The office building's annual fire drill was supposed to be routine. The alarm sounded. The voice message played: "This is a drill. Please evacuate using the nearest stairwell." What occupants on the 15th floor heard: "This is... [garbled]... Please... [garbled]... nearest... [garbled]." The word "drill" was completely lost in reverberation. What remained was "This is... evacuate... nearest" — which sounded exactly like a real emergency to everyone who couldn't hear the qualifying word.

Three employees called 911. One person grabbed their personal belongings, their lunch, their colleague's lunch, and a potted plant. Someone on the 12th floor started running, which triggered a cascade of running on floors 11 through 8. The stairwell — a concrete tube with RT60 approaching 5 seconds — made the PA system even more unintelligible, so people emerging from each floor had no idea if this was real or practice. The "orderly drill" became a semi-panicked evacuation that took 22 minutes and resulted in two sprained ankles.

The stairwell STI was 0.18. The open office floors measured 0.38. Neither met the EN 54-16 requirement of 0.50 for voice alarm systems. The word "drill" has a short vowel and a liquid consonant that are particularly vulnerable to reverberation masking.

Voice evacuation must be intelligible in the worst acoustic conditions: stairwells, corridors, and mechanical rooms.

The Moral: If "drill" and "real" are indistinguishable on your PA, you have an intelligibility crisis. Measure stairwell STI with SonaVyx STI — that's where it matters most and performs worst.