Conventional foam earplugs are known. These plugs have a continuum of foam, are cylindrical or slightly tapered and are larger that the acoustical canal of the ear. By wedging the plugs into the ear canal, the exterior surface of the plugs conforms to the ear canal and the central portion of the plug inhibits the penetration of at least treble sounds to the car canal.
Regarding so-called "treble sounds," it is the acoustical "highs" or treble sounds that both make listening in a high noise environment painful to the ear and permanently damaging to hearing sense.
In Carrigan U.S. Pat. No. 5,332871 issued Jul. 26, 1994 entitled Sliding Valve Ear Plug an earplug is disclosed. It includes an earplug body having successive and larger rubber hemispheres mounting a valve body to the ear. The valve body defines an aperture centrally of the plug body and central to the ear canal. A slide acts as an acoustical valve seat and selectively opens and closes the valve body. This slide is linear in motion; its size and excursion is dependent upon the physiology of the ear. It brings a sound-transmitting aperture in the slide into and out of registry to permit sounds to directly reach the ear.