Persistence of Sonic Deposition on Smoke Alarms in Forensic Fire Investigations

Smoke alarms have been shown to develop sonically-deposited regions of acoustically agglomerated soot particles when they sound in smoke-filled air. These sonic depositions can be examined forensically post-fire to determine if the smoke alarm sounded during the incident. However, it is not clear how these sonic depositions are affected by common firefighting and post-fire actions. To determine the effects of post-fire forensic smoke alarm testing and environmental conditions on the persistence of the existing sonic deposition of soot on the horns of a smoke alarm, sixty (60) smoke alarms were subjected to smoke from fires of several different fuel types and common post-fire conditions or actions. Initially, each alarm was exposed to smoke in a small-scale experimental fire to develop sonic deposition around the horn. The fuel types for the fires were smoldering wood, flaming toluene–heptane, smoldering polyurethane foam, flaming polyurethane foam, and a combination of smoldering and flaming polyurethane foam. The alarms were then subjected to four common post-fire actions: pressing the test button, exposure to synthetic canned smoke, exposure to standing water, and exposure to running water. Each detector was visually inspected before and after the post-fire action. Results varied from no soot removed to almost all soot removed depending on the fuel type and post-fire test. An objective evaluation system was used to rank the degree to which soot was removed from the alarm horns: 0 (no soot removed), 1 (some soot removed), and 2 (all soot removed) based on visual inspection. The smoldering wood and smoldering polyurethane foam fires left behind a sticky resin that was essentially unaffected by any of the post-tests. The flaming foam and flaming toluene–heptane fires left powdery soot on the horn which could be easily wiped off. This soot was almost completely washed off by running water (1.067 average degree of removal) while the canned smoke and standing water post-tests removed a significant portion of the soot (0.533 and 1.000 average degrees of removal, respectively), which could lead an investigator to an errant sounding determination. Pressing the test-button appeared to make minimal impact on the amount of soot around the alarms horns regardless of the fuel type (0.067 average degree of removal).