Mapping the Echo Chamber

Advancing down the hallway led me straight to a heavy fire exit door, propped open just an inch with a rubber wedge. I slipped through onto the concrete stairwell landing! The acoustic resonance in this vertical shaft was unbelievable. I stepped toward a dropped brass key lying flat on the grey floor, gripped its notched shaft, and tapped it firmly against the hollow metal railing post. *CLANG!* The ring echoed up three floors like a starting bell! I noted the sound cadence—three solid seconds of vibration. Perfect strategic relay channel established!