- Ensuring Clear Communication: The Role of Speech Intelligibility in Life Safety
In emergency situations, every second counts. Whether it’s a fire, a hazardous material event, or another life-threatening scenario, people rely on voice communication systems to guide them to safety. If those messages aren’t clear, understood, and actionable, the consequences can be catastrophic. That’s why speech intelligibility—the measure of how clearly speech is understood in a space—is an essential component of modern fire alarm and emergency communication systems.
Despite its importance, speech intelligibility is often overlooked during inspections and system approvals. Enforcing regular intelligibility testing isn’t just a technical requirement; it’s a public safety imperative. Backed by NFPA codes and supported by reliable tools like Bedrock’s measurement systems, speech intelligibility testing should be a standard requirement in every fire protection and life safety evaluation.
What Is Speech Intelligibility?
Speech intelligibility refers to the clarity and comprehensibility of spoken messages in an environment. It determines how well occupants can understand emergency announcements over a fire alarm or public address system. It’s not just about whether a message is audible—it’s about whether it’s understood.
Many factors affect intelligibility, including background noise, reverberation, speaker placement, and system quality. In an emergency, muffled, distorted, or unintelligible messages can delay evacuations, increase panic, and lead to injuries or worse.
Why It Must Be Measured
- NFPA Compliance
Speech intelligibility is addressed in NFPA 72, 2025 edition, Annex D, which outlines standardized methods and performance benchmarks to ensure voice communication systems are effective during emergencies. It provides the industry-recognized framework for how speech clarity should be measured and validated in fire alarm and emergency communication systems.
"The intelligibility of an emergency communication system is considered acceptable if at least 90 percent of the measurement locations within each ADS have a measured STI of not less than 0.45 (0.65 CIS) and an average STI of not less than 0.50 STI (0.70 CIS)."
This guidance uses the Speech Transmission Index (STI), a standardized measurement ranging from 0 to 1, where values above 0.5 are considered suitable for life safety communication. The criteria in Annex D are based on international standards and widely used by system designers, inspectors, and technicians to ensure voice messages are both audible and understandable.
While Common Intelligibility Scale (CIS) may still be referenced, STI is the preferred and more current method for measuring speech clarity in today’s testing environments.
2. Life Safety
Clear voice messages help people evacuate faster, follow instructions accurately, and avoid hazardous areas. Confusing or inaudible messages increase risk, delay egress, and may lead to dangerous misinterpretations. Ensuring that systems are intelligible saves lives, especially in large buildings, high-rises, airports, schools, hospitals, and public venues.
3. System Validation
Speakers degrade, acoustic conditions change, and systems evolve over time. Renovations, added partitions, and new furnishings can all affect speech clarity. Measuring intelligibility ensures that what worked during commissioning still works today. It’s a critical part of system maintenance and lifecycle safety.
4. Reduced Liability
When speech communication fails during an emergency, investigations often reveal that testing wasn’t thorough—or wasn’t done at all. Intelligibility testing provides documentation that a system meets required standards. In the event of an incident, this becomes vital in demonstrating due diligence and reducing legal exposure.
How to Measure Speech Intelligibility
SDi offers solutions from Bedrock Audio that make speech intelligibility testing simple, accurate, and code-compliant. The Bedrock AM100 is a high-end Class 1 analyzer built for professionals who need precision, wireless control, and integrated signal generation—all in a rugged, portable package. For straightforward and efficient field use, the SM50 provides an easy-to-use touchscreen interface, bundled with essential features like STIPA and full STI testing, real-time analysis, and a protective hard case. Paired with either analyzer, the BTB65 Talkbox delivers standardized STIPA signals at calibrated levels to simulate human speech, making it easy to conduct repeatable, valid tests across any voice evacuation system. These solutions are designed to streamline intelligibility testing without sacrificing accuracy or compliance.
Benefits of Using Bedrock for Intelligibility Testing
- Accuracy – Bedrock tools are calibrated to international standards, ensuring reliable STI/CIS results that meet or exceed code requirements.
- Efficiency – Testing can be performed quickly across multiple zones. Results are immediate and easy to interpret, reducing labor time and simplifying inspection workflows.
- Documentation – Bedrock software generates intelligibility reports automatically, making it easier to provide compliance documentation during audits or occupancy approvals.
- Proactive Risk Management – Regular intelligibility testing helps identify and resolve issues before they become safety risks. Whether due to speaker failure, acoustic changes, or system adjustments, problems can be detected and corrected early.
Speech intelligibility testing isn’t just another box to check—it ensures that when an alarm sounds, the message is clear and understood. With reliable tools like those from Bedrock, testing is simple, standardized, and effective. It should be a baseline requirement for commissioning, maintenance, and annual life safety inspections—because in an emergency, every word must be heard and understood.
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