Final answer:
The NFPA 72 requires signals to have an intelligibility level not less than 0.45 on the STI scale. The STI measures the effectiveness of speech transmission and assesses the quality and intelligibility of speech in different environments.
Step-by-step explanation:
The NFPA 72 requires signals to have an intelligibility level not less than 0.45 on the STI (Speech Transmission Index) scale. The STI is a measure of the effectiveness of the transmission of speech in a given environment. It is designed to assess the quality and intelligibility of speech in various settings. For example, if a signal has an STI value of 0.45, it means that 45% of the speech is intelligible in that environment. Higher STI values indicate better speech intelligibility.