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Both theoretical studies and experience with active systems for noise reduction disclose constraints necessary for successful systems. These include the following. (1) Topological fitness: A net of antisources suitable to the primary noise source. (2) Density of cancellers: Quite different results are produced by dense than by sparse arrays. (3) Directivity: Different results are achieved by directional cancellers than by nondirectional ones. (4) Causality: Cancellation requires repetitive patterns of the noise to be attenuated, or the ability to sample the noise to be canceled earlier than the production of the antinoise by the time interval needed for processing the data. Some consequences of these constraints are discussed. a)Present address: L’Aragon F-13008, Marseille, France.