Adaptive indicator (C and Ctr)

Professional Definition

According to the standard PN-EN ISO 717-1, the spectral adaptation index is a value added to a single-number weighted index (e.g., Rw), aimed at accounting for the spectral characteristics of a specific external noise. This is a necessary correction, as building materials isolate low and high sounds differently.

The spectral adaptation index (C or Ctr) – a value determined according to PN-EN ISO 717-1, which is added to the weighted index to account for the spectral characteristics of noise in the assessment of sound insulation.

There are two main types of this index:

C Index – used when the noise source is predominantly mid and high-frequency sounds (e.g., conversations, TV, fast train movement, children's play).

Ctr Index – used when low frequencies dominate (e.g., urban traffic noise, slow train movement, disco music with prominent bass). The abbreviation “tr” comes from the word traffic.

Acoustics in Simple Words

Imagine that the acoustic insulation of your wall is a shield. The value Rw tells us how thick this shield is on average. However, the attacks (noise) can vary. C Index represents an attack with light blows (conversation, shouting) – the shield usually copes with them as the standard predicts. On the other hand, Ctr Index represents a heavy hammer strike (low tones, rumble of a truck engine).

Most walls and windows perform worse against “heavy hammers” (low frequencies). Therefore, the Ctr index is usually a negative value. If your window has Rw = 40 dB and Ctr = -5 dB, it means that for street noise, real protection is only 35 dB. It’s like buying a jacket for the frost (Rw), but the adaptation index tells you how much colder you will feel when a strong wind blows (Ctr).

Summary

The spectral adaptation indices C and Ctr are key corrective parameters that allow for a realistic assessment of the effectiveness of a partition. The C index refers to residential noise (“pink”), while Ctr is critical in designing buildings near busy streets where difficult-to-explain low frequencies dominate.