Windows can provide up to 32% reduction in perceived loudness for a standardized mixture of aircraft, railroad and vehicular traffic noise versus a dual pane window. Noise reduction percentages can be misleading in some instances. The basic problem is the human ear. As the environment gets quieter, the ear becomes more sensitive to noise. This means that some of the reduction provided is lost. A window manufacturer can say they reduce the current noise levels by 80% or by 50% and both statements will be true. One is the instrument measurement and the other is the perceived reduction. Additionally, depending on actual noise levels, the reduction may be perceived as 100%. Dual pane window sales people can say they stop the noise (in some noise level environments they do). They can also say they cut the noise over 50% (true by instrument reading compared to a poorly sealing window).
Logic would dictate that the higher a window’s STC rating, the better it should be at reducing traffic noise. But this is often not the case. The STC (sound transmission class) was developed to control noise for speech, not planes, trains and automobiles. Speech is quite different from the low frequency roar of traffic or the high pitched whine of a jet engine. The good news is that OITC (outdoor-indoor transmission class) was specifically developed to address transportation noise. It is used to specify the sound transmission loss properties of exterior building elements such as walls and windows. The OITC uses outside noise sources such as traffic, aircraft and trains to calculate a single number rating. As a general rule, an increase in OITC means a corresponding decrease in interior noise level.