On Mon, 09 Jul 2012 10:54:18 +0100, "Dave Plowman (News)"
wrote:

Normal double glazing isn't a very good sound insulator as the gap is too
small. If you fit secondary glazing the thickness of the wall away from
the original you'll get very good results. Glass will be better than
plastic because it is denser - the thicker the better. And it needs a good
air seal all round.

Bingo all around, there.
I rented a house on a busy main road - double-glazed with secondary
glazing of fairly thick glass in an aluminium frame which could be
removed for cleaning. The whole assembly sat in a channel arrangement
which allowed the halves to slide and overlap. When shut, the noise
was reduced to below the point of normality - instead of a main road
it was like being in a cul-de-sac. In fact, it was so good at sealing,
there was an extra wall powervent fitted.