uneffected, and anything above it has audio compression applied to them. The higher this number goes, the more audio that gets compressed – but if you turn it up too high then all of your audio may end up being a single volume level!
A good rule-of-thumb for finding an appropriate threshold setting is to experiment with values between about 60dB (which compresses everything) and 0dB (where no audio ever gets compressed).
Ratio
The ratio controls how much audio will be compressed. A higher ratio means more compression, and a lower ratio means less compression.
For example, a setting of 4:1 means that for every four decibels the input level reaches, only one decibel would make it to the output. Some of the most common ratios are 1:1 (no compression), 2:1, 4:1, 8:1, 16:1 and 32:1 or 64:2 depending on the desired effect.
It’s a good idea to start with a lower ratio before working your way up if you’re not sure where to start as too much compression can sometimes lead to unwanted distortion in the sound or make it difficult for an instrument or voice track to be heard clearly.
Knee
An odd-sounding term, but makes sense. The knee parameter refers to how the compressor will act after it has been triggered. A “hard knee” means that the change in amplitude is abrupt, while a “soft knee” results in a gradual reduction of volume.
If you set a low value for the knee, then the compressor will slowly ramp up as volume increases. This is good if you want a subtle effect or do not want your music to be squashed too much. A high value for the knee means that there is no gradual increase and instead, it slams down when the threshold is reached. You might use this setting if you are looking for an intense sound like in dubstep tracks or electronic dance music (EDM).
Attack Time
Attack time is simply the amount of milliseconds it takes for the compressor to reach its full effect after a signal exceeds the threshold.
The faster it reaches its max value, the more intense and aggressive your compressed signal becomes. A slower attack time results in a smoother transition for louder signals because of less compression applied when they’re quieter or lower-volume passages.
Release Time
The release time dictates how long it will take before the audio returns to its original level after being compressed. A quick release time helps with punchy sounds while longer releases help tame dynamics for smoother mixes.
Makeup Gain
Makeup gain is the adjustment of volume to compensate for the change in dynamics. This is an important feature because compressed sounds can become noticeably quieter, which may not be desirable when you’re making a mix that includes loud and quiet sections.
The amount of makeup gain should depend on what you need it for – a smaller amount will help with any sudden changes in volume while a higher one helps make up volumes lost through compression.
Types of compression
There are four main types of compression, each with its own characteristics. They are:
- VCA
- Optical
- FET
- Tube
VCA
A VCA (Voltage Controlled Amplifier) compressor is a type of compressor that is generally considered to be the most transparent and natural sounding.
The VCA is a form of variable resistor with one side connected to the output and the other side connected to ground. When no audio signal is applied, there’s no change in current flowing through the circuit so it has no effect on volume; but when an audio signal enters, its amplitude will cause changes in current flow which are then amplified by the circuit.
Optical
An optical compressor’s design is based on a photocell that detects fluctuations in light. When it detects these fluctuations, it turns into a signal which then activates the device to compress and limit sound waves going through the system. The result is less volume variation between loud and soft sounds and more even dynamics overall without any distortion or unwanted effects like pumping or breathing.
FET
A FET (Field Effect Transistor) compressor is a type of compression that uses an active electronic device to control the gain between two stages. It was first invented by Harold Black in 1948 and it became popular in the 1960s because it could be used as an automatic volume controller, which was a new concept at the time. The circuit consists of three parts: input impedance, feedback network, and output impedance.
In contrast with other types of compressors, FET compressors have an uncolored sound because they use solid-state components instead of tubes (valves). However, this can also make them less responsive than valve models and more difficult to work with in some ways.
Tube
Tube compressors are a classic type of compressor. They can be used to control the dynamics and tonal qualities of an audio signal, as well as act as a limiter, which helps prevent peaks from clipping and distorting.
Tube compressors tend to deliver more warmth than their solid state counterparts because they color the sound by adding harmonic content in order to make it sound richer or smoother.
A tube compressor works differently than other types of compressors because instead of using electronics to amplify its input signal, it uses vacuum tubes that change the voltage coming into them so that when amplified, the output is louder.
Compression tips and techniques
Compressors are often the most misunderstood of all audio processing tools. It’s not uncommon for a producer to use them improperly or even abuse them, with little or no understanding of what they’re doing and why.
In many cases, compression is necessary but be mindful that it can also be overused and abused. This can yield unpleasant results, such as making the track sound flat or over-compressed.
As with most audio processing, there isn’t a black-and-white approach to audio compression, but there are a few things to keep an eye on when using it in your music:
- Too much compression can cause distortion in your music, so it’s important to use with caution. If you are getting distortion after applying compression, try backing off from the threshold level until it goes away.
- It’s important to use compression sparingly or you’ll end up with a squashed and lifeless sound
- You should compress individual sounds rather than applying it across the whole mix – leave that for the mastering process
- Use compression sparingly when recording vocals as it can make them sound unnatural and robotic
- Don’t compress drums too heavily or they’ll lose their punchiness and ‘pop’, and will be more difficult to hear in the mix.
- If things are sounding too thinned out and flat after compressing them, lower the ratio setting and turn up the threshold again.
- When compressing an instrument or voice, make sure not to use too much of the spectrum – play with your attack time until you find what sounds best.
What is compression in music? Some say the “black art” of music production
Compression is a mystery to many musicians and producers. It’s been called “the most misunderstood effect in music production”, and rightly so! However, I hope that this article has helped clarify any misconceptions you had on the topic and will help get you started with exploring its applications for yourself.
After you’ve come to grips with compression, why not learn about another essential music production tool in my guide to reverb? Also, you’ll find some great compressors in my 50 best free vst plugins guide if you’d like to practise what you’ve learned in this article.