Secondary Glazing for Traffic Noise

Below 30mph, engine and exhaust noise dominates and sits low in the spectrum. Above 30mph, tyre-road interaction takes over and moves energy up into the 500-1500Hz range. Buses and HGVs add a 60-125Hz component that travels furthest and is hardest to stop, which is why living on a bus route feels different from living on a car-only street.

For an A-road or bus route we specify 10.8mm acoustic laminate at a 150-200mm cavity. The laminate mass handles the mid band, the acoustic PVB damps the coincidence dip, and the deep cavity provides the mass-spring-mass decoupling that low-frequency bus rumble requires.

A shallower 50-75mm cavity — common in cheaper installations — costs you 6-8dB precisely in the low band where the problem is worst.

A 25-30dB reduction takes a 60dB bedroom to around 32dB, which is below the WHO 35dB night-time guideline. Perceptually, a 10dB reduction is heard as roughly half as loud, so 27dB is heard as a fall to well under a quarter of the original intrusion.

Traffic noise in London is dominated by tyre roar, engine load and the body-panel rumble of buses and HGVs, and most of that energy sits below 500Hz. Low frequencies are the hardest thing a window has to stop: they pass through light, stiff materials easily and they are barely affected by the small air gap in a sealed double-glazed unit. This is why homeowners on the Euston Road, the A40 or a bus corridor often report that new double glazing made almost no difference to the rumble.

Secondary glazing attacks the problem differently. A wide, unsealed-from-the-outer-pane air cavity of 100-150mm decouples the two layers so the mass-air-mass resonance drops well below the frequencies that matter, and asymmetric glass thicknesses stop the two panes sharing a coincidence dip. That is the mechanism, and it is why cavity depth is worth more here than glass thickness alone.

For a front elevation on an arterial road or bus route we would normally specify 10.8mm acoustic laminate in the secondary panel against the existing 4mm primary glass, with the deepest cavity the reveal will carry, and continuous compression sealing. In suitable configurations that combination reaches up to 54dB reduction, and a realistic expectation for most London reveals is somewhere in the mid to high 40s.

Where the reveal will only take 60-80mm we say so before quoting rather than after installation. A shallow cavity with heavy glass still performs well at mid and high frequencies — you lose the sirens, the voices and the tyre hiss — but the deepest bus rumble will remain partially audible, and it is better to know that in advance.

We measure rather than estimate. A free sound survey records dB(A) at the window over a representative period, identifies whether the profile is steady traffic or peaky (a bus stop, a junction, a speed table), and produces a specification aimed at that profile. A stop-start junction with air-brake events needs different treatment from a steady 40mph flow.

The specification is delivered through our acoustic glazing London service, which is where the heavier laminate build-ups and deeper cavity designs live. Post-installation we can re-measure at the same position so the improvement is documented rather than remembered.