Modulating effects, such as flanger and chorus, are powerful tools for adding depth to an instrument or spicing up a mix. Modulating effects, by nature, cause the tonality of the sound to sweep through the frequency spectrum at a set speed. While this makes for cool sounds, modulation can sometimes cause the signal to apparently "disappear" as comb filtering causes some frequencies to cancel each other out. You can combat the varying strength of the sound by duplicating the original track and applying the effect to duplicate.

For example, if you apply a flanging effect to a vocal track. Create a duplicate track and apply flanging. Turn the level down on the original (dry) track so that it's only audible when the flanged track drops in volume. You'll be able to hear the vocal at a consistent level, but the flanged effect will be easily audible.