Those who expect to replace windows, especially those with insulated glass, will like to know this:
Typically when you order a window, you have the option to order it with glass panes that are different thickness. If you don't ask for this, likely the panes will be the same thickness.
The reason is resonance. If you hold a tuning fork a few feet from another and tap one, if the other tuning fork is for a different frequency, nothing happens. However, if they are the same frequency, the other tuning fork will also emit sound even though you never touched it.
Same thing happens with window panes. If the two panes are different thicknesses, you vastly reduce, or entirely eliminate the ability of the sound to pass through the window. If they are the same thickness, the window will act as a sound conductor, so what you hear on one side, you will hear on the other.
I used this strategy when I converted older single pane windows to insulated glass. Although there are additional steps you can take to increase the soundproofness of a window, this is a big one that most people don't know about and entirely overlook.
So it pays to know this when you order the windows. It sure did work for me.
Dx G