Acoustic performance is not just a number of decibels. Depending on the terminology quoted after the dB value, the meaning will be different.

dB (Rw) - this is the weighted sound reduction index derived from a lab test of a particular building element. The higher this value, the better the performance. 

dB (DnTw) - This is the onsite tested performance of a building element which also takes into account flanking paths through the building structure that the element is connected too as well as other noise paths possible between the two rooms in question.

Often associated with these two forms of acoustic performance is the term Ctr which can be written as:

dB (Rw + Ctr) or dB (Dntw + Ctr)

Ctr is a correction factor defined for residential buildings separating walls or floors/ceilings. Ctr will vary per system and test.

 Siniat systems are typically quoted as either dB (Rw) or dB (Rw + Ctr) performance and can advise on appropriate systems to try and achieve the onsite requirements.