Limiting

From sinedesign

Overview

Limiting is a special case of compression where the compression ratio is set high enough to prevent audio from surpassing a certain volume level. There is no exact ratio that separates a compressor from a limiter, but it is commonly accepted that anything above a 12:1 ratio could be considered limiting. [1] Limiters may have the same parameters as compressors, but commonly have fewer settings. Their main use is in the mastering stage.

Definitions

  • Threshold - The signal volume (in dB) that triggers when the compressor is activated.
  • Output Ceiling - The highest volume (in dB) of the output signal.
  • Attack - Determines how long the signal needs to be over the threshold before the compressor reduces it.
  • Release - Determines how long the signal needs to be under the threshold before the compressor stops reducing it.

Guidelines

Avoiding audible compression artifacts

Similar to the compressor, any gain reduction of more than 3 dB will sound unnatural. Although some may choose to produce a “pumping” effect for creative purposes, it’s generally recommended to keep gain reduction to a minimum. [2]

Setting the attack and release parameters

Start with the attack set to its slowest/highest setting and the release set to its fastest/lowest setting. This allows all transients through and shuts off the compressor immediately when the signal crosses back down over the threshold. From there, you can gradually shift the parameters deliberately to your liking.

Set the release time to be no longer than the most common interval between transients (usually between 50-150ms). This allows the compressor to fully shut off by the time the next hit triggers it and creates a more musical effect, similar to synchronizing a delay time. [3]

Metering

Limiters are both very powerful and very sensitive to small parameters shifts. Therefore it is recommended to use them in combination with meters that can help illustrate - in objective terms - how the limiter is affecting the track. Some of the common metrics that are metered when using limiters (especially in the mastering process):

  • Volume Units (VU) - Reflects the perceived loudness of audio over time by measuring the average signal level over short periods of time (usually around 300ms).
  • Root-Mean-Square (RMS) - Another methodology for showing the 'average' levels over time.
  • Peak Value - Measures the instantaneous level of an audio signal.

Techniques