Sound intrusion though windows can also be a source of great annoyance if the sound is excessive, and in some cases developments have failed the sound insulation testing due to poorly specified and installed windows. Sound intrusion may be down to a variety of factors. If the windows are operable types, the first thing to check is to see if the widows close properly against their seals and any weather stripping is continuous and in good order. If the window leaks air, then more often than not it will usually leak sound. If the window closes to form a tight seal and the weather-stripping seems adequate the next thing to check is the window frame. Windows are frequently installed in to the wall opening with plastic shims to insure the unit is plumb and level. In high traffic areas you will normally require a double glazed unit as a minimum and in some cases a triple glazed unit will be required.
The space between the frame and the wall should be insulated and sealed before the window casings are installed. If this was not done correctly you can probably detect the sound leakage by placing your ear close to the frame and listening. The sound may be able to run along the cavity and back into the adjacent property, It is case of letting your ears do the walking and listening. If you detect distinguishable differences in sound you may have found the problem; the solution will be to remove the casing and insulate and caulk all leaks. In some cases the thickness of the glass maybe the problem in which case it may be necessary to install another layer of glass on the inside of the existing frame making sure that the additional pane of glass is well sealed into the opening.
In some instances you may be able to lower excessive sound by installing a heavy curtain – especially if the house is in a conservation area and as a result the local authority will not let you change or upgrade the windows.