Audacity Phaser Effect: Sweeping Notch Controls

The Audacity Phaser effect creates its signature sweeping, "whooshing" sound by applying phase-shifting all-pass filters that cause destructive interference when mixed with the original audio. This guide explains the specific parameters in Audacity's Phaser plugin—including Stages, LFO Frequency, Depth, Feedback, and Dry/Wet—that control the number, movement, and intensity of these sweeping notches.

Stages

The Stages parameter determines the number of all-pass filters used in the processing chain, which directly dictates the number of notches in the frequency response. Each pair of stages typically introduces one notch. Setting a low number (such as 2 or 4) creates a subtle, gentle sweep with fewer notches, while higher settings (up to 24) produce a dense, complex comb-filter sweep with multiple distinct notches moving simultaneously.

Dry/Wet

The Dry/Wet control balances the unprocessed (dry) signal with the phase-shifted (wet) signal. The sweeping notch sound relies entirely on phase cancellation between these two signals. Setting this parameter to a 50/50 balance produces the deepest cancellation, maximizing the prominence of the notches. Shifting too far toward 100% dry or 100% wet eliminates the phase cancellation, removing the notch effect entirely.

LFO Frequency (Hz)

The LFO Frequency controls the speed of the sweep. A low-frequency oscillator (LFO) modulates the center frequencies of the notches up and down over time. Lower values (e.g., 0.1 to 0.5 Hz) result in slow, dramatic, evolving sweeps, whereas higher values (above 2.0 Hz) cause rapid, vibrato-like frequency modulations.

LFO Start Phase (Degrees)

The LFO Start Phase sets the starting position of the modulation cycle when playback or rendering begins. Adjusting this from 0 to 360 degrees dictates whether the sweep begins at the bottom, middle, or top of its frequency path, which is useful for synchronizing the sweep with specific transients or beats in the track.

Depth

The Depth parameter controls the modulation width of the sweep. It dictates how far the notches travel across the audio spectrum. A low depth setting restricts the notches to a narrow frequency band, resulting in a subtle movement, while a high depth setting forces the notches to sweep across a wide frequency range, from deep lows to crisp highs.

Feedback

The Feedback control routes a portion of the processed output back into the filter input. Increasing feedback sharpens the peaks between the notches, giving the sweep a sharper, more metallic, and resonant character. High feedback settings accentuate the edges of the notches, making the sweeping movement significantly more pronounced and aggressive in the mix.