Theory of Acoustical Resistance Enclosures
Loudspeaker enclosures in which an acoustical resistance component plays a significant role have been employed in three different functions: as "conventional" enclosures, where the resistance is used to control the response near the resonance frequency; as directive enclosures, where the acoustical resistance is intended to function over a wide frequency range providing polar pattern control for the loudspeaker; and as internally divided enclosures, where a resistive element is inside a more conventional enclosure and the function of the resistance is to control the driver parameters and decouple the enclosure at mid frequencies. Usually the acoustical resistance is realized as a resistive material (cloth, mesh, felt, etc.) at a loudspeaker port, but also the absorbent inside the enclosure plays a significant role.: So far, the theories published to describe such systems have been concentrating on distinct aspects of the behaviour. This paper provides a general model for all the types of the acoustical resistance enclosure described above. Slightly different approaches, however, are possible for these types to simplify the practical design, since for discussing the low-frequency only applications the geometry of the system can be ignored to a large extent, whereas in wide-band applications the geometrical configuration of the ports must be modelled. Depending on the configuration, the interior of the speaker can be described either as a lumped component or as a transmission line; both of these approaches are discussed.
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