Home » Part 1. Audio Compressor: What Is it And How Does It Work

Part 1. Audio Compressor: What Is it And How Does It Work

This post is part one of a five part series on audio compression. In the post, we will what is an audio compressor and how it works. We will go over each control on the compressor so you can have more understanding of how the audio compressor works.

audio compressor

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Audio compressors are a vital tool in modern music production. Nowadays, music is getting louder and louder (louder means better) now there is a competition to be the loudest.

Without the compressor mix and mastering engineers can’t achieve that big loud sound that we hear in popular music.

Learning how to use the compressor can be a very daunting task for young engineers and producers. They often misuse the compressor, which makes their mix sound dull or too punchy.

Using the compressor properly comes from knowing how it works and knowing the variety of functions they offer.

What Is An Audio Compressor

An audio compressor is a dynamic processor used in audio production to compress a sound’s dynamic range.

More specifically, it is used to reduce the difference between the loudest part and the quietest part of an audio signal during mixing or mastering.

Once the sound’s dynamic range is more consistent, the mix engineer can turn up the overall loudness of the audio without the burst of loud transients.

The compressor does much more than controlling dynamic range and maximise loudness.

Professional mix engineers also use the compressor for there unique sounds and the colourisation it adds to the mix.

Mix engineers also use the compressor to add warmth, punch, add presence to their mixes.

How Does An Audio Compressor Work

Just running an audio signal through a compressor without compressing can change the tone, add sonic effect.

Some compressors are more natural and you have to

Audio Compressor Basic Parameters

The list of parameters below is the basic controls you will find on most compressors.

There are compressors with more advance controls, but I won’t cover that here.

Threshold

Set the threshold level just about the average level in the audio signal. Once the loudest parts surpass the threshold, the compressor is activated, and it gets compressed.

Threshold levels measured in dB, and it can be confusing to see the measurement in negative numbers. But remember in the digital domain, anything above 0dBFS is clipping.

Setting the threshold at -20dB means that audio signals -19 and above will get compressed and signals that fall below -20 is unaffected by the compressor.

Ratio

The ratio determines by home much the audio signal that exceeds the threshold gets compressed.

This works on a principle of gain ratio, that is, it measures input level to output level. A ratio of 4:1 means that for every 4dB above the signal gets compressed the 1dB.

For example, let’s compress an audio signal that peaks at -8dB. If we set the threshold at -20dB, the input signal is 12dB above the threshold.

A ratio of 4:1, the audio signal gets compressed to 3dB above the threshold.
That is an 8dB gain reduction, and the output signal is -12dB.

Attack

The attack controls the speed to which the compressor reacts to the audio signal once it passes the threshold level. It is the time the compress takes to go from no compression to full compression.

A fast attack means the compressor kicks in as soon as the audio signal passes the threshold. This is usually used the tackle the initial transients that are too loud.

A slower attack time allows more of initial transients over the threshold before the compressor starts compressing.

On most compressors, the attack is a knob control, which allows you to dial in how fast or slow the attack time. Some compressor has a switch between fast or slow attack time.

Release

The release controls how quickly the compressor stops compressing once the signal drops below the threshold level.

Setting a very slow release time can cause a ‘pumping’ effect, which are artefacts that of the compressor clamping down on the sound for an unnaturally long period.

Some compressor plugins have auto-release control, which can be useful when you are just learning how to use the compressor.

Makeup Gain

Sometimes label as output-gain on some compressor, the make-up gain is the less complicated parameter to understand and use. It allows you to ‘make up’ the gain you lost through gain reduction.

Take our example from above we got 8dB of gain reduction after compression, which we can add back with the make-up gain.

But instead of increasing the peaks, the overall audio signal increases now we have more consistent and less dynamic sound.

Knee

audio compressor knee

The knee determines how aggressively the sound gets compressed. Along with the ratio and the threshold, the knee decides when to start applying compression and by how much.

A soft knee tells the compressor the gradually apply compression as the audio signal approached the threshold.

The compressor gently starts applying compression just before the audio surpasses the threshold.

Compression is gradually applied until the audio signal reaches a full level above the threshold then full compression is applied.

Soft knee sounds more subtle and natural and often used on melodic instruments such as vocals, keys and strings.

Set at hard knee, full compression kicks as soon as the audio signal passes the threshold level.

Hard knee is suitable for rhythmic instruments, such as drums and percussions.

Compressor Variations

Not all compressors will have all the controls listed above, especially hardware or hardware emulations.

For example, Waves CLA 1176 doesn’t have a threshold or makeup gain. The input and output control works as threshold and makeup-gain respectfully.

Some compressor plugins will have more controls than the basic ones, usually used as or to add effects.

Go to part 2