Circumaural ear defenders require a cushion which is highly adaptable to the irregular contours of the head so as to provide a good air seal, and yet behaves like a stiff spring so as to minimize low‐frequency cup vibration. These apparently contradictory requirements are met by a cushion consisting of an annulus of highly flexible yet rather inextensible material partially filled with an incompressible fluid. A theory of cushion behavior is given and is supported by experimental data. Experiments with real heads show that the attainable low‐frequency attenuation is limited by the viscoelastic properties of the flesh. Values of circumaural flesh impedance derived from the experimental data are compared with the mastoid data of other workers, and the relationship between sound transmission and threshold shift data is discussed. The design of a rigid cup and flexible frame suitable for use with the cushion is also discussed.