The indicator of sound insulation from airborne sound that takes into account the adaptive index Ctr

Professional Definition

The sum of the weighted index of the appropriate sound insulation parameter against airborne sound and the spectral adaptive index Ctr. NOTE – the index refers to any parameter defining sound insulation against airborne sound and is marked as ‘A2’ in the symbol index of the given parameter.

In engineering practice, this parameter is most commonly encountered in the form of the sum Rw + Ctr (or RA2). The standard index Rw (weighted sound insulation index) is optimized for human speech sounds and mid frequencies. The spectral adaptive index Ctr is a negative value that adjusts this result downward, showing the actual effectiveness of the partition against noise with a predominance of low tones, such as street traffic, railway noise, or club music.

Acoustics in Simple Words

Imagine that the sound insulation of your window is a shield. The standard index Rw tells us how well the shield protects against arrows (neighbors' conversations, high sounds). However, street noise – the roar of buses and trucks' engines – is not arrows but heavy cannonballs. A shield that works great against arrows may crack under the pressure of a cannonball.

The index taking into account Ctr provides information on how the shield performs against those “cannonballs.” If the window manufacturer states Rw = 40 dB, but Ctr is -5 dB, then the actual protection against street noise is only 35 dB. This is a key difference that determines the comfort of living in the city.

Summary

This index is essential when designing building facades and windows in cities. It allows for a realistic assessment of the acoustic insulation of the partition under conditions of low-frequency noise predominance, preventing the overestimation of the protective properties of materials.