What is a Weighted Sound Reduction Index (Rw) and how is it determined?

The Weighted Sound Reduction Index (Rw) is a single-number rating used to describe how effectively a building element reduces airborne sound transmission. It is derived from laboratory testing and represents the overall sound insulation performance of a product across a range of frequencies.

Rw is determined by measuring the sound reduction of a system in controlled test conditions, typically in accordance with standards such as BS EN ISO 10140. During testing, sound is generated in one chamber and measured in an adjacent chamber separated by the test specimen. The difference in sound level between the two spaces—corrected for reverberation and other factors—produces a frequency-based sound reduction curve.

This curve is then compared against a reference curve defined within the standard. The position of the reference curve is adjusted until it best fits the measured data, resulting in the final Rw value, expressed in decibels (dB).

It is important to understand that Rw is a single-number summary, and while useful for comparison, it does not show how performance varies across different frequencies. For plant noise applications, low-frequency performance is often critical, and therefore octave band data should always be reviewed alongside the Rw value.

In the context of acoustic louvres and acoustic cladding systems, Rw values are typically more relevant to solid panels or enclosure systems. Ventilation louvres are more commonly assessed using insertion loss data, as airflow must pass through the system.

At Solinear, our SoundEX acoustic systems are supported by detailed laboratory data, including octave band performance, allowing consultants to model real-world noise reduction accurately rather than relying solely on single-number ratings.