Compression

Compression in audio is the process of reducing a sound’s dynamic range – the difference between its quietest and loudest parts. A compressor automatically turns down loud peaks and can boost quieter sections, making a track sound more even. For example, if a singer suddenly belts out a high note, a compressor will catch that peak and lower it so it doesn’t clip, keeping the performance at a more consistent level. Compressors are fundamental in mixing and mastering because they help every element sit together smoothly in the mix.

Key Elements of a Compressor

All compressors share several core controls:

Input: Controls how much signal is sent into the compressor. The behavior of the compressor depends on the level it receives.
Threshold: The level above which compression starts to take effect.
Ratio: Determines the amount of gain reduction. For example, 4:1 means that if the signal goes 4 dB over the threshold, only 1 dB gets through.
Attack: How quickly the compressor reacts after the signal exceeds the threshold.
Hold: Keeps the gain reduction active for a brief moment after the signal drops below the threshold.
Release: How quickly the compressor stops reducing the gain after the signal falls back below the threshold.
Knee: Controls how gradually or abruptly compression starts when the signal crosses the threshold.
Output: Adjusts the final level after compression.
Dry/Wet (Mix): Blends the compressed signal with the original unprocessed signal (often used in parallel compression).
Detection Style: Feed-forward or feedback detection affects how the signal is analyzed.
Detector Source: Filters like high-pass or low-pass can shape what frequencies trigger the compressor.
RMS Window Size: Controls how the average loudness is measured over time.
Pre-Attack: A short lookahead feature that allows the compressor to catch peaks right as they happen.

Why Compression Matters

Compressors shape a mix for both technical and musical reasons. The primary effect is dynamic control: bringing quieter sounds up and louder sounds down so that nothing gets buried or overly dominant.

Benefits:

Consistency – Keeps vocals, instruments, or entire mixes even in volume.
Sustain – Lengthens the perceived duration of notes.
Glue – Makes different elements feel cohesive, especially on buses or full mixes.
Loudness – Allows the track to be made louder without clipping, improving perceived energy and impact.

Even subtle compression can make a track sound more professional and listener-friendly across different playback systems.

Different Types of Compressors

VCA (Voltage Controlled Amplifier): Known for precision and fast response, VCA compressors are versatile and often used for drums and buses. Famous examples include the DBX 160 and SSL G-Series Bus Compressor. They offer clean gain reduction and tight control.

Optical Compressors: Use a light sensor and an optical element to control gain reduction, resulting in smooth, musical compression with slower attack times. The Teletronix LA-2A is a classic optical compressor prized for its warm, natural sound, especially on vocals and bass.

FET (Field Effect Transistor) Compressors: Known for their fast attack and aggressive character, FET compressors add punch and presence. The Urei 1176 is the iconic example, famous for its distinctive coloration and quick response, widely used on vocals, drums, and guitars.

Tube Compressors: Utilize vacuum tubes to create smooth, warm compression with harmonic distortion that adds pleasant coloration. The Fairchild 670 and Manley Variable Mu are classic tube compressors known for their rich, creamy sound favored in mastering and vocals.

PWM (Pulse Width Modulation) Compressors: These are less common and typically digital or hybrid units that use pulse width modulation circuits for gain control, offering unique tonal characteristics and often used for special effects or creative sound shaping.

Digital Compressors: Fully software-based or digital hardware compressors offering precise, transparent compression with extensive control options. They often include advanced features like lookahead, multiband processing, and customizable curves.

Sidechain Compression: A Creative Tool

One powerful technique in compression is sidechain compression. Normally, a compressor reacts to the signal it’s processing. But with sidechaining, the compressor listens to a different signal as its trigger. For example, you can compress a bass guitar every time a kick drum hits. This creates space in the low end and helps the kick cut through the mix-common in electronic and pop music.

Multiband Compression

Multiband compression splits the audio signal into separate frequency bands, allowing independent compression settings for each band. This is especially useful for complex mixes or mastering, where you might want to tame harsh highs without affecting the lows or control bass frequencies without squashing the mids.

For example, you can apply gentle compression to the bass frequencies to tighten the low end, while using a faster attack on the high frequencies to reduce sibilance in vocals.