Intelligibility--What Did You Say?

How many times have you been to a large corporate complex and struggled to hear an announcement on the public address system? Now consider the same sound system and the dangers when the listener is instructed in emergency evacuation procedures. You thought the announcement was to go to the "rear," when the actual announcement was to stay "here." Given several high-profile cases where people misunderstood instructions during fires, the authorities have responded with new standards, and every contractor should have at least a core understanding of the issues.

The requirement for intelligibility exists in the Appendix to the National Fire Protection Act, NFPA 72 (Voice Evacuation Systems). The appendix refers to various methods of measuring intelligibility, but first one must agree on what "intelligible" means.

The simplest definition of intelligibility would be that people can understand speech when heard over the sound system. In the early days of measurement, the only way to prove that a system was intelligible was to gather a large group of people and have them mark on sheets of paper what word they thought they heard. The testing and grading procedure for these "word score" methods was time consuming, and bringing large groups of people to every area to be tested was not practical.

Another option was to measure the level of the noise as compared to the level of the speech. This was a step in the right direction, and typically evacuation systems are required to produce a level which is 15dB(a) sound pressure level (SPL) louder than the ambient noise. The advantage was that the test could be done quickly with an inexpensive SPL meter. The downside was that under some common conditions, it did not accurately predict intelligibility. In large rooms that lack absorption, reverberation makes speech difficult to understand. As Ken Jacob of Bose noted, "audibility does not imply intelligibility." What this means, is that making a system louder will make it sound louder but not necessarily easier to understand. In many cases, making a system louder will actually make it less intelligible as it merely drives more energy into the reverberant field. And, if the sound were distorted due to clipping or other electrical problems, making it louder did nothing to recognize the underlying problem.

The military was concerned about this situation as it related to soldiers and pilots not understanding critical commands. In response they began funding laboratory research into how humans perceived speech. This research eventually made it to the civilian world and was adopted by various international standards. International Electrotechnical Commission IEC standard 60268-16 provides for the leading methodology, which is known as the Speech Transmission Index (STI). There are then variations of the STI data to make it easier to convert to other standards. The most universal of these is the Common Intelligibility Scale (CIS), which was adopted by IEC 60849. And the ISO 9921 final draft provides for a new, faster method using a test tone known as STIPA. This method can measure STI in as little as 15 seconds, using a simple handheld analyzer.

There are various products on the market to measure intelligibility, but in general a test tone is played through the sound system and the analyzer is placed in the part of the room to be checked for intelligibility. It would make no sense to measure up near the roof, if people sit 30 feet below. So, the contractor defines the areas where people are located and makes a series of measurements. If the room scores more than .50 STI, which is the same as .70 CIS, then he or she can be confident that people will understand a message. If the system measures below this acceptable level, then re-examining the factors that reduce intelligibility is necessary: Is the system producing 15dB above the ambient noise levels within the range of human speech? Is the sound system creating excessive distortion or clipping? Is the room very reverberant? Each of these factors can be corrected, once the problem is discovered.

Modern technology is making sound measurement easier and more affordable. Designing a complex system is still a job best left for the professional. The verification of intelligibility, however, is quickly becoming something that nearly any competent contractor should be able to achieve. The World Wide Web has much more information on the topic by searching for "Intelligibility" and "STI." EC

MILLER, an audio expert, can be reached at

About the Author

Gregory J. Miller

Freelance Writer
Gregory J. Miller, an audio expert, can be reached at .

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