Compressor

A compressor provides automatic control over an audio signal’s level, creating a smoother, more balanced sound. Explore how compressors can enhance audio quality in your next project.

A compressor reduces the dynamic range of a sound, controlling the loud peaks and boosting quieter parts. This results in a more consistent audio level, making it ideal for applications where an even sound is desirable, such as vocals or rhythm sections in music.

Compression ratios determine the amount of compression applied. A higher ratio (e.g. 8:1) applies stronger compression, while a lower ratio (e.g. 2:1) is more subtle. See figure 2 for a visual representation of different compression ratios and their impact on output levels.

Compressor chart — Figure 2 - Graph illustrating the impact of various compression ratios on output

Compressor Controls

Input Gain

Definition: Controls the initial signal level sent to the compressor.

Purpose: Adjusting input gain ensures that the signal level matches the compressor’s intended threshold.

Soft Knee/Hard Knee

Soft knee: This type of compression gradually applies gain reduction as the signal approaches the threshold. The transition from uncompressed to compressed is smooth, which creates a more natural, subtle effect. Soft knee compression is often used for musical or vocal tracks where a gentle touch is desirable.

Hard knee: This type applies compression abruptly once the signal exceeds the threshold, creating a more immediate and noticeable change in dynamics. Hard knee compression is typically used when a stronger, more aggressive effect is needed, as in certain instruments or genres that benefit from pronounced dynamics control.

Soft vs hard knee compression chart — Figure 3 - Graph comparing hard knee and soft knee compression, highlighting differences in how each compression type affects the audio signal at various thresholds

Threshold

Definition: Sets the level at which compression begins.

Example: If set to -15dB, signals below this threshold remain unaffected, while those above it are compressed according to the selected ratio.

Ratio (Amount)

Definition: Determines how much compression is applied to signals above the threshold.

Example: At a 2:1 ratio, a 2dB increase in input above the threshold results in only a 1dB increase in output, reducing loudness spikes for a more even sound.

Attack

Definition: Controls how quickly the compressor responds once the signal exceeds the threshold.

Short Attack: Responds quickly, often catching percussive transients.
Long Attack: Responds slowly, preserving initial transients like drum hits.

The attack time is measured in milliseconds.

Release

Definition: Determines how long it takes for the compressor to stop affecting the signal after it falls below the threshold.

Fast Release: Can result in a “pumping” sound as dynamics shift too quickly.
Slow Release: Maintains a smooth sound but may compress signals unintentionally if held too long.

The release time is measured in milliseconds.