Traditional residential smoke alarms producing a high-frequency T-3 sound have been shown to exhibit sonic deposition of acoustically-agglomerated soot when they sound in the presence of smoke produced in a typical residential fire. The intense acoustic field generated by alarm sounders, which produce these traditional smoke alarm tones, causes colliding soot particles to adhere to one another and eventually settle onto surfaces as their mass increases. The sonic field generated by the horn creates a pulsed flow around the alarm openings that can lead to increased soot deposition in this area. The agglomerated particles then can deposit on the smoke alarm proximate to the horn. The presence of the acoustically-agglomerated soot deposition on alarms has become an accepted forensic technique used to determine if an alarm sