There are essentially two categories of sound:
As the name implies, airborne sound travels through the air often starting from a point sources such as TV, Radio, person(s) talking
Impact noise travels through the building structure. It is generated by vibrations and frequently creates noise in nearby rooms or hallways. Typically this type of noise comes from hitting or impacting the structure directly. For example, footsteps on the floor, vibrations from plumbing, moving furniture etc
Sound Transmission Class (STC)
The noise perfomance of a building is referred to as the Sound Transmission Class
The Sound Transmission Class is a single number rating of the effectiveness of a material or construction assembly to retard the transmission of airborne sound. The sound transmission loss between the source and receiving rooms are plotted on a graph by frequency and sound level in decibels. The STC curve is a sliding contour that is fitted to the performance data plotted in a manner that will allow no more than 32 deficiencies below the appropriate contour. The maximum deficiency at any given frequency shall not exceed 8 decibels.
The higher the STC, the better the structure is at isolating airborne noise. An STC rating of 50 means that the sound passing through the building is reduced by 50dB.
Building Code of Australia (BCA) Requirements
the BCA specifies the minimum STC wall and floor requirements between adjoining dwellings but uses a sound reduction index (Rw), which is equibavalent to STC.
The BCA requires a Rw for floors above dwelling and walls bewteen a bathroom, laundry , kitchen or a habitable room in adjoing dwellings of 50 while otherwall, 45.
The BCA specifies only the minimum requirements. In 2005 the guide was updated because occupant expectations were not being met. This guide should be treated as a starting point and is not really appropriated for higher end, quality developments.