Sound Reduction Index

The weighted sound reduction index is derived with the help from a standardized curve In the frequency range 100 – 3150 Hz. The reference curve comes from the International Standard 717-1.

The reference curve is shifted up or Down on The Sound Reduction (transmission loss) curve with increments of 1 dB until the unfavourable deviations (deviations under the reference curve) is as large as possible but not exceeding the total of 32 dB.

A rough sketch is shown below with the general idea, the reference curve (shown In red Line) is shifted on the Sound Reduction ‘R’ curve (shown In Black Line) until the total deviations is as large as possible but not exceeding 32 dB. The deviations are shown In the red marked area.

The value of the shifted reference curve at 500 Hz is taken as the single number rating. This single number rating is usually what building regulation Will refer to when setting minimum requirements.

Flanking Sound Transmission Paths

Just remember, you will rarely be measuring only the separating building element, you will most likely also be measuring all there flanking transmission paths as well!

Acoustic flanking transmission refers to the indirect path through which sound travels between different spaces or structures, bypassing conventional soundproofing measures.
Understanding flanking transmission is important for effective acoustic design and noise control in buildings and other structures.

Flanking transmission of airborne sound energy is an issue that can occur with stiff acoustic coupling of separations such as walls, floors and facades. The dominant sound transmission path is amplified with all the other flanking paths and in turn, increases the sound transmission to the adjacent room.