Ducting Reducers are great to use for long ventilation systems as the supply of fresh air into the grow room can be provided through the required size ducting and just before the grow room you can increase the size to reduce the flow of air to a more gentle flow as it enters and it hits your plants and you can also do the same with your extraction system. Increasing your ducting size near to your outlet will reduce the air escaping to a more gentle flow, which will help to reduce some noise.