London living brings culture, convenience and character, but it can also bring traffic, buses, sirens and late-night street noise into rooms that should feel restful. If you are turning up the television or waking to the same road noise each night, acoustic glass and secondary glazing may be worth investigating.

There is no single glass specification that makes every home silent. Sound reduction depends on the existing window, the frame and seals, the noise source, the air gap or cavity depth, and how the complete system is installed.

This guide explains how acoustic laminated glass works, why a wide independent cavity can help, and how to compare soundproof secondary glazing with replacement windows without over-promising what any one system can deliver.

What is acoustic glass?

Acoustic glass is usually laminated glass made from glass layers bonded with a specialist interlayer. That interlayer is designed to damp vibration, so less sound energy is transmitted through the pane than with a comparable piece of standard float glass.

How the interlayer helps

When traffic noise reaches a window, the glass can vibrate. An acoustic interlayer helps absorb and dissipate some of that vibration. It is one part of the answer rather than a standalone promise: the result still depends on the pane, the supporting frame, the seals and the rest of the building envelope.

Why glass specification matters

More glass mass is not automatically better. Different panes can respond differently to different frequencies, and an acoustic laminated pane may be selected to complement the original window rather than simply match it. The right thickness and make-up should be specified for the opening, available space and sound you are trying to address.

Illustration showing sound waves being dampened through layered acoustic glass
Acoustic laminated glass can damp vibration, while the independent frame and cavity help separate the two window layers.

Why the air gap or cavity depth matters

Secondary glazing adds a separate internal window in front of the original one. The space between them creates an independent air cavity, which helps decouple the two layers. That is a different arrangement from a sealed replacement unit, where the panes sit close together in one frame.

For demanding traffic noise, installers often discuss a cavity in the region of 100mm to 150mm. That is a useful design reference, not a universal rule. The best air gap depends on the reveal depth, shutters, radiators, ventilation, opening style and how the original window is maintained. Our guide to why the air gap matters in soundproof windows explains the principle in more detail. For a closer look at the 150mm question, read the 150mm cavity science article.

A larger cavity is not enough on its own. Poor seals, a leaking original sash, or sound travelling through walls, floors, roofs and vents can limit the improvement. Read this practical look at air gaps and Red Route noise for more context on the trade-offs.

Acoustic glass vs standard glass

Standard glass can be a sensible choice where the main need is light, appearance or a general secondary layer. Acoustic laminated glass is considered when sound transmission is a priority, particularly where traffic includes low-frequency engine rumble as well as sharper sounds such as braking and sirens.

  • Standard glass: a straightforward pane whose acoustic performance depends heavily on its thickness, the frame and the cavity around it.
  • Acoustic laminated glass: a laminated construction with a damping interlayer intended to reduce vibration through the pane.
  • The complete system: glass, seals, frame, cavity depth and the original window must work together. A premium pane cannot compensate for a significant air leak.

For a closer comparison of the two materials, see our acoustic glass versus standard glass comparison.

Classic living room bay window with discreet secondary glazing
The acoustic upgrade can sit quietly behind the period room and original window you want to keep.

Secondary glazing vs replacement windows

Secondary glazing and replacement windows solve different problems. Replacement windows remove the existing frame and install a new complete unit. Secondary glazing keeps the original window in place and adds an independent internal layer.

For a period home, retaining the original window may matter as much as performance. Secondary glazing can preserve external character and may be a less disruptive route, but it uses some internal reveal space and leaves you with two layers to operate and maintain. Replacement windows can offer a simpler single-window operation, but they may alter original fabric, sightlines or the external appearance.

There is no blanket answer on planning or listed-building consent. The position depends on the property, the proposed work and the relevant local authority, so check before committing. Compare the practical trade-offs in our 2026 guide to secondary glazing versus double glazing and this secondary glazing versus full replacement comparison.

Choosing noise reduction windows for London traffic

London traffic noise is not one sound. A Red Route may combine bus engines, tyres, horns, deliveries and emergency vehicles. A window specification that helps with one frequency range may be less effective against another.

When assessing noise reduction windows, look at the whole opening rather than a single headline rating:

  1. Identify the source: note whether the problem is steady road noise, occasional peaks, rail, aircraft or voices.
  2. Check the existing window: condition, draughts, glazing, shutters and opening method all affect the starting point.
  3. Measure the reveal: cavity depth must fit around handles, trim, radiators and ventilation needs.
  4. Specify the glass and seals together: acoustic laminated glass is only useful when the frame closes and seals properly.
  5. Consider flanking paths: noise can bypass the window through masonry, doors, roofs, floors, vents and adjoining openings.

