Technical Guide

The Physics of Silence: How to Block Low-Frequency Bus and Lorry Rumbling

Published 18 February 2026 · 16 min read

You can hear it right now if you live on a London bus route: that deep, persistent rumble that seems to pass straight through your walls and windows. High-pitched sounds — sirens, car horns — are annoying, but they're relatively easy to block. Low-frequency noise is the real enemy. It has longer wavelengths, more energy, and a nasty habit of vibrating through solid materials like they're not even there.

This guide explains the actual physics behind why certain glass types block noise and others don't. No marketing fluff — just the science that determines whether your windows work or not.

Understanding Mass Law

The single most important principle in acoustic glazing is the Mass Law: doubling the mass of a barrier increases its sound insulation by approximately 6dB. It sounds simple because it is. Heavier glass = more noise blocked.

Glass Type	Thickness	Mass (kg/m²)	Rw Rating
Standard float	4mm	10	29dB
Laminated	6.4mm	16	34dB
Stadip Silence	10.8mm	27	42dB

But Mass Law has a catch: it works best at higher frequencies. Below about 200Hz — exactly where bus and lorry engines operate — mass alone isn't enough. That's where the next two principles come in.

The PVB Acoustic Interlayer: Damping What Mass Can't

Detailed cross-section of acoustic laminate glass showing PVB damping interlayer between glass panes

Every rigid material has a coincidence frequency — a point where sound waves align with the natural vibration of the material and pass straight through it. For standard 4mm glass, this happens around 3,000Hz. For thicker glass, it drops into the speech-frequency range, which can actually make things worse.

The 10.8mm Stadip Silence glass solves this with a special polyvinyl butyral (PVB) acoustic interlayer. Unlike standard PVB used in safety glass, this acoustic-grade film is specifically engineered to convert sound energy into heat energy through viscoelastic damping.

How the PVB interlayer works:

Sound waves hit the outer glass pane and cause it to vibrate
The PVB film absorbs these vibrations and converts kinetic energy to thermal energy (heat)
The inner glass pane receives significantly reduced vibration
The effect is most pronounced at the coincidence frequency, exactly where standard glass fails

The result is a glass that performs consistently across the entire frequency spectrum — from the 80Hz rumble of a diesel engine to the 4,000Hz screech of brakes. No weak spots. No frequency gaps.

The 100mm Air Gap: Why Distance Matters

Here's the part most people get wrong: they assume double glazing and secondary glazing work the same way. They don't. A standard sealed double-glazed unit has a 12-20mm gap between panes. That's fine for thermal insulation, but acoustically, it's almost useless at low frequencies.

Secondary glazing creates a 100-150mm air gap — ten times wider than double glazing. This is critical because of the mass-air-mass resonance principle: the resonant frequency of the system drops as the air gap increases. With a 100mm gap, the resonance frequency falls below 80Hz — below the range of most urban noise sources.

Double Glazing (16mm gap)

25-30dB

Resonance at ~400Hz — right in the traffic noise band. Poor low-frequency performance.

Secondary Glazing (100mm gap)

45-54dB

Resonance at ~60Hz — well below traffic noise. Excellent across all frequencies.

This is why secondary glazing outperforms double glazing for noise reduction every single time. It's not marketing — it's physics.

The Complete System: How All Three Work Together

When you combine all three principles — mass, damping, and air gap — you get a system that's greater than the sum of its parts:

Original window (4mm glass)

Provides initial mass barrier. Reduces noise by ~25dB on its own.

100mm air cavity

Decouples the two glass layers. Drops resonance frequency below audible urban noise.

10.8mm Stadip Silence (acoustic laminate)

Heavy mass + PVB damping layer. Eliminates coincidence dip and absorbs residual vibration.

Combined result: up to 54dB noise reduction. A busy London road at 78dB drops to 24dB inside — quieter than a whisper. This is the specification we recommend for properties near Heathrow flight paths, railway lines, and major A-roads.

Where This Matters Most in London

Low-frequency noise is concentrated along specific corridors. If your property is near any of these, standard glazing simply won't cut it:

•Bus routes — diesel engines produce 80-100Hz rumble that travels through walls
•HGV corridors — the A40, A2, A13 carry heavy goods vehicles through residential areas 24/7
•Rail lines — Overground and mainline trains produce sustained low-frequency vibration
•Aircraft — Heathrow approach paths affect a wide swathe of West London

For heritage properties in these areas, secondary glazing with acoustic laminate is often the only permitted solution that actually works. Listed building owners and conservation area residents consistently choose this specification because it delivers real-world results without compromising their property's character.