Your IIC Rating, or Impact Insulation Class, is a major factor in how much sound tenants will hear directly below a floor-ceiling assembly. The rating is determined by using a tapping machine on an existing assembly, and measuring the amount of sound that comes through below. This test measures structure-borne, or vibrational sound – the more sound detected below, the worse the assembly. While Concrete Floors are usually much better than Wood Joist for STC ratings, they do not always perform much better for structure-borne noise; the assembly should be reviewed on a case-by-case basis.
Often-times, soft flooring such as carpet is much more effective at blocking structure-borne noise (such as footfall) than hard flooring, such as tile. However, there are a number of other variables, including the ceiling system below, adequate use of acoustic underlayments, and workmanship in the floor system itself.
IIC TESTING: The IIC value is determined by ASTM E492 (IIC), and field tested by ASTM E1007 (AIIC). While the lab test is conducted under more controlled conditions, both tests may provide value in determining how much structural sound you may expect to attenuate. However, note that sub-gypcrete underlayments may perform much worse in the field if proper edge isolation is not used.
Target IICs by Building Type
In addition to performing field testing and ensuring proper installation and isolation of sound mats, the other critical component is to first design a structure that will have an acceptable Acoustical Factor of Safety in regards to IIC. Common IIC Targets may be found below, in accordance with LEED v4 and Architectural Acoustics by David M Egan, a leading acoustic specialist:
Apartments: 50+ (meets Building Code)
Condos: 50-60+ (depending on level of luxury)
Schools: 40-45 (for ancillary and core learning spaces, respectively)
Expected IIC of Your Assembly Design
IIC Ratings are very difficult to model, and therefore, you should strongly consider reviewing relevant lab test data. This is available from the Canadian Noise Reduction Council (CNRC) and California Department of Health Services. Reputable underlayment suppliers will also have lab- and field-test data available upon request.