each recess faces a corresponding one of the filtering modes when the device 1 is filtering the sound according to the wearer 39 s needs the recess 22 a might include a marking 26 intended to indicate on what filtering mode the device 1 is operating in fig3 the marking 26 is shaped to resemble an arrow 39 s tip indicating the device 1 is filtering sound according to the properties of the filtering material 25 u2032 inside canal 24 u2032 more preferably the marking 26 is a protrusion that will allow the wearer to determine just by sensing with the tip of his fingers on what filtering mode the device 1 is operating the thickness of the surroundings of the recesses 22 a u2014 the one indicating the operating filtering mode u2014 is always smaller than the rest of the body of the knob 22 thus defining an open end 28 in fig5 the knob 22 is depicted in the filtering mode i and the open end 28 at the knob periphery is formed due to the differences in thickness in the surroundings of the recess 22 a in order to let the sound enter inside the open end 28 and consequently inside the inner ear of the.