The ER20 is fitted with a flanged rubber tip that creates an isolating seal in your ear canal to block out sound so that it can then control what gets in and what doesn’t. The central chamber of the plug creates an acoustic channel that feeds the sound up to the filter at the base of the ear tip where the ER filter shaves an average of 16dB off across all frequencies, giving the effect of turning down the volume on the world around you with no muffling.