The four things that decide the result
1. Pane asymmetry
Every pane of glass has a frequency at which it resonates and passes sound efficiently. Two identical panes in a sealed unit share that frequency, so at that pitch the pair behaves almost as though the cavity were not there — and traffic noise contains a great deal of energy there.
Making the panes different thicknesses gives them different weak points, so what passes through one is resisted by the other. This is the single most important decision in the glass build. See asymmetrical glazing.
2. A laminated inner leaf
The interlayer in laminated glass is viscoelastic: it converts vibration into a tiny amount of heat rather than passing it along. A laminated pane outperforms a solid pane of the same weight, which is why every serious acoustic unit we build has one.
3. Cavity width
Wider cavities perform better, particularly at low frequencies. A sealed unit is limited to what its edge seal can span — which is why secondary glazing, with a gap of 100 mm or more, often beats a replacement sealed unit acoustically.
4. The frame and the seal
This is where most acoustic upgrades fail.
Sound and air take the same path. A window that leaks air leaks noise, and no glass specification compensates for a gap. So the frame type matters: a casement compresses a gasket all round when it locks; a slider seals by proximity and has an inherent path at the meeting stile.
If quiet is the objective and there is room to swing a sash, hinged beats sliding every time. Where a slider is unavoidable, a lift-and-slide mechanism closes most of the gap by dropping the panel onto its seals.
Matching the build to the noise
Different sources need different builds:
- Road traffic — broadband with substantial low-frequency content from engines and tyres
- Rail — low-frequency rumble plus high-frequency wheel squeal
- Aircraft — predominantly low frequency, which favours wide cavities
- Construction — impulsive and broadband, and usually temporary
A useful conversation starts with what you hear rather than how loud it is.
The honest limit
Once the window becomes the strongest acoustic element in the room, the next weakest one governs. Commonly that is:
- an air-conditioning sleeve or ventilator through the wall
- the entrance door and its undercut
- a lightweight partition or a service shaft
- the ceiling, in a flat below a terrace
We look at the whole opening rather than quoting glass in isolation, because a window that leaves the room only slightly quieter is a bad outcome for everyone.
The options, ranked by disruption
Acoustic inserts — fastest, reversible, keeps the existing window.
Secondary glazing — best low-frequency performance, original window stays.
Acoustic windows — full replacement, best single-system result.
Tell us the room, the direction it faces and what you can hear from it. That is enough to say which of the three is right.