For many instances of road traffic noise, the value of Rw + Ctr for the glass can be considered as the most relevant.  

Rw is the weighted sound reduction, in decibels (dB), incorporating a correction for the ear’s response. Ctr is an adjustment that is added to Rw to take account of characteristics of particular sound spectra, such as urban road traffic.

If a glass had a sound insulation quantified as Rw (C, Ctr) = 41 (-2, -4) dB, then its performance (i.e. by how much the noise would be reduced) against typical urban road traffic would be Rw + Ctr = 41 – 4 = 37 dB.

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