For another grounded overview of tackling busy roads, read these practical steps for Red Route noise and the science behind soundproofing London’s Red Routes.

What noise does secondary glazing work best against?

Secondary glazing is often investigated for noise arriving through a window, including steady road traffic, buses, trains, aircraft and voices. Acoustic laminated glass and a suitably separated inner frame may help with parts of that sound spectrum, but low-frequency vibration can also travel through walls, floors, roofs, doors, vents and adjoining windows. A window upgrade cannot promise silence or remove every peak.

Source comparisons sometimes describe around 45–50 dB or a 70–80% perceived-noise reduction for suitable arrangements. Treat those as indicative, test- and property-dependent references. Ask what was measured, with which glass and complete window, at which frequencies and under what installation conditions.

What soundproof secondary glazing should include

“Soundproof” is often used as shorthand, but no window can promise complete silence in every room. A well-specified system should consider:

  • an independent frame that fits the reveal without compromising the original window;
  • careful perimeter seals and junctions, because small gaps can undermine acoustic performance;
  • an acoustic laminated glass specification suited to the noise source;
  • an air gap or cavity depth that the room can accommodate; and
  • access for cleaning, ventilation, future repairs and normal use.

Our soundproof secondary glazing guide covers the system as a whole, including the practical compromises that a brochure headline can miss.

What a measured survey should establish

A survey should give you a property-specific recommendation rather than a guaranteed percentage. Ask the installer to explain:

  1. which noise sources the proposed glass and frame are intended to address;
  2. what cavity depth is realistic in each room and why;
  3. how the unit will be sealed around the opening;
  4. how original sashes, casements, shutters and finishes will remain accessible; and
  5. what limitations remain if sound is also entering through another part of the building.

Published case studies can be useful for understanding possibilities, but their results should not be treated as a promise for another property. For a sense of the different homes and constraints involved, browse this Westminster noise case study or these London period-flat examples with that qualification in mind.

Five questions to ask a secondary-glazing company

  1. What noise source is the proposed build-up designed to address? Ask how the glass, cavity and frame relate to traffic, rail, aircraft, voices or vibration in your property.
  2. What complete-window evidence supports the recommendation? Check whether any dB or percentage figure comes from the installed assembly, not glass in isolation.
  3. What air gap is realistic in each reveal? The answer should account for shutters, curtains, radiators, handles, ventilation and sightlines.
  4. How will the unit be sealed and kept accessible? Ask how junctions, the original window, cleaning, future repairs and normal operation will be handled.
  5. What is included in the quotation? Confirm survey, repairs, glass, frame, installation, access, making good, aftercare and any limitations in writing.

A careful company should be comfortable explaining what the proposed system may improve and what it cannot address. A survey is the point at which those questions become specific to your rooms.

FAQs about acoustic glass and secondary glazing

Is acoustic glass the same as soundproof secondary glazing?

No. Acoustic glass is one component: usually laminated glass with a damping interlayer. Soundproof secondary glazing refers to the wider system of independent frame, seals, cavity depth, original window and installation. Even then, “soundproof” should not be read as a guarantee of silence.

How much noise can acoustic glass reduce?

There is no reliable single figure for every home. Results depend on the glass, the window, the air gap, the seals, the building and the noise source. Ask for a specification and test evidence that relate to the complete system, not just the glass in isolation.

Is a 100mm or 150mm air gap always necessary?

No. A generous cavity can be helpful for traffic noise, but the useful depth depends on the existing reveal and the details of the room. A survey should balance acoustic aims with ventilation, access, sightlines and practical space.

Will acoustic laminated glass stop London traffic or Red Route noise?

It can reduce noise transmitted through a suitable window system, but it cannot remove every sound. Heavy traffic may also be heard through walls, floors, roofs, vents and doors. The best recommendation starts by identifying how the noise is reaching the room.

Is secondary glazing better than replacement windows?

That depends on your priorities. Secondary glazing is often considered where retaining original windows and external character matters, while replacement windows provide a new single unit. Compare the condition of the existing window, available space, heritage constraints, operation, cost and acoustic goals before choosing.

Can secondary glazing help create peace and quiet in the city?

It can make a room feel calmer when the specification addresses the main route for sound transmission. A measured survey is the sensible next step because the outcome depends on the specific window, air gap, glass, frame and building.

A considered next step

Acoustic glass can be an important part of a quieter home, but the best result comes from matching the glass, cavity depth, frame and seals to the room you actually live in. That measured approach can improve comfort while keeping original period windows and architectural character in view.

Contact SecondaryDoubleGlazing for a measured survey or bespoke quote. We can review your windows, the noise source and the available reveal, then explain what soundproof secondary glazing may realistically achieve for your home.