I am interested in understanding the key factors in sound proofing a window.
I once stayed in an old hotel (built about 1815) in New York City on Canal Street and the room was absolutely silent which may be hard to believe for anyone who has been to Canal Street, but it was actually perfectly silent.
The reason for this was that the windows were doubled (not double pane); ie there were two layers of windows. The outer window was some kind of tempered glass and was sized exactly to the outer frame of the window and had been caulked into place. The inner windows were the original casement windows from the 19th century. Due to the thick width of the window frame there was a separation of perhaps 10 inches between the air tight window on the outside and the inner window.
I have been in houses with normal modern double paned windows and they are definitely not sound proof, so I am wondering what was special about the arrangement in the hotel. My guesses are:
(1) The outer window was absolutely airtight, but modern windows are not perfectly air tight but have joints and so forth that let sound past, and that made the difference.
(2) The distance between the panes was 10 inches and for some reason that is far more effective than two panes that are close together.
Does anybody know if either of these two factors (or another) is what is critical in making a window sound proof?