Understanding NIC and STC in Acoustics

In architectural acoustics, achieving optimal sound isolation is crucial for creating comfortable and functional spaces. Two primary metrics used to assess sound isolation are Sound Transmission Class (STC) and Noise Isolation Class (NIC). While both ratings aim to quantify a partition’s ability to block airborne sound, they differ in their measurement methods and applications.

STC is a laboratory-measured rating that evaluates a partition’s ability to attenuate airborne sound under controlled conditions. According to ASTM E90, STC is determined by measuring the sound reduction across various frequencies and calculating a single-number rating noise isolation class that represents the partition’s overall performance. This method provides a standardized comparison of different materials and assemblies. However, STC does not account for real-world variables such as flanking paths, room absorption, or installation quality, which can significantly impact actual performance.

In contrast, NIC is a field-measured rating that assesses the actual sound isolation between two spaces after construction. ASTM E336 outlines the NIC measurement process, which involves measuring the sound level difference between two adjacent rooms under typical usage conditions. This approach captures the effects of real-world factors, including flanking noise, room finishes, and construction quality. As a result, NIC values often differ from STC ratings, typically being 3 to 7 points lower, reflecting the influence of these variables.

While STC provides a useful benchmark during the design phase, NIC offers a more accurate representation of a partition’s performance in the finished building. Therefore, it is advisable to use STC ratings for initial design and specification purposes, and to conduct NIC testing after construction to verify that the acoustic goals have been met.

In summary, both STC and NIC are valuable tools in the field of architectural acoustics. Understanding their differences and appropriate applications enables architects and engineers to design spaces that meet desired acoustic standards and provide occupants with the intended level of sound isolation.