Noise FAQs: Measurement, Decibels and Glazing

Noise is measured in decibels (dB) with a calibrated sound-level meter. For homes, the meter is usually placed at the façade and then inside the room, recording an average over a representative period. Measurements are normally A-weighted (written dB(A)) so they reflect how the human ear hears different frequencies.

The decibel scale is logarithmic: every 10 dB increase sounds roughly twice as loud. So a 70 dB street sounds about twice as loud as 60 dB, and a reduction from 70 dB to 40 dB feels dramatically quieter — roughly eight times less loud.

For restful sleep, internal night-time levels in the low 30s dB or below are generally desirable. World Health Organization guidance suggests keeping night-time bedroom levels around 30 dB where possible. Many London homes without treatment sit well above this.

Traffic, buses and lorries produce strong low-frequency energy, which travels through glass and frames more easily than higher-pitched sound. A façade reading of 70 dB dominated by low-frequency rumble is harder to treat than the same level from general street activity, which is why the noise source matters as much as the number.

Secondary glazing adds a second, independent glazed panel inside the existing window. Sound is reduced by three mechanisms: the extra mass of the glass, the deep air gap between the two windows (typically 100–200 mm for acoustic work), and the decoupling of the two panes so vibration is not passed directly through. Laminated acoustic glass adds a damping interlayer for further reduction.

The wider the air gap between the existing window and the secondary glazing, the better the low-frequency performance. Double glazing with a 12–20 mm cavity cannot match the acoustic performance of a 100 mm or deeper secondary glazing gap, which is why secondary glazing usually outperforms replacement double glazing for noise.

Specialist acoustic systems can reduce external noise by up to 54dB in suitable configurations. Actual performance depends on the existing window, glass specification, air-gap depth, installation quality, room construction and noise frequency. A sound survey sets a realistic target for your property.

Yes. Heavier glass blocks more sound, and laminated acoustic glass (for example 6.4 mm or 10.8 mm) uses a soft interlayer that damps vibration. Using a different thickness from the existing window also helps, because mismatched panes do not resonate at the same frequency.

Yes. Aircraft noise arrives as short, high-level peaks rather than a constant level. A well-sealed secondary glazing system with laminated acoustic glass reduces those peaks substantially, and because flights pass intermittently, opening styles that preserve ventilation and access are usually straightforward to specify.

Enormously. Sound behaves like water — it finds the smallest opening. Acoustic seals around the secondary glazing frame, and attention to trickle vents, letterboxes and gaps around the original window, are essential to achieving the measured performance of the glass itself.

Yes. Secondary glazing is fitted inside the existing window and is fully reversible, so original sash, casement and period windows are retained. This makes it well suited to listed buildings and conservation areas, subject to confirming any consent requirements for the specific property.

Request a free sound survey. We record the façade and internal levels with a calibrated meter, identify the dominant noise source and recommend a glass, air-gap and opening specification with a realistic target level. You can also build an indicative noise profile online using our noise profile tool.