How Audacity Compressor Threshold and Ratio Work

Dynamic compression in Audacity allows you to control the dynamic range of an audio track by reducing the volume of the loudest peaks. The entire process of how much volume is reduced relies fundamentally on two core parameters: the Threshold and the Ratio. Together, these settings establish an input baseline and a mathematical formula that dictate exactly when gain reduction begins and how intensely it scales back audio peaks.

The Role of the Threshold

The Threshold, measured in decibels (dB), is the level that the incoming audio must exceed before the compressor activates.

• Below the Threshold: Any audio signal quieter than the set threshold passes through the compressor completely unaffected.
• Above the Threshold: Any audio signal louder than the threshold triggers the compressor and is subject to attenuation (gain reduction).

For instance, if the Threshold is set to -12 dB, audio playing at -18 dB or -14 dB remains untouched. Once a sound spikes to -6 dB, the compressor engages because the signal has surpassed the -12 dB limit by 6 dB.

The Role of the Ratio

The Ratio determines how aggressively the audio is attenuated once it crosses the threshold. It is expressed as a proportion (e.g., 2:1, 4:1, or 10:1), comparing the input level above the threshold to the resulting output level above the threshold.

• 1:1 Ratio: Applies no compression. The input level equals the output level.
• 2:1 Ratio: For every 2 dB the signal exceeds the threshold, only 1 dB is allowed through to the output.
• 4:1 Ratio: For every 4 dB the signal exceeds the threshold, only 1 dB is allowed through.
• High Ratios (e.g., 10:1 or greater): Flattens peaks heavily, approaching standard audio limiting.

How Threshold and Ratio Determine Gain Reduction

Gain reduction is the total number of decibels subtracted from the original signal to bring it down to the target output level. It is calculated exclusively on the portion of the audio that overshoots the threshold:

1. Calculate the Overshoot: Overshoot = Input Level - Threshold
2. Calculate the Allowed Output Above Threshold: Output Above Threshold = Overshoot / Ratio
3. Calculate Total Gain Reduction: Gain Reduction = Overshoot - Output Above Threshold

Practical Example

Consider an audio track with the following Audacity compressor settings:

• Threshold: -10 dB
• Ratio: 4:1
• Audio Peak (Input): -2 dB

Here is the step-by-step calculation:

1. The signal exceeds the -10 dB threshold by 8 dB (-2 dB minus -10 dB).
2. With a 4:1 ratio, the compressor divides that 8 dB overshoot by 4, allowing only 2 dB above the threshold to pass through.
3. The resulting peak level becomes -8 dB (-10 dB threshold + 2 dB allowed overshoot).
4. The total gain reduction applied to that peak is 6 dB (the original 8 dB overshoot minus the 2 dB allowed output).

By adjusting the Threshold, you choose which parts of the performance receive compression. By adjusting the Ratio, you determine how strictly those dynamic peaks are tamed.