The first wall (2) is porous. Inside is a film (4). This is sufficiently thin to avoid acoustic effects; its superficial weight is less than 100 g/m2. The film is fixed to the porous wall, leaving at least 50% of the area which faces the wall unattached.

An air filter comprises a female portion of a connecting duct having two parallel longitudinal and diametrically opposed openings to house flanges that project to the outside of the wall of an intake duct; and removable fastener for securing the filter to ...

The intake duct (1) has two ducts (2,3) joined together end to end. Each duct comprises of two duct sections joined and welded together by the flanges (6-9). Bellows portions (4,5) are formed on the outer surface of each duct. Each bellows portion has two ...