4.1 The purpose of the LIR rating is to describe the low-frequency impact noise, often referred to as "thudding," primarily generated by footfall on lightweight structures. The rating increases as the impact sound attenuation of the floor-ceiling structure increases. This rating has been shown to correlate with subjective reaction3^,4 and can be used to evaluate and compare the low-frequency impact noise performance of assemblies.

4.2 The LIIC rating can be used for relative comparison and to rank-order the low-frequency performance of assemblies within a given test laboratory.5 The reproducibility of LIIC between laboratories has not been measured, and the relationship between laboratory and field ratings (LIIC and LIR) is not assumed or established.

Note 1: The tendency for measurement uncertainty to increase at low frequencies should be considered when utilizing metrics of classification standard.

Note 2: This classification does not replace Classification E989 (Impact Insulation Class) and is not interchangeable with it. For example, the LIIC rating of an assembly does not determine its IIC rating and cannot be used to show compliance with an IIC requirement. The expectation is that the low-frequency ratings would be reported alongside the existing ratings.

4.3 This classification shall only be used with one-third-octave-band data.