Rw is the weighted sound reduction index in dB (decibels) and it describes the airborne sound insulating power of a building element. It is a laboratory-measured value as defined in ISO717 Part 1. It can apply to walls, ceiling/ floors, ceiling/roofs, doors, or windows. The higher the number, the greater the sound insulating power of the building element. It is measured over the frequency range 100 to 3150Hz
An increase in the Rw of a wall by 6 points will reduce the perceived loudness of sound passing through the wall by about half. Detailed below is how the sound insulating effectiveness of walls depends on their Rw (or Rw + Ctr values).