Most traditional single glazed windows, in Listed buildings, with poorly sealed frames offer little defence against noise. New developments on brownfield sites with new double glazing, strain against the high sound levels.
A single glazed primary window generally provide 20dB to 30dB reduction, which when you take the following figures into account, is not a very good barrier to everyday noise pollution. An acceptable sound pressure level for a bedroom is 30/35dBA and 45/ 50dBA for a classroom and traffic on busy roads reaches 70-80dBA. Well sealed secondary glazing with a cavity of at least 100mm from the primary window, can reduce noise in excess of 45dB. Combine secondary glazing with double glazed windows and a 56dB reduction is achievable.