Halliday & Matthiessen (2014: 349):
The way the voice system works is as follows. A clause with no feature of ‘agency’ is neither active nor passive but middle. One with agency is non-middle, or effective, in agency. An effective clause is then either operative or receptive in voice. In an operative clause, the Subject is the Agent and the Process is realised by an active verbal group; in a receptive [clause] the Subject is Medium and the Process is realised by a passive verbal group.