What Is ΔLw (Delta Lw)? Acoustic Underlay Ratings Explained
Written by our acoustic insulation specialist — 15+ years experience supplying soundproofing to UK homeowners, developers and contractors. About our experts.
If you've been comparing acoustic underlay products, you've almost certainly encountered the ΔLw rating — sometimes written as Delta Lw or ΔLw. It's the standard measure of acoustic underlay performance, yet it's rarely explained clearly on product pages. This guide gives you a complete, technically accurate explanation — so you can compare products confidently and specify the right underlay for your project.
What Does ΔLw Stand For?
ΔLw stands for Weighted Impact Sound Improvement Index. The symbols break down as:
- Δ (Delta) — Greek letter meaning "change" or "difference"
- L — level (in this context, sound level)
- w — weighted (a frequency-weighted single-figure value)
In simple terms: ΔLw tells you how much an acoustic underlay improves impact sound insulation compared to a bare concrete reference floor. It's an improvement value — not an absolute measurement of the floor's final acoustic performance.
How Is ΔLw Measured?
The test is standardised under EN ISO 10140-3 (formerly EN ISO 140-8). In the laboratory:
- A bare concrete reference slab (150mm dense concrete) is measured for impact sound level using a standardised tapping machine
- The acoustic underlay is laid on the slab and the floor finish is added
- The system is re-measured
- The reduction across frequencies is calculated and weighted to produce a single ΔLw figure in decibels (dB)
The higher the ΔLw figure, the more impact noise the underlay reduces relative to the bare slab.
Our Product ΔLw Ratings Explained
- 3mm Recycled Rubber: 54dB ΔLw — solid entry-level performance; 20dB above Part E minimums
- 6mm Recycled Rubber Pro: 74dB ΔLw — industry-leading; 40dB above Part E minimums
- AcoustiLay 8 System: 30dB + 58dB — 30dB impact, 58dB airborne (different metrics, see below)
ΔLw vs DnT,w vs L'nT,w — What's the Difference?
This is where acoustic specification gets confusing. Here's a clear breakdown:
| Rating | What It Measures | Higher = Better? | Used For |
|---|---|---|---|
| ΔLw | Improvement in impact insulation (lab test, underlay) | ✅ Yes | Comparing underlays |
| L'nT,w | Actual impact sound level in a building (field test) | ❌ No (lower = better) | Building Regs Part E compliance |
| DnT,w | Airborne sound insulation in a building (field test) | ✅ Yes | Building Regs Part E compliance |
| Rw | Weighted sound reduction (lab, for products) | ✅ Yes | Comparing walls, floors, windows |
Part E Requirements in Numbers
UK Building Regulations Part E requires separating floors between dwellings to achieve:
- L'nT,w ≤ 62dB (impact sound — lower is better, this is the maximum permitted)
- DnT,w + Ctr ≥ 45dB (airborne sound — higher is better, this is the minimum required)
Note that ΔLw (underlay product rating) and L'nT,w (building field test result) are different metrics. A high ΔLw underlay contributes to achieving a low L'nT,w in the building — but the final field test result depends on the whole floor system, including the subfloor construction and any flanking paths.
Why ΔLw Numbers Look Large
You might wonder how an underlay can achieve "74dB" when Part E only requires L'nT,w ≤ 62dB. The answer is that ΔLw and L'nT,w are different measurements on different reference baselines. ΔLw is measured against a bare 150mm concrete slab — an artificially poor starting point. The final L'nT,w in a real building will be different (and will be higher or lower depending on the complete floor system).
The key point: a higher ΔLw underlay always performs better than a lower ΔLw underlay — the relative comparison is valid even though the absolute numbers don't directly translate to field test results.
Ctr — What Is the Spectrum Adaptation Term?
The "Ctr" in "DnT,w + Ctr ≥ 45dB" is a correction factor for low-frequency noise (traffic, bass music). It's always a negative number (e.g., Ctr = -5 means the effective value is 5dB less than the Rw figure alone). Building Regs require the combined DnT,w + Ctr to meet the threshold — not just the Rw or DnT,w alone.
FAQs
Is a 74dB ΔLw underlay 20dB better than 54dB?
Yes — in acoustic terms, 20dB more is very significant. In human perception, 10dB is roughly twice as loud, so 20dB is roughly 4× the perceived loudness difference. The 6mm Rubber Pro (74dB) will produce a dramatically quieter result than the 3mm (54dB) for serious noise problems.
Do I need an acoustic test certificate to comply with Part E?
Yes — for notifiable works (new builds, conversions, material changes of use), pre-completion acoustic testing by a UKAS-accredited firm is required. Product data sheets showing ΔLw values are useful for specification but do not replace in-situ field testing.
What is BB93 and is it relevant to acoustic underlay?
BB93 (Building Bulletin 93) is the UK acoustic standard for schools, published by the Department for Education. It specifies acoustic performance criteria for different school spaces. Acoustic underlay can contribute to BB93 compliance in school floor installations, particularly in corridors and activity spaces above classrooms.
What is BS 8233 and how does it relate?
BS 8233 is the British Standard for sound insulation and noise reduction in buildings — it sets recommended indoor ambient noise levels for different building types (homes, offices, schools). It complements Part E (which sets minimum isolation between dwellings) by providing guidance on the overall acoustic environment. Acoustic underlay contributes to achieving BS 8233 targets by reducing impact noise levels in rooms below.