Noise Science Blog

How noise is measured

Sound is a pressure wave, and the decibel scale is logarithmic: a change of about 3dB is just noticeable, while 10dB is generally heard as roughly a doubling of loudness.

Most readings are taken as dB(A), a weighting that discounts very low and very high frequencies to match how the ear responds. That matters in London: a bus idling outside may read only a few decibels above a car, yet its low rumble passes through single glass far more easily.

• 30–35dB: a quiet bedroom at night.
• 40–50dB: traffic noise with a window slightly open.
• 60–70dB: a busy main road or bus route.
• 75dB and above: railway lines, sirens and heavy plant.

How sound gets in through a window

Glass resists sound mainly through mass. A wave strikes the pane, the pane vibrates, and a smaller wave leaves the other side carrying the rest of the energy. Thin single glass is light, so it moves readily.

The space behind the glass matters just as much. Air in a sealed cavity behaves like a spring and, with the pane, forms a resonant system. Where that cavity is shallow, the resonance lands in the middle of road and rail noise and can help sound through.

What secondary glazing changes

Secondary glazing adds a second, independent window inside the existing frame, separated by a deep cavity — commonly 100–200mm rather than the 12–16mm gap of sealed double glazing. The panes are decoupled, so the resonance moves lower and the cavity absorbs energy before it reaches the room.

Acoustic laminated glass adds damping that turns a little vibration into heat, and perimeter seals close the gaps where most noise leaks. Together these give reductions of up to 54dB in suitable configurations; results depend on your existing window, wall and installation.