FAQ: Insertion Loss, Sound Insulation and Absorption Coefficients

What is insertion loss (DL)?

Insertion loss (DL in dB) is the reduction in sound pressure level at a receiver when a noise barrier is placed between source and receiver, compared with the bare-path scenario without the barrier. DL is a project-level performance metric because it depends on barrier height, source-receiver geometry, terrain profile and edge diffraction. It is the primary specification metric for any noise barrier project because it determines whether regulatory noise targets at receivers are met. DL is predicted using acoustic models such as CNOSSOS-EU or Harmonoise and validated in-situ to EN 1793-2.

What is sound insulation rating (Rw or DnT,w)?

Sound insulation rating (Rw in dB for buildings, or DL in the noise barrier context) is a laboratory-measured intrinsic property of a noise barrier panel. It quantifies how much airborne sound is prevented from passing through the panel itself, independent of project geometry. Rw is measured in a transmission suite with the panel mounted in a test opening between a source room and a receiving room. EN 1793-1 defines the equivalent test for noise barrier panels. A high Rw value means the panel itself is a good sound insulator. However, a high Rw alone is not sufficient because project-level insertion loss is often limited by edge diffraction rather than by transmission through the panel.

What is absorption coefficient (alpha-w)?

Absorption coefficient (alpha-w, a dimensionless value between 0 and 1) measures how much sound energy is absorbed by the panel face when sound impinges on it. An alpha-w of 0.0 means the panel reflects essentially all incident sound. An alpha-w of 1.0 means the panel absorbs essentially all incident sound. Absorption coefficient is important on the source-facing side of a noise barrier because absorption reduces the sound field reflected back toward the source and into adjacent receivers. EN 1793-5 classifies absorption from class A1 (low) through A4 (high). Yukings metal-louvered absorption panels typically achieve class A2 to A3 absorption.

How do DL, Rw and alpha-w work together?

On a typical highway noise barrier project, the noise barrier design relies on all three mechanisms working together. The panel itself must provide sufficient sound insulation (Rw/DL) to prevent sound from passing through the panel material. The barrier height and length must provide sufficient diffraction attenuation to achieve the target insertion loss (DL) at receivers. The source-facing face of the panel should provide sufficient absorption (alpha-w) to reduce reflection back toward the carriageway and to improve the net insertion loss. Yukings optimises all three parameters simultaneously when designing a noise barrier for a specific project.

Common misconceptions to avoid

Three misconceptions are particularly common. First, believing that a high Rw (good sound insulation) alone guarantees a good project-level insertion loss. It does not because edge diffraction around the barrier may dominate and limit the DL achievable on site. Second, specifying panels by depth alone without verifying both the Rw/DL and alpha-w values - the depth tells you little without the acoustic classification. Third, comparing performance values from different standards or from tests performed in different laboratories without normalising. Always request ILAC-accredited EN 1793 test reports from your noise barrier supplier.