Sometimes the extractor inlet doesn't fit the filter or the same diameter of the pipe, and reductions are needed. For this purpose, we use the appropriately sized reduction coupling.
It's also used for Y or T connections, to extract two pipes from one, as it's advisable to reduce the diameter of the pipe.
When there are multiple branches in a pipe, only reductions in the flow can control the flow so that it draws equally from one side as from another. For example, extracting two 150mm or 125mm pipes from a 200mm pipe, because if you continue with two 200mm pipes, the air can be sucked from only one pipe, rendering the other unusable, thus affecting the ventilation of the crop.
If we use a reduction, but in reverse, to expand it, for example, from 125mm to 150mm at the extractor air outlet, we will achieve much lower pressure when it reaches the outside and make less noise.