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Compression Explanation

Compression is one of the most important tools in music production and mixing. It controls the dynamics (the difference between the loudest and quietest parts) of an audio signal, making the sound more consistent and polished. Here’s a step-by-step explanation of how compression works, key parameters, and practical techniques.

What is Compression?

Compression reduces the dynamic range of a sound. It makes the loud parts quieter and can bring the quieter parts closer to the forefront, creating a more balanced sound. This helps individual instruments or elements sit better in a mix and makes a track sound smoother and more controlled.

Key Compression Parameters

  1. Threshold:

    • Definition: The threshold sets the level at which the compressor begins to reduce the signal’s volume. Any sound above this level will be compressed.
    • How It Works: If you set the threshold at -10dB, only the parts of the signal louder than -10dB will be compressed. Signals below -10dB will pass through unaffected.
    • Practical Example: In vocals, you might set the threshold to catch only the loud peaks, reducing sudden volume jumps.

  2. Ratio:

    • Definition: The ratio determines how much compression is applied once the signal exceeds the threshold.
    • How It Works: A ratio of 2:1 means that for every 2dB the signal exceeds the threshold, only 1dB will be output. A ratio of 4:1 means that if the signal exceeds the threshold by 4dB, only 1dB will be output.
    • Common Ratios:
      • 2:1 to 4:1 for gentle compression (vocals, guitars, piano).
      • 4:1 to 8:1 for more aggressive control (drums, bass, punchy instruments).
      • Above 10:1 is considered limiting, which is extreme compression.

  3. Attack:

    • Definition: The attack time determines how quickly the compressor starts reducing the volume after the signal exceeds the threshold.
    • How It Works: A fast attack (e.g., 1-5ms) means the compressor kicks in immediately, which can reduce the initial “punch” or transient of a sound. A slow attack (e.g., 20-30ms) allows more of the transient through before compression starts.
    • Practical Example: On a snare drum, you might use a slower attack to let the initial “snap” through, keeping it punchy while compressing the body of the sound.

  4. Release:

    • Definition: The release time determines how long it takes for the compressor to stop compressing after the signal falls below the threshold.
    • How It Works: A fast release (e.g., 50-100ms) lets the sound return to its original level quickly, which can be good for keeping energy in the track. A slow release (e.g., 500ms or more) smooths out the sound over a longer period but can make it sound overly compressed or unnatural.
    • Practical Example: On vocals, you might use a medium release so the compression doesn’t affect every small fluctuation but still smooths out the performance.

  5. Knee:

    • Definition: The knee controls how abruptly the compression kicks in once the signal crosses the threshold. There are two types:
      • Hard Knee: The compressor kicks in immediately when the threshold is crossed, resulting in a more noticeable compression effect.
      • Soft Knee: The compressor gradually begins reducing volume as the signal approaches the threshold, creating a smoother and more natural compression.
    • Practical Example: Soft knee compression is commonly used on vocals and instruments for a subtle effect, while hard knee compression might be used on drums for a more pronounced control.

  6. Makeup Gain:

    • Definition: After compression, the overall volume of the signal is usually reduced. Makeup gain compensates for this by boosting the compressed signal back up to a desired level.
    • How It Works: If compression reduces your signal’s loudness by 5dB, you can apply 5dB of makeup gain to restore the overall volume, while retaining the dynamic control of compression.
    • Practical Example: Use makeup gain to keep the vocal at the right level in the mix after compression has been applied.

  7. Output Gain:

    • Definition: The output gain is used to control the final level of the compressed signal before it leaves the compressor.
    • How It Works: This is similar to makeup gain but typically used to set the final volume of the compressed signal in the mix.

How Compression Works (Visualization)

  1. Uncompressed Signal: Imagine a vocal track with some words being sung softly and others loudly. Without compression, the louder parts can overpower the mix, and the quieter parts might get lost.

  2. Apply Compression: By setting a threshold, the compressor will only reduce the volume of the louder parts that exceed this level. The quieter parts remain unchanged unless they cross the threshold.

  3. Reduce Dynamics: The compressor lowers the volume of those louder peaks. You can then use makeup gain to raise the overall volume of the track, making the soft and loud parts more balanced.

Types of Compression

  1. Downward Compression: The most common form of compression, where the loud parts are reduced in volume, and the quiet parts remain unaffected.

  2. Upward Compression: Less common, but this type of compression raises the volume of the quieter parts while keeping the loud parts untouched. This helps bring out detail without reducing the impact of the loud parts.

  3. Parallel Compression (New York Compression):

    • Definition: A mixing technique where a heavily compressed signal is mixed back in with the original, uncompressed signal. This helps maintain the original dynamics while adding more body and sustain.
    • Practical Example: Used on drums or vocals to make them sound bigger and more powerful without losing their punch.

  4. Multiband Compression:

    • Definition: Compressors that apply compression differently to different frequency ranges (e.g., low, mid, high frequencies).
    • Use: Allows you to compress only certain parts of the frequency spectrum (e.g., compressing bass frequencies more than highs).
    • Practical Example: Great for mastering or complex signals like drum buses or full mixes.

  5. Sidechain Compression:

    • Definition: Compression triggered by an external audio source (the “sidechain”) rather than the signal being compressed itself. When the external source (like a kick drum) plays, the compressor reduces the volume of the target signal.
    • Practical Example: In dance music, sidechain compression is often used to make the bassline “duck” every time the kick hits, giving the kick more space in the mix and adding a pumping effect.

Practical Compression Settings for Different Instruments

  1. Vocals:

    • Threshold: Set to control the loudest peaks.
    • Ratio: 2:1 to 4:1 for gentle control.
    • Attack: Medium (5-10ms) to retain natural dynamics.
    • Release: Medium to fast, so the compression follows the natural phrasing of the vocals.
    • Makeup Gain: Use to maintain vocal level in the mix.

  2. Bass:

    • Threshold: Set to even out dynamic range.
    • Ratio: 4:1 to 6:1 for more consistent volume.
    • Attack: Slow to allow the transient of the note to come through.
    • Release: Fast to medium, depending on the style of the bassline.

  3. Kick Drum:

    • Threshold: Set to control the punch and volume peaks.
    • Ratio: 4:1 or higher for punchy compression.
    • Attack: Slow (20-30ms) to preserve the initial impact of the kick.
    • Release: Fast to keep the kick snappy.

  4. Snare Drum:

    • Threshold: Set to control loud hits.
    • Ratio: 4:1 for punch and control.
    • Attack: Medium to slow to allow the transient (initial hit) to come through.
    • Release: Fast for a tighter sound.

  5. Drum Bus (all drums together):

    • Threshold: Set to control the peaks of the combined drum elements.
    • Ratio: 2:1 to 4:1 to glue the drum elements together.
    • Attack: Slow to allow transients through.
    • Release: Medium to fast to maintain rhythm and punch.

Common Mistakes with Compression

  1. Over-compressing: Too much compression can make a track sound flat, lifeless, and lacking in dynamics. Use compression subtly and focus on controlling dynamics without destroying the natural feel of the performance.
  2. Setting Attack Too Fast: A fast attack can squash the initial transients of a sound, making drums or vocals sound dull. If you want punch and clarity, use a slower attack to let the transients through.
  3. Setting Release Too Slow: If the release is too slow, the compressor might not have time to “reset” between notes, causing an unnatural pumping sound. Test different release settings to match the rhythm of the track.

Conclusion

Compression is a powerful tool to control dynamics, add punch, and glue elements together in a mix. It’s essential to understand how each parameter affects the sound, so you can make informed decisions when compressing different instruments. Through practice, you’ll learn to use compression to enhance clarity, consistency, and energy in your mixes.