A quieter room depends on the complete sound path, not simply on thicker glass.
Start with the source
Write down the noise that bothers you, the times it occurs and the rooms affected. Passing traffic, voices and low-frequency mechanical noise can behave differently. A short recording can help explain the issue, but a phone app is not a substitute for a professional acoustic assessment.
Look for other routes: vents, doors, roof details and gaps around the wall can limit improvement even when the glazing performs well.
The importance of the cavity and seals
The separation between the original and secondary glazing, glass selection and perimeter sealing all influence acoustic performance. A larger separation can be useful, but the available reveal and the tested system determine what is practical.
Air gaps around an inner frame can undermine performance. Ask how the installer will seal uneven masonry or timber and how the moving panels seal when closed.
Read acoustic figures carefully
A laboratory sound-reduction rating describes a tested specimen under stated conditions. It does not guarantee the same reduction in a furnished room with vents and other sound paths.
Ask for the test report and the exact glass build-up, cavity, frame and opening style used. If traffic noise is the concern, ask whether the evidence includes an appropriate traffic-noise adjustment rather than quoting a single unexplained number.
Plan for fresh air
Opening a window to ventilate changes the acoustic situation. Discuss how the room will receive fresh air when the secondary glazing is closed, especially in bedrooms.
For severe or unusual noise, an acoustic consultant can help define a realistic target before money is committed. Avoid promises of silence or a universal percentage reduction.