There are five stages of filtrations in your Dyson.
The first stage of filtration is the outer cyclone where incoming air is accelerated to high speeds subjecting particles of dust and dirt to enormous g-forces. The air escapes from the top of the cyclone, but the dirt which has assumed a great mass, falls to the bottom of the bin.
In second stage air passes through the shroud and prevents the coarse particles entering the inner cyclone.
The third stage is specially designed inner cyclone which removes as minute as 0.5 microns by spinning the air at hundreds of mph.
The 4th stage is placing filters before the motor so that filter  membranes trap microscopic particles of dust in their tiny pores thus air expelled is completely clean and dust-free.
The 5th stage is placing the filter after the motor for added filtration and over all protection .
 

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