Hearning protection devices (HPDs),constructed from muffs that seal against the skin around the external ear (circumaural HPDs)_ and devices tha trest against the pinna (supra-aural HPDs),reduce noise less at frequencies below 500 Hz than at frequenices abouve.In recent years,protabel analog activenosie reduction (ANR) HPDs have been devleoed to increase the total noise reduction at low frequencies [1,2].
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