L'nT,w (Standardized Impact Sound Level)

L'nT,w is the weighted standardized impact sound pressure level, the primary field measurement parameter for impact sound insulation of floor assemblies. Defined in ISO 717-2:2020 and measured per ISO 16283-2, it represents the sound level in the room below a floor excited by a standardized tapping machine, corrected for the receiving room's reverberation time. Unlike IIC, lower L'nT,w values indicate better performance (less impact noise transmitted). The prime symbol (') indicates that flanking transmission is included. Building regulations specify maximum L'nT,w values: UK Building Regulations require L'nT,w ≤ 62 dB between dwellings, with 56 dB or lower for enhanced performance. Typical values range from 75 dB (bare concrete) to 40 dB (high-performance floating floor). Impact sound insulation is improved by adding soft floor coverings (carpet), floating screed systems, resilient interlayers, or suspended ceilings. L'nT,w is the key metric for resolving impact noise complaints in multi-storey residential buildings.