Chapter - 5

Key Words: Overview, Compressors, Ratio, Attack, Gain

Compressor

🎯 Learning Objectives

By the end of this topic, you should be able to:

  • Comprehend the fundamental principles of audio compression.
  • Identify the key parameters: threshold, ratio, attack, release, knee, and make-up gain.
  • Apply this knowledge to manipulate audio dynamics effectively.
  • Understand advanced techniques: sidechain compression, multiband, ducking.

📘 Explanation of Compressor Parameters

Threshold

The threshold is the level at which the compressor begins to act. Signals above this level are reduced according to the ratio.

Ratio

The ratio defines how much gain reduction is applied.

  • 4:1 ratio → For every 4 dB above threshold, output increases by 1 dB.
  • 10:1 ratio → Heavy compression (limiting).
  • R < 1 → Expansion (used for gates).

Attack

Controls how quickly the compressor engages when the input exceeds the threshold. Fast attack tames transients, slow attack preserves punch.

Release

Controls how quickly gain returns to normal after signal drops below threshold. Short release avoids pumping; long release smooths dynamics.

Knee

  • Hard Knee: Compression starts immediately at threshold.
  • Soft Knee: Compression gradually begins near threshold, producing smoother dynamics.

Make-up Gain

After compression, the overall level may be reduced. Make-up gain restores the desired loudness.

Types of Compressors:

  • VCA (Voltage Controlled Amplifier): Precise and fast.
  • Opto (Optical): Smooth, musical compression.
  • FET (Field Effect Transistor): Aggressive, punchy.
  • Vari-Mu: Smooth leveling, vintage vibe.
  • Single-band / Multiband: Multiband splits frequencies for independent control.
  • Sidechain (internal/external): Compression triggered by a separate signal.

Level Detection Methods:

  • Peak: Responds instantly to signal peaks.
  • RMS: Responds to average signal energy.
  • Loudness/K-weighted: Mimics perceived human loudness.

Applying Compression to Specific Audio Sources:

Vocals

  • Threshold: catch peaks.
  • Ratio: 3–4:1.
  • Attack: 5–10 ms.
  • Release: 50–100 ms.

Drums

  • Kick: fast attack, medium release, high ratio (6–8:1).
  • Snare: medium attack/release, ratio 4–5:1.

Bass

  • Threshold for louder notes.
  • Ratio: 4–5:1.
  • Attack: 10–20 ms, Release: 50–100 ms.

Advanced Compression Techniques:

Sidechain Compression

Triggers compression based on another input signal. Common in ducking and rhythmic pumping.

De-essing

Band-limited sidechain targeting sibilance frequencies to reduce harsh 's' sounds.

Multiband Compression

Splits signal into frequency bands; compress independently, then recombine.

Auto-release, Look-ahead

  • Auto-release: release time adapts to signal.
  • Look-ahead: detects peaks before they occur to apply smooth compression.

🎨 Visual Demonstration

Compressor Demo

Compressor Sine Wave Demo

Gray: original wave | Blue: compressed | Red: threshold

🧠 Key Takeaways

  • Threshold: Sets compression start level.
  • Ratio: Determines compression amount.
  • Attack & Release: Control timing of compression.
  • Knee: Smooths or sharpens onset of compression.
  • Make-up Gain: Restores overall signal level.

🧠 Quick Quiz

Test your understanding of compressors:

1) Which parameter sets the level above which a compressor starts reducing gain?

2) What happens if you set a 4:1 ratio on a compressor?

3) Which of these affects how quickly the compressor reacts to a signal exceeding the threshold?

4) What is sidechain compression typically used for?

5) Which statement is true about multiband compression?