Sound isolation between apartments is often poor, especially in older buildings; this results in complaints of noise intrusion from neighbors. The problem is avoided by modern building codes that specify allowable limits for noise insulation between adjacent dwellings. Apartments that meet or exceed the minimum standards of the building codes usually are free from routine noise intrusion complaints.
Field noise insulation tests are used to verify insulation quality for code compliance assessment of new or renovated buildings.  Existing buildings may be tested before renovation work to assist in design of partition upgrade features.
Airborne noise insulation test results are rated as Sound Transmission Class (STC). Effects of floor surfaces on impulsive noises such as footfalls are rated as Impact Insulation Class (IIC). IIC rating is directly related to STC rating for any assembly, except for the effect of floor surface hardness.
Numerous noise isolation test reports have been compiled by the California Department of Noise Control in order to provide a reference for partition selection. These are published in their STC-IIC Catalog.
The following construction detail shows features needed to provide ratings that meet or exceed the minimum allowable limits required by the building code. These include "floating" or resilient isolation of floor, ceiling and wall surfaces from the structure, along with sound absorbing fill materials in joist and stud cavity spaces. The figure also shows critical edge gap details that prevent direct contact between adjacent floating surfaces at corners.
The features shown above meet the code minimum requirements, but this does not assure resident satisfaction or absence of noise intrusion complaints.  Insulation that just meets the standards is in fact poor quality. Ratings higher than the code minima are recommended for good or very good quality of noise insulation. These can be achieved by adding features such as lightweight concrete sub-floor topping, additional gypsum board panels attached under the sub-floor, viscoelastic damping compound between the multiple gypsum board layers, or independent framing for both faces of a partition.