Nonlinear passive acoustic filtering

Abstract From the Canadian patent

In an ear muff intended for protection against excessive noise exposure, the passive means according to this invention automatically and continuously varies the amount of reduction provided for the sound arriving at the ear of the wearer; it retains substantially unaltered the performance of the ear muff for high sound levels but effects a prescribed lesser value of sound reduction at low sound levels and maintains coherent phase relations between the sound waves arriving at the two ears as needed for reception of directional information.