A headphone assembly for an active noise cancellation system has a rigid moulded plastics shell having a rigid baffle dividing the shell interior into a front volume and a closed rear volume, both of which may be filled with an acoustic foam. The baffle has a central opening in which is mounted a headphone transducer, and a seal cushion is disposed around the mouth of the shell, to effect an air-tight seal against the user's head. The headphone transducer has two effective sound radiating surfaces of different sizes, and may be constructed as a form of orthodynamic drive unit. In the latter case, the orthodynamic drive unit diaphragm may have two separate coils of different radial extents.