Sound transmission reduction relies on two main principles:
Sound Absorption - Removing sound energy from within a room by using materials such as acoustic tiles or open cell foam to soak up sound.
Sound Insulation - Preventing the transmission of sound waves by introducing a barrier. Examples include brick, concrete, metal, heavy doors, etc.
Sound waves will travel the path of least resistance. A high performing door will not compensate for less performing materials in the walls around it. In order to reduce the transmission of sound, materials are added between the source room and the receiving room. The resulting change in decibel level is the sound transmission loss and is given a Sound Transmission Class (STC) rating. The higher the rating number, the lower the transmission of sound. The STC rating is roughly the decibel reduction a partition can provide. For instance, if a 100 dB noise is reduced to 55 dB, that product roughly has a STC 45 rating.