In designing ear defenders of the cover type for protection against sound the cushion presents a serious problem. To provide an airtight seal against an irregular object like the side of the head, the cushion is usually made quite soft. This, however, contradicts the requirement of a cushion with a high spring constant which is necessary to prevent the cup from vibrating at low frequencies. This contradiction is resolved by the use of an annular sheath which is filled with a liquid or semisolid. The high bulk modulus of the liquid prevents appreciable volume changes of the cushion due to the sound pressure while the high Young's modulus of the sheath prevents area change. The vibration of the cup is greatly reduced while the thin sheath offers no great rigidity to impede fitting the head. Low frequency attenuations as high as 34 db have been obtained when a solid material is substituted for the head. This is much higher than the limit set by the compliance of the skin and flesh on the head.