Technical Guide to Acoustic Underlay — Performance Data & Specifications
This technical guide explains how acoustic underlay performance is measured, how to read acoustic data sheets, and how to specify the right product for Building Regulations Part E compliance. Written by our acoustic insulation specialist with 15+ years of UK industry experience.
How Acoustic Performance Is Measured
ISO 140 Laboratory Testing
Acoustic underlay performance is measured in ISO-accredited laboratories in accordance with BS EN ISO 10140. The test involves installing the underlay in a standardised floor construction and measuring sound transmission using calibrated equipment. Results are expressed as a single-number weighted value.
Key Metrics Explained
| Metric | What It Measures | Higher/Lower = Better? | UK Regulation Target |
|---|---|---|---|
| ΔLw (Delta Lw) | Impact sound improvement index | Higher = better | System must achieve L'nT,w ≤ 62dB |
| Rw | Airborne sound reduction (lab) | Higher = better | System must achieve DnT,w ≥ 45dB |
| DnT,w | Airborne sound (field test) | Higher = better | ≥ 45dB (new build), ≥ 43dB (conversion) |
| L'nT,w | Impact sound (field test) | Lower = better | ≤ 62dB (new build), ≤ 64dB (conversion) |
Product Performance Data
| Product | Thickness | ΔLw | Best Application |
|---|---|---|---|
| 6mm Recycled Rubber | 6mm | Up to 74dB | Flats, Part E, all hard floors |
| 12mm Acoustic System | 12mm | 30dB impact + 58dB airborne | Full Part E compliance |
| 3mm Recycled Rubber | 3mm | Up to 54dB | Budget, low-rise, minor improvement |
| SilentMat 15mm | 15mm | Variable | Heavy use, commercial, gyms |
Understanding ΔLw (Delta Lw)
ΔLw is the single most important number when comparing acoustic underlays for impact noise. It represents the improvement the product adds to a standard floor construction — the higher the number, the better.
Important: ΔLw is a product rating, not a system rating. The actual in-situ impact noise level (L'nT,w) of a completed floor depends on the whole assembly — substrate, underlay, floating platform, and ceiling treatment below. Part E compliance is assessed against L'nT,w, not ΔLw.
Part E Compliance — Specification Guide
New Build Separating Floor (Concrete)
- Structural concrete slab (minimum 300mm reinforced)
- Acoustic resilient layer (ΔLw ≥ 17dB minimum) — our 6mm or 12mm system
- Floating screed (65mm sand:cement) or floating chipboard platform
- Perimeter isolation strip at all wall junctions
Conversion Separating Floor (Timber Joist)
- Acoustic resilient layer above joists
- Floating chipboard platform
- 100mm acoustic mineral wool in joist void
- Independent ceiling below (resilient bars + 2 layers plasterboard)
Specification Checklist for Specifiers
- ☐ Identify floor type (concrete slab / timber joist / screed)
- ☐ Identify performance target (Part E new build / conversion / general improvement)
- ☐ Identify noise type (impact / airborne / both)
- ☐ Select underlay with appropriate ΔLw for target
- ☐ Check UFH compatibility if applicable
- ☐ Specify perimeter isolation strips
- ☐ Specify acoustic sealant at all junctions
- ☐ Arrange pre-completion testing if required (Part E)
Need a full specification? Contact our technical team for a free consultation. Browse our full acoustic insulation range.