HOW TO USE FAN SILENCERS
You will want to attach your silencer just after the fan in the extract duct run, to soak up any noise generated by the motor. Ideally it will be connected with a fast clamp to ensure a nicely sealed, air tight connection. Then simply connect your ducting as your normal and run it to the usual place.
Bear in mind that the acoustic foam will slightly reduce the air flow rates throughout your system. It will not be by much, but there will be a small drop in the overall airflow rates.
Also worth considering if noise is a concern is to make sure that your ducting is pulled as straight and as tightly as possible. Not only will this reduce any extra noise generated from turbulence in the ducting, it will also help to improve the overall air flow rates.