Rw (Weighted Sound Reduction Index)
Rw describes the airobrne sound insulating power of a building element. It is a laboratory measured value that can apply to walls, ceilings/floors, ceilings/roofs, doors or windows. The higher the number, the greater the sound insulating power of the building element.
Rw+Ctr (Rw Plus Spectrum Adaptation Term)
Rw+Ctr is equal to Rw with the addition of a low frequency sound correction, Ctr. The us of Rw + Ctr has been adopted due to the increase in low frequency sound sources such as surround sound systems, traffic and aircraft noise, drums and bass guitars… Two walls can have the same Rw rating but have different resistance to low frequency sound.
DnTw and DnTw+Ctr (Measured On Site)
These values are the equivalent of Rw and Rw+Ctr, but are measured on site. An on site measured value is permitted to be 5 points lower.
Above are the basic terms and definitions that are commonly used to describe abilities of different walls in terms of acoustic insulation. There are quite many more terms and definitions but they are not particularly relevant for this particular topic.