Measures Relating to Sound Blocking (Transfer) Between Closed Spaces

Ceiling Attenuation Class (CAC)—A measure for rating the performance of a ceiling system as a barrier to airborne sound transmission through a common plenum between adjacent closed spaces such as offices. A ceiling system with a CAC ≤ 25 is considered low performance, whereas one with CAC ≥ 35 is high performance. CAC is measured according to ASTM E1414.

Sound Transmission Class (STC)—STC is the wall equivalent of CAC and is a measure for rating the performance of a wall system as a barrier to airborne sound transmission between closed spaces such as closed offices, corridors, and conference rooms, and even in open offices with furniture and dividers, etc. An STC ≤ 35 is considered low performance, while an STC ≥ 55 is considered high performance. STC is measured according to ASTM E90.

Articulation Index (AI)—The AI is a measure of the intelligibility of speech in the presence of background noise, and is related to the signal-to-noise ratio, S/N. The AI ranges from 1.0 which means that the speech is highly intelligible, to 0.0 meaning that speech cannot be understood. Speech Privacy (PI) is calculated from the AI to indicate the level of speech privacy according to the following: PI = (1-AI) * 100%, and is generally rated as PI > 95% is confidential, and PI = 80 to 95% is non-intrusive.

These measurements will inform architects as they contemplate and calculate the acoustic qualities needed for various environments. We'll begin with offices.