Decibel Reduction Explained: dB, Rw and What Actually Sounds Quieter

Acoustic data sheets are written for engineers, not homeowners. You'll see numbers like 'Rw 42 dB' or 'STC 38' or '37 dB at 500 Hz' and have no way of translating that into 'will I sleep through the bin lorry'. This guide cuts the jargon down to the three things you actually need to understand before picking a glass spec.

Decibels Aren't Linear — They're Logarithmic

A 10 dB reduction sounds roughly half as loud to the human ear, not 10% quieter. A 20 dB reduction sounds about a quarter as loud. This is why a quoted 'only 23 dB extra reduction' from secondary glazing is in fact transformative — it's the difference between motorway-loud and library-quiet inside the same room, with nothing else changed.

Rw, STC and the Numbers That Actually Matter

Rw (Weighted Sound Reduction Index) is the European spec figure — the single number you'll see on glass datasheets. STC (Sound Transmission Class) is the American equivalent, broadly comparable within 1-2 dB. Both compress 16 frequency bands into one number using a standard reference curve. The honest catch: Rw flatters performance against speech but understates performance against bass-heavy noise. For street and pub noise, also ask for the Ctr correction figure — a -5 to -8 dB adjustment that gives a more realistic picture for low-frequency sources.

The 10.8mm Acoustic Laminate Standard

For demanding noise environments, 10.8mm Stadip Silence acoustic laminated glass has become the industry benchmark. Two layers of 5mm float glass are bonded with a special acoustic PVB interlayer that absorbs vibrational energy across the full speech and traffic frequency range. Compared to the more common 6.4mm laminate, the 10.8mm spec delivers an additional 4-6 dB of reduction — modest on paper but transformative in a real room.

For a deeper technical breakdown of why this specific glass works, see our companion guide on the science of silence behind acoustic laminate glass, or compare specs across all glass options on the acoustic secondary glazing page.

What Real Installations Actually Deliver

Marketing claims for acoustic glazing range from honest to wildly optimistic. The realistic benchmarks from our portfolio: a 10.8mm acoustic laminate system with a 150mm cavity achieves 42-46 dB total reduction (versus 22-26 dB for original single glazing), eliminates condensation on the inner pane within one heating season, and reduces heat loss through the treated window by 60-65%.

For thermal performance specifically, our thermal performance guide sets out the U-values; for acoustic numbers, the noise reduction guide shows real before-and-after measurements from London projects.

Customers consistently describe the post-installation experience in two phrases: 'I didn't realise how loud it was before' and 'the bedroom finally feels separate from the street'. Both are honest reactions to a 20+ dB drop in ambient noise — perceived by the brain as a fourfold reduction in loudness.

The Heritage and Planning Angle

For listed and conservation-area properties — which describes the majority of pre-1939 housing in London and the Home Counties — secondary glazing is the only thermally and acoustically meaningful upgrade that's reliably approvable. Replacement double glazing is refused as a matter of routine for listed buildings; even slimline heritage-style units rarely clear conservation review.

Our listed buildings guide sets out the reasoning conservation officers use, and our secondary glazing vs double glazing for listed buildings explainer breaks down the specific 'reversibility' test that decides which alterations get approved. The short answer: internal, removable, no damage to original fabric — and you're almost always fine.

For projects in particularly sensitive locations — within sight of a Grade I building, in a designated cathedral conservation area, or within a national park — we'll always advise a courtesy notification to the conservation officer before installation, even where strict consent isn't required.

Realistic Project Pricing for 2026

Per-window pricing for a single specification-grade secondary glazing unit in 2026 typically ranges from £550 (small thermal-spec casement) to £1,750 (large bay sash with 10.8mm acoustic laminate), all installation included. Whole-house projects unlock 15-25% discounts versus single-unit pricing thanks to fixed-cost amortisation across multiple windows.

For honest project budgeting, see our whole-house cost guide or run your own figures through the bespoke estimator — both tools include the per-window discount that whole-project quotes attract. Compare against the 2026 London price breakdown for benchmark per-window figures.

All quotes from our network include detailed survey, bespoke manufacture, professional installation, perimeter sealing, manufacturer's glass guarantee, and a 10-year installation warranty. Hidden extras (decorating reinstatement, scaffolding, structural alterations) are rare with secondary glazing because the work is internal and non-structural.

How to Take the Next Step

The standard project timeline runs roughly four weeks from first enquiry to completed installation: 3-5 days to first survey, 5 working days to detailed quote, 2-3 weeks for bespoke manufacture, then 1-2 days on site for fitting. Survey visits are free and non-obligation; quotes are detailed and itemised window by window.

Start with the online estimator for a tailored installed-price figure based on your specific window count and postcode, or browse our case studies to see comparable London and Home Counties projects with before-and-after photographs and measured acoustic data.

For specific local context — including which London areas and Home Counties towns we cover most frequently and the typical project specs in each — see our location pages. Every project we take on includes a manual survey verification before the final quote, so the figure you sign off on is the figure you pay.