An acoustic device includes an acoustic assembly that defines a first acoustic volume. The acoustic assembly includes at least one passive radiator in acoustic communication with the first acoustic volume. A first electro-acoustic driver is configured to acoustically energize the first acoustic volume so as to drive the piston in oscillatory motion. The acoustic device also includes a housing within which the acoustic assembly is disposed. The housing has a curved inner surface. The passive radiator has a curvature which conforms to the curvature of the curved inner surface of the housing so as to reduce unused space between the at least one passive radiator and the curved inner surface of the housing.