The 'Soundproofing' Fail: Why Your New Double Glazing Still Lets Traffic Noise In

It is a common scenario for homeowners in London: you invest thousands in high-spec, modern double glazing, expecting a "silent" home, only to find that the rumble of the morning bus and the shrill of a passing siren are almost as loud as before.

The reason for this failure isn't necessarily poor quality, it is physics. Standard double glazing is designed primarily for thermal insulation, not acoustic isolation. When it comes to the low-frequency noise common in urban environments (traffic, aircraft, and construction), standard double-glazed units (IGUs) often suffer from a phenomenon known as Mass-Air-Mass (MAM) resonance.

In this guide, we break down the technical reasons why standard double glazing fails and why acoustic secondary glazing is the only laboratory-proven method to achieve noise reduction up to 54dB.

Skip the reading? Get an instant decibel estimate for your home with our Acoustic Calculator (pre-filled for failed double glazing) — your results will hand off straight to our team if you'd like a tailored quote.

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What You'll Learn

• Why the 16mm-20mm gap in double glazing acts as a "drum" for traffic noise.

• The role of Mass-Air-Mass resonance in sound transmission.

• Why a 100mm–150mm air gap is the mandatory minimum for low-frequency attenuation.

• A technical comparison of glass types, from standard 4mm to 10.8mm Stadip Silence.

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The Physics of Failure: Why Standard Double Glazing "Leaks"

Sound travels in waves. Higher frequencies, like birdsong or human voices, have short wavelengths and are relatively easy to block with simple barriers. However, traffic noise, specifically the "rumble" of engines, consists of low-frequency sound with long wavelengths.

1. The Drum Effect (MAM Resonance)

Standard double glazing typically features two 4mm glass panes separated by a 16mm to 20mm cavity. In the world of acoustics, this creates a "Mass-Air-Mass" system. The air trapped between the two panes acts as a spring. At a specific frequency, the resonance frequency, the two panes of glass actually vibrate together, significantly reducing the window's ability to stop sound.

For most standard double glazing, this resonance occurs precisely within the frequency range of traffic noise. Instead of blocking the sound, the window essentially becomes a drum, transferring the vibration from the outside world directly into your living room.

2. Symmetrical Panes

Standard windows often use two panes of the same thickness (e.g., 4mm glass / 16mm gap / 4mm glass). Because the panes are identical, they share the same "critical frequency", the point at which they are most transparent to sound. This creates a "weak spot" in the window's acoustic profile.

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The Laboratory Solution: The 100mm Rule

To effectively decouple the noise from your home, you must move the resonance frequency out of the range of human hearing or, at the very least, out of the range of common urban noise.

Why Cavity Depth Matters

The most effective way to lower the resonance frequency is to increase the air gap. While double glazing is restricted to small gaps to maintain thermal efficiency (convection currents in large gaps can actually reduce U-values), secondary glazing allows for a much larger cavity.

By installing a second, independent internal window at a distance of 100mm to 150mm from the primary window, we achieve a "decoupled" system. This massive air gap shifts the resonance frequency so low that it no longer interferes with the window's ability to block traffic noise.

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Head-to-Head Comparison: Acoustic Performance Data

The following table illustrates the performance gap when comparing glazing technologies head-to-head.

Quick check: How is dB reduction actually measured?

The Rw (Weighted Sound Reduction Index) is measured under BS EN ISO 10140-2 in a certified acoustic chamber, averaged across 100 Hz–3.15 kHz. The figures in this table are system Rw values (full installed window), not glass-only ratings — which is what you actually experience at home.

Quick check: Is the 54 dB figure realistic for my London terrace?

Yes, provided two conditions: (1) a 100 mm+ air gap between primary and secondary unit, and (2) compression seals around the full perimeter. If your reveal is shallow (<80 mm), expect 45–48 dB — still a perceived halving of noise versus standard double glazing.

Window Type	Glass Configuration	Air Gap	Noise Reduction (dB)	Low-Frequency Success
Single Glazing	4mm Float	N/A	~22dB	Extremely Poor
Standard Double Glazing	4/16/4mm	16mm	~31dB	Poor (Resonance issues)
Acoustic Double Glazing	6.8mm / 16 / 4mm	16mm	~36-38dB	Moderate
Soundproof My Window	10.8mm Stadip Silence	100mm+	Up to 54dB	Excellent

Note: A 10dB increase represents a perceived halving of noise to the human ear. A jump from 31dB to 54dB is not just an incremental improvement; it is a transformative change in the living environment.

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The Engine Room: 10.8mm Stadip Silence Glass

At Soundproof My Window, we don't use standard glass. Every installation features 10.8mm Stadip Silence, a high-performance acoustic laminated glass manufactured by Saint-Gobain.

The Role of the PVB Interlayer

Unlike standard laminate, which uses a basic plastic film to hold glass together if it breaks, Stadip Silence uses specialist PVB acoustic interlayers. This interlayer acts as a dampening core, absorbing vibrational energy as it attempts to pass through the glass.

Quick check: What's the difference between acoustic PVB and standard PVB?

Standard PVB (used in car windscreens) is a single rigid layer designed for impact safety. Acoustic PVB is a three-layer visco-elastic structure that physically converts sound vibration into trace heat. The acoustic version adds roughly 3–5 dB at the coincidence dip — the exact range where standard glass fails.

When you combine this high-mass, dampened glass with a 100mm–150mm air gap, the results are industry-leading. You can view our detailed decibel guide to see how this glass handles different noise profiles, from Heathrow flight paths to sirens in Belgravia.

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Real-World Benefits for London Homeowners

While noise reduction is the primary goal, our systems provide secondary benefits that are particularly valuable for owners of Victorian, Georgian, and Edwardian properties.

• Thermal Efficiency: Our installations deliver up to 65% heat savings, often outperforming new double glazing because they eliminate the drafts common in original sash windows.

• Planning Permission: Because secondary glazing is an internal modification, it typically requires no planning permission, even for Grade II listed buildings or properties in conservation areas like Chelsea or Richmond.

• Aesthetics: We specialize in heritage and listed building installations. Our frames are designed to align perfectly with your existing meeting rails and glazing bars, making them virtually invisible from the street.

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Choose Acoustic Secondary Glazing When...

• You live on a main road or bus route where low-frequency rumble is the primary issue.

• You are located under a flight path (Heathrow, Gatwick, or London City Airport).

• You own a period property with original sash windows that you are legally required to preserve.

• You have already installed double glazing and found it insufficient for noise control.

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Frequently Asked Questions

Q: Can I just put thicker glass in my existing double-glazed frames?

A: While thicker glass helps slightly with mass, you are still limited by the small (16-20mm) air gap. You will still experience the MAM resonance that lets traffic noise through. True soundproofing requires the decoupling provided by a 100mm+ gap.

Q: Will secondary glazing make my room feel smaller?

A: Not significantly. The system is installed within the existing window reveal. For most London period homes, the depth of the reveal is more than enough to accommodate a 100mm gap without encroaching on your living space.

Q: Is it better than "Acoustic Double Glazing"?

A: Yes. Even the best acoustic double-glazed units (which use asymmetric laminated glass) are limited by their narrow cavity. Laboratory tests show that secondary glazing with a large air gap consistently outperforms high-end double glazing by 10-15dB.

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Summary of Technical Specifications

• Glass Type: 10.8mm Stadip Silence (Laminated with Acoustic PVB).

• Target Air Gap: 100mm - 150mm.

• Maximum Noise Reduction: 54dB.

• Thermal Improvement: Up to 65% reduction in heat loss.

• Guarantee: 10-year performance guarantee.

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Get a Professional Assessment

Soundproofing is a science, not a guesswork game. If your current windows are failing to provide the peace and quiet you expect, the solution lies in the physics of the air gap.

We offer free on-site noise surveys across London. Our experts use precision equipment to measure the specific frequency of the noise entering your home and can provide a bespoke configuration to eliminate it.

Book Your Free Noise Survey Today or use our Acoustic Calculator to estimate your potential savings.

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Sources & References

• Saint-Gobain Technical Data: Stadip Silence Acoustic Performance Ratings.

• BS EN ISO 10140-2: Laboratory measurement of sound insulation of building elements.

• The Physics of Sound: Understanding Mass-Air-Mass Resonance in Glazing Systems.