I've not got secondary glazing (or double glazing either) but I've been googling to try to find a
reliable source of information. (It seems that almost everyone selling secondary double glazing would like you to think that, once you've had it installed, you could sleep through a bomb going off outside your house!).
This page, even though it's from a sales company, seems to be more honest and accurate than many others:
https://clearviewsg.co.uk/noise-reduction/effective-secondary-glazing-noise-reduction/
It correctly states that
(a) secondary glazing is generally more effective at cutting out sound than double glazing ; and
(b) if sound insulation is important to you, you ought to be looking for a supplier who uses acoustic glass (which uses two, or more, layers of glass, with a layer of PVB in between). It will cost more but be far better at reducing sound.