NFPA 72 requires signals to have an intelligibility level not less than ? on the STI.

• A. 0.5
• B. 0.7
• C. 0.9
• D. 1.0

Answer: (B)

Intelligibility of audible alarm signals is assessed with the Speech Transmission Index (STI). STI is a 0 to 1 measure of how well a listener can understand speech through a channel, with higher values indicating clearer understanding despite room noise and acoustical effects. NFPA 72 sets a minimum requirement of 0.70 on the STI for alarm signals, meaning the message should be understood at least 70% of the time at the typical listening locations in the protected space. This threshold strikes a practical balance: it ensures sufficient clarity for occupants to grasp the instructions quickly in real-world conditions, while remaining achievable through proper speaker design, placement, and signal optimization. If STI falls below 0.70, understanding can drop, increasing the risk during an emergency. The other values are either too low to guarantee reliable comprehension or more stringent than necessary for standard installations.