walls where caulking has not been applied effectively Partitions that are inadequately sealed and contain back-to-back electrical boxes, untreated recessed lighting and unsealed pipes offer flanking paths for sound and significant leakage. Acoustic joint tapes and caulking have been used to improve sound isolation since the early 1930s. Although the applications of tapes was largely limited to defense and industrial applications such as naval vessels and aircraft in the past, recent research has proven the effectiveness of sealing gaps and thereby improving the sound isolation performance of a partition. Flanking Building codes typically allow for a 5-point tolerance between the lab-tested and field-measured STC rating; however, studies have shown that even in well-built and sealed installations the difference between the lab and field rating is highly dependent on the type of assembly. Special variations of STC By nature, the STC rating is derived from lab testing under ideal conditions. There are other versions of the STC rating to account for real-world conditions. Composite STC The net sound isolation performance of a partition containing multiple sound isolating elements such as doors, windows, etc. Apparent Sound Transmission Class (ASTC) The sound isolation performance of a partition measured in the field according to ASTM E336, normalized to account for different room finishes and the area of the tested partition (i.e. compare the same wall measured in a bare living room and an acoustically dry recording booth). Normalized Noise Isolation Class (NNIC) The sound isolation performance of a partition measured in the field according to ASTM E336, normalized to account for the reverberation time in the room. Noise Isolation Class (NIC) The sound isolation performance of a partition measured in the field according to ASTM E336, not normalized to the room conditions of the test. Field Sound Transmission Class (FSTC) The sound isolation performance of a specific elements in a partition, as measured in the field and achieved by suppressing the effects of sound flanking paths. This can be useful for measuring walls with doors, when you are interested in removing the influence of the door on the measured field STC. The FSTC testing method was historically prescribed by ASTM E336, however the latest version of this standard does not include FSTC. Door Sound Transmission Class (DTC) The sound isolation performance of doors when measured according to ASTM E2964. Legal and practical requirements Section 1206 of International Building Code 2021 states that separation between