What is "intelligibility"?

Intelligibility is a word that most people have never used.  Until a few years ago, I'd never heard the word either.  This is a measurement of how "understandable" a fire alarm voice evacuation signal is to the human ear.  To perform a test of intelligibility, we play a special recording into the voice microphone and then measure the output of the speakers on the system.  Unlike a decibel test, we are not measuring the sound level.  We are measuring how well that message can be heard by the test meter.  We test and record our readings at multiple locations in the building to ensure our recorded message can be understood throughout the space.  After we've done all this, and received acceptable readings throughout, we can feel confident that the fire department will be able to use that system to get people out of the building if needed.  This type of system is normally required in high-rise buildings over 7 stories and in places of assembly.  Just another part of the science of life safety.

For more info, see the Society of Fire Protection Engineers website http://www.sfpe.org/?page=FPE_ET_Issue_94