Glass - is it any good for soundproofing?
Glass is an excellent material for soundproofing. The reasons for this are firstly that it is dense, meaning that you get a lot of mass in a fairly small thickness Secondly that it is non-porous - it doesn't have holes where sound can leak through.
The question is then why don't we use more of it? If glass is so great, then why don't we build whole see-through studios?
The main answer to that is that glass is expensive. Much more expensive than concrete, bricks, gypsum board and particle board - the soundproofing staples. The same applies to lead, which is also an excellent soundproofing material. So because glass is expensive, we only use it where we need to - in windows of course. Lead is used to line soundproof doors.
Glass is of course also fragile, unless it is toughened or reinforced, which makes it even more expensive.
But there are two other problems...
Since glass is only ever used as a window, and windows need to be transparent, it is impossible to apply any kind of acoustic treatment to it, only to the frame.
Also, windows are generally flat. Making glass curved doesn't necessarily make it into a lens, and you can see perfectly well through a curved window, but they are once again more expensive. Flat windows cause strong reflections that potentially can degrade the sound image in the studio control room.
In its place however, glass is a very good soundproofing material. Come to think of it, it's just as well really. What would we do if it wasn't?