Rw (weighted sound reduction index)

“Rw” is calculated from a wall’s sound transmission losses over 16 frequencies. It is measured in decibels (dB) as a type of average of low to high frequencies.

Low frequency sound waves (E.g. from a bass guitar) are large and intrusive. Their sound transmission loss is below Rw (average). At the other end of the scale, high frequency sound waves (E.g. violin, piccolo) are short wave, less intrusive and their sound transmission loss is above average.

A typical sound rating is expressed as Rw 55 (-1; -5). The first figure in the brackets is “C” which indicates irregular performance in the high frequencies. It is not addressed in the BCA. The second figure in the brackets is “Ctr” and is essential in BCA requirements for party walls.

Ctr (low frequency spectrum adaptor)

“Ctr” is an indicator of how much lower the wall’s performance would be if the noise source was mainly low frequency. Ctr is a negative number. The sound rating above of Rw 55 dB with a Ctr of -5 gives Rw + Ctr = 50.