Wave Dynamics GuidesRw, DnT,w and L′nT,w Explained
What the common sound insulation ratings mean, why laboratory Rw values differ from on-site DnT,w results and how to read a test report.
In Short
Rw is a laboratory rating for how well a product or construction blocks airborne sound. DnT,w is the airborne rating measured on site between two rooms, and L′nT,w is the on-site impact sound rating for floors. For airborne ratings higher is better; for impact ratings lower is better.
- Rw: laboratory airborne rating of a product or construction.
- DnT,w: on-site airborne rating, standardised to a reference reverberation time.
- L′nT,w: on-site impact rating, where a lower number means a quieter floor.
- Expect site results to be several decibels below laboratory values.
Laboratory Ratings: Rw and Ln,w
Manufacturers test walls, floors, doors and glazing in a laboratory where flanking paths are suppressed. The airborne result is the weighted sound reduction index, Rw, and the impact result for floors is Ln,w. These values are useful for comparing products, but they describe the element on its own, not the finished building.
On-Site Ratings: DnT,w and L′nT,w
On site, sound travels through the separating element and every connected path around it. The airborne result, DnT,w, is the level difference between two rooms adjusted to a standard reverberation time of 0.5 seconds, so a furnished and an empty room give comparable results. The impact result, L′nT,w, is the noise level in the room below when a standard tapping machine runs on the floor above, again standardised. The prime symbol shows the result includes flanking.
Spectrum Adaptation Terms: C and Ctr
ISO 717 also defines adaptation terms. C adjusts for mid to high-frequency noise such as speech, and Ctr for low-frequency noise such as traffic and bass music. Approved Document E in England and Wales uses DnT,w + Ctr, which is why results there look lower than Irish results for the same wall.
Reading a Test Report
- Check the rating used matches the guidance for that jurisdiction.
- Look for the frequency curve, not just the single number, to see where a construction is weak.
- Confirm the room volumes and partition area are stated.
- Check background noise was measured and corrected for.
Frequently Asked Questions
Why is the site result lower than the product Rw?
Because site results include flanking, junctions, services and workmanship. A drop of several decibels between laboratory and site is normal.
What is a good DnT,w?
In Ireland, 53 dB is the minimum for separating walls and floors between dwellings. Higher values give better privacy, and higher-specification developments often target more.
This guide gives general information and is not a substitute for project-specific advice. Requirements change, so always check the current regulations, guidance and your planning conditions. Ask our team about your project.
Jon Maguire (BSc Hons MIOA) brings over 8 years of expertise in acoustic consultancy, specialising in environmental acoustics with a focus on 3D modelling, assessments, and detailed design for industrial, commercial, and energy projects. He has lead noise assessments for key clients while thriving in multidisciplinary environments. Jon’s diplomatic communication skills are ideal for stakeholder consultation.
Jack is a Field Engineer at Wave Dynamics, supporting the delivery of environmental monitoring and acoustic consultancy services through detailed on-site fieldwork. He has hands-on experience in noise monitoring, vibration monitoring, and dust monitoring across a variety of infrastructure, construction, and environmental projects. Jack is involved in the setup, operation, and maintenance of monitoring equipment, ensuring accurate data collection in line with relevant standards and project requirements. He works closely with consultants and project teams, providing reliable field support that underpins high-quality assessments and reporting.
Aidan is the Director for all U.K Operations and has worked in multiple industries including for clients in the UK and internationally operating in the residential, industry, healthcare, education, and construction sectors.
