In multifamily building construction you need to insulate the dwelling units for sound. When using the International Building Code all common interior walls, partitions and floor/ceiling assemblies between adjacent units and between units and hallways, stairs and service areas need to be insulated. There are two types of sound that need to be addressed, the first is sound transmitted through the air and the second is sound transmitted through the building structure.
Sound Transmission Class (STC) is a rating of how well a building partition attenuates airborne sound. The STC number is roughly the reduction in decibels that a partition creates in the 125 Hz to 4,000 Hz range. Walls and floor/ceiling assemblies need to be insulated for airborne sound to achieve a STC of 50 or more. Typically you can find the sound rating for assemblies in the same location that you found the fire rating for the assembly. You can also contact manufacturers for more information on assemblies that use their products. If you choose to create your own assembly and have it tested, it must have a minimum STC of 45 after it is tested. Where you have openings and penetrations in the assembly they must be adequately insulated to maintain the STC with the only exception being entry doors.
Impact Insulation Class (IIC) is a rating of how well a floor/ceiling assembly attenuates structure borne sound. The IIC number is roughly the reduction in decibels that a partition creates in the 100 Hz to 3,150 Hz range when tapped by the testing machine. The easiest way to achieve the required IIC rating of 50 is to use carpet and underlayment as your flooring which can give you 20+ points. There are also a number of underlayment options for harder surfaces. You can also have the assembly field tested if you don’t have an official rating and again you need to achieve a 45.
Now on the subject of field testing vs using an established rating, it is my opinion that field testing should only be chosen if the particular assembly is necessary to the building design or will create serious savings. Otherwise it isn’t worth the brain damage and testing costs.