8.5: Amplitude Processors

Amplitude (Dynamics) Processors

Amplitude processors — also known as dynamics processors — affect the dynamic range of an audio signal. The dynamic range is the difference between the loudest and softest parts of the sound. By controlling these variations, amplitude processors make audio more consistent, polished, and easier to manage in recording, mixing, or broadcasting.

Amplitude processors modify the level or dynamic response of signals. They are crucial in maintaining consistent loudness, preventing distortion, and shaping the expressive contour of performances.

Key Types Include:

• Limiter
  A limiter prevents a signal from exceeding a set level. It acts as a fast, high-ratio compressor to protect speakers, amplifiers, or transmission systems from overload or distortion.

• Expander
  An expander increases the dynamic range by making quiet sounds quieter and loud sounds louder, often used to restore dynamics after heavy compression.
  

• Noise Gate (sometimes considered a noise processor)
  A gate mutes or reduces signals that fall below a certain threshold, effectively removing unwanted background noise or hum when no useful sound is present.

• De-Esser (sometimes considered a noise processor)
  A de-esser targets and reduces harsh sibilant sounds (such as “S” and “T”) in speech or vocals. It acts like a frequency-selective compressor, reducing only the offending high-frequency range rather than the entire signal.

• Compressor
  A compressor is an amplifier that reduces the dynamic range of an audio signal. It works by automatically lowering the level of loud sounds and/or raising soft sounds. In essence, it squeezes the difference between the loudest and softest parts of a signal.
  
  

Compressors are used to:

• Control excessive peaks in audio.
  

• Smooth out variations in vocal or instrumental performances.
  

• Maintain consistent volume levels.
  

• Enhance sustain and presence in instruments.
  

• Improve speech intelligibility.
  

• Provide stable output for broadcasting and live sound applications.
  

• Compressors are excellent tools for speech, voiceover, and broadcast work, helping to maintain a smooth, professional sound and fixing erratic audio levels.
  

Compressor Parameters

To control how and when compression occurs, several parameters define a compressor’s behavior:

• Threshold:
  The level (measured in dB) above which compression begins. Signals below the threshold pass unaffected; those exceeding it are reduced.
  Example: If the threshold is set at –10 dB, only sounds louder than –10 dB will be compressed.
  

• Ratio:
  Determines the amount of compression applied once the signal crosses the threshold. A 3:1 ratio means that for every 3 dB the input rises above the threshold, the output increases by only 1 dB. Higher ratios produce stronger compression (e.g., 10:1 acts nearly as a limiter). Lower ratios (2:1 or 3:1) offer gentle, musical control.
  Effect: Reduces peaks and evens out performance dynamics without sounding unnatural.
  

• Attack Time:
  The speed at which the compressor reacts once the signal exceeds the threshold. Fast attack (1–10 ms) catches sharp transients like drum hits; slow attack (20–100 ms) lets initial transients pass, preserving punch and articulation.
  

• Release Time:
  The speed at which the compressor stops reducing gain after the signal falls below the threshold. Short release times restore dynamics quickly but may cause “pumping.” Pumping is the audible effect of pulsating as the compressor is turning on and turning off. Longer releases sound smoother and more natural.
  

• Make-Up Gain:
  Compression often lowers overall signal level. Make-up gain restores lost loudness, allowing the compressed audio to match or exceed its original volume while maintaining dynamic control.
  

• Knee (Hard vs Soft):
  The knee describes how abruptly compression begins at the threshold.
  

  • Hard Knee: Compression starts immediately once the signal crosses the threshold — more aggressive and precise.
    

  • Soft Knee: Compression is introduced gradually as the signal approaches the threshold — smoother and more musical, common in vocals and mix-bus compression.