Glass is a good soundproofing material if it is thick enough.
There is one type of window that is absolutely useless for sound insulation however, and that is an open window.
If a window has to be opened to provide ventilation then all of the rest of the soundproofing in the studio is rendered worthless.
Some windows have to be opened only for cleaning, in which case the opening section should be surrounded by a compression seal, not a brush seal (or no seal at all).
A properly constructed window would have two panes of differing thicknesses (to avoid resonance effects allowing the same band of frequencies through each pane), set in mastic in a well constructed frame.
The flexible mastic decouples the panes from the frame and from each other, reducing the opportunities for sound transmission.
Since this is in effect a double leaf partition, it would be sonicly advantageous to fill the gap between the panes with mineral wool.
Unfortunately, the window would now no longer function as intended. The best compromise is to line the reveals (the edges between the panes) of the window with absorbent material to soak up the energy that would otherwise bounce around inside until it found an outlet.
Often, windows are constructed so that the panes are angled to each other.
This has some value in preventing standing waves being set up between the otherwise parallel panes, but its greater value is in cutting down on the visual reflections there would be otherwise.
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