Audio Compression Explained

• Amplitude is the perceived loudness of a sound wave, measured in decibels (dB).
• Dynamic range is the ratio between the loudest and softest signals in an audio recording.
• Dynamic processors automatically control volume by measuring amplitude changes over time, using a set threshold.

Compression

• A compressor is a type of dynamic processor.
• A 1:1 ratio means no compression is applied.
• Lower ratios offer subtle compression.
• Higher ratios provide more aggressive compression.
• In a 4:1 ratio, think of it as sound compressed by a quarter (1/4).
• A 10:1 ratio is considered a limiter.
• Compressor attack determines how quickly the compressor reduces the volume after the signal exceeds the threshold.
• Compressor release dictates how long it takes for the signal to return to its uncompressed level.

Knees

• The knee is located at or near the threshold.
• It determines how much and at what point the compressor will reduce gain.
• A hard knee applies the full compression ratio immediately when the signal crosses the threshold.
• A soft knee eases the signal into the set ratio over a wider range.

Limiters

• A limiter has a ratio of 10:1 or higher.
• Limiters are effective for controlling quick peaks.
• A brick wall limiter prevents the signal from exceeding the threshold.
• A brick wall limiter has an infinite ratio
• A brick wall limiter maximizes loudness without clipping
• A brick wall limiter has an instant attack time.
• Placing a limiter before a compressor controls peaks, allowing the compressor to manage the average signal level.

Expander

• An expander acts like a gate with ratio control.
• An expander increases the difference between the loudest and quietest parts of the audio.
• It is the opposite of a compressor.
• When a signal drops below the threshold, the expander reduces gain based on a ratio.
• Expanders provide a smoother transition than gates.

De-essers

• De-essers attenuate a narrow band of frequencies, typically between 6kHz and 8kHz, to reduce sibilance.
• Excessive de-essing can introduce a lisp.
• De-essers have threshold and target frequency controls.
• Target frequency selects the frequency range where the de-esser identifies sibilance.

Gate

• Gates reduce or eliminate unwanted background noise.
• Gates open and close based on the incoming audio signal level.
• The threshold level must be set for the gate
• The gate opens when the audio signal crosses the threshold
• The gate closes when the audio signal drops below the threshold.

Multi-band compressor

• Multi-band compressors divide an audio signal into multiple frequency bands
• Each band can be compressed independently.
• Multi-band compressors offer precise control over the dynamic range of different frequency ranges.
• Multi-band compression works well on sibilance across entire frequency range.

Envelope shaper

• Envelope shapers modify the gain of a sound's attack and release phases.
• It can be used to boost or attenuate the gain of each frequency band in a signal. 

Side chaining

• Side chaining uses one audio track to control the volume of another track via a compressor.
• Side chaining creates space in a mix, allowing a sound to "duck" when a different sound reaches a certain level.

EQ or compress first?

• The order of EQ and compression is flexible.
• When subtracting frequencies, EQ is best to use first.
• When adding frequencies, compression can be performed first, however fixing a frequency after compression is generally recommended