Secondary glazing on sash window

[Sue]

Rather than replacing a sash window with one with acoustic glass, I'm considering having secondary glazing fitted on the existing sash window.

Has anybody done this? If so, was it effective and who did you use?

Also how does the cost compare with replacing the window?

Thanks for any help anybody can give!!!

It is a bedroom window and I would want still to be able to open it, however it might be a trade-off between noise reduction and air :(