Mass Loaded Vinyl (MLV) is a flexible and effective soundproofing content for windows, produced from PVC and vinyl which has a weighty material for instance barium sulphate or calcium carbonate.  Although double glazed windows offer great thermal efficiency by manufacturing facility-sealed models, secondary glazing can actually provide improved sound reduction https://sash-windows80998.azzablog.com/39071125/the-ultimate-guide-to-acoustic-secondary-glazing