Audio Compression: Basic Application and Techniques 

Audio
Gabriel Maybank || October 25, 2016

What is Compression?

A compressor is a fine-tune processor that controls the dynamic range of an instrument/voice. Dynamics are the variations in volume between softer and louder parts of a song, recording, instrument, etc.  The goal of a compressor is to maintain musical dynamics. Have you ever listened to a live singer and noticed that when he or she sings quiet or talks it’s almost not audible, but when he or she sings they are near blowing the speakers? A compressor is used to control that range between soft and loud by bringing up the quiet levels and compressing or lowering the louder levels. The compressor acts as an automatic fader. The compressor reduces or boosts the signal at a specified ratio above or below a set threshold.

Compressor Controls

Input Gain: The amount of the signal that is sent to the compressor’s input stage.

Threshold: Level at which the compressor proportionately begins to reduce or boost the signal. If the threshold is set for -15dB, all signals above this level will be attenuated and anything below will be unaffected.

Output Gain: The amount of signal sent to the device’s output or speakers after compression.

Slope Ratio: The input-to-output gain ratio determines how much signal is needed to cause the signal to be reduced by a certain –dB. For example, a 2:1 ratio will produce a 1-dB increase in the output for every 2-dB increase in the input.

Attack: Determines how fast or slowly the compressor will reduce the signals beyond the threshold.

Release: Determines how slowly the compressor will restore the signal back to its dynamic level once it has fallen below the threshold.

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Knee: “Soft-Knee” is the gradual and smooth transitioning between no treatment and full treatment of the compressor making the compression more transparent. “Hard-Knee” is the strict limit and sharp transition between no treatment and full treatment of the compression and is a more obvious transition to achieve a desired effect.

Gain Reduction Meter: Displays how much gain reduction is applied, when the reduction starts and stops, and a visual indication of the attack and release activity.

Input Meter:  Displays input gain

Output Meter: Displays what we hear and ensures output does not exceed a specific limit.

Types of Compressors

Downward Compressor: Reduces the level of signals above the threshold, making loud sounds quieter.

Upward Compressor: Boost the level of signal below the threshold, making quiet sounds louder.

Limiter: Ensures no signal exceeds the threshold by reducing any signals above the threshold down to the threshold level

Expander: Reduces any signal below the threshold, making quiet sounds quieter.

Upward Expander: Boost any signal above the threshold, making loud sounds louder

Gate: Attenuates all signals below the threshold by a fixed amount.

Ducker: Attenuates all signals above the threshold by a fixed amount.

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Setting Up a Compressor

Setting the compressor is not a visual action; an audio engineer has to train their ears to hear the differences being made while adjusting settings.

DO NOT MIX WITH YOUR EYES.

Initial Settings

  • Threshold all the way up so there is no compression
  • Ratio all the way up or 1:1
  • Low or short attack and release

Extent: Set the threshold below the highest signal

Degree: Set the ratio down from maximum

Attack: Set the attack longer

Release: Set the release longer

Application

  • Compression can be used to bring instruments forward or backward in the mix by bringing up or lowering the attack.
  • Compression can be used to lengthen the sustain of instruments for a creative effect. Result is unnatural reverb decay.  This achieved by having a low threshold, high ratio, fast attack and fast release so the gain reduction recovers as quickly as possible
  • Compression can be used as a De-esser, or sibilance attenuator.
  • Compression can be used to add punch to an instrument like a bass guitar by starting from a medium attack moving towards a slower attack and medium threshold and slower release time.
  • Use a fast attack of 1 to 5ms and a ratio between 6:1 to 15:1 and moderate to heavy gain reduction to control brass instruments.
  • A faster attack time and a slower release can add to an electric guitar’s “bite” in the mix, but because of the guitar amp and pedals they do not need much compression.
  • For the kick, a 4:1 ratio with an attack setting of 10ms or slower can emphasize the initial attack and add depth and presence.
  • A snare’s attack settings should be faster to catch the initial transient (the hit). The threshold should be set for minimal reduction during a quiet part and a larger amount of gain reduction during the louder sections.
  • Compression on vocals should begin with a threshold setting of 0dB, a ratio of 4:1, and the attack and release at their midpoints. Gain reductions that are somewhere in between 3 and 6 dB will often sit well in the mix.
  • A gate can be used to reduce or remove a hum coming from a faulty direct box or over bleed from another louder instrument
  • A ducker can be used to combat masking. It is an effective way to clear space for more the important instruments.

PROTECT YOUR EARS; TAKE A BREAK!

The Audio Expert: Everything You Need to Know About Audio

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