How to block a window gap against train noise

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Hi,

I hope you can help me. I live by a train track and have double glazed windows which do a great job of blocking out sound. The building I live in is very energy efficient and so keeps the heat in. During the current summer months I am forced to have a portable air conditioner running which requires my windows to be open to vent hot air. I have found a company that will cut a hole in my window for me to vent the hose out of but I want to ensure that when the winter hits I can cover or fill the hole with some sound proofing material that will mimic the effect of blocking the sound from the train track.

Do you supply any material and how would you recommend I install it to maximise effect?

The trains make between 50 - 70 db of noise. With my windows shut I hear maybe 25-30db.

The hole is 125mm diameter and the material needs to be able to exposed to outside environment.

Hope you can help in this unique instance.

Kind regards,