A bistable display having one or more permanent hideable mark, and a method of forming the permanent hideable mark, are described. The protective elements have thermoplastic elements arranged together to form at least one layer helically wound around the said cable. The driver circuit further includes a capacitor arranged to form a timer. The flange has holes for mounting the box to a structure. Via a communication link a signal which depends on both input converter’s output signals is transmitted to a second device for the other ear which comprises at least an output electricalmechanical converter.