The ExpanderNode collects audio data from all of its inputs and applies an expander filter before passing the data along to its outputs.
Common Uses
- Increasing the contrast between the soft and loud parts of the audio
- Removing quiet sounds from the audio
Using the ExpanderNode
Expander filters reduce the volume of sounds that drop below the threshold, according to a specified ratio. The volume is calculated according to this formula:
(NOTE) VolumeAdjustmentDecibels = ((1.0 / Ratio) - 1.0) * (ThresholdDecibels - CurrentVolume)
In other words, if the Ratio is 2, the ThresholdDecibels is -10 dB, and the current volume of the sound is -14 db, the volume adjustment will be -2 dB and the sound will be reduced to -16 db. If the Ratio is set to 0.0, the (1.0 / Ratio) - 1.0 value will be set to 0.0, so audio that goes below the ThresholdDecibels value will have their volume reduced by the same number of decibels that they are below the threshold. This will create an effect like a Gate filter.
The AttackMillisec and ReleaseMillisec properties control how quickly the filter eases in and out as the sound goes below the ThresholdDecibels then goes back above it. The AttackMillisec should generally be short and the ReleaseMillisec should be longer, but this will depend on the desired effect. The KneeWidth property sets the number of decibels, centered at the ThresholdDecibels, over which the volume change eases in.