Reducing noise in a project to a level that is acceptable is variable depending on the project and the surrounding amenity, taking into consideration factors like traffic, train or planes etc. However no matter how good the window is at keeping noise out, if it is not installed and sealed properly during construction, noise will still penetrate the home.
Sound Transmission Class (STC) reflects the amount of noise that is reduced when sound passes through the window. If the noise outside is 70dB and inside is 40dB, the window is said to have an STC rating of 30. So the outside noise is reduced inside by 30dB. A 10dB decrease in sound is best described as halving of the sound – e.g. a 40dB noise seems half as loud as a 50dB noise. The average spoken conversation makes a 50dB noise, while common street traffic and neighbourhood sounds make about 70dB noise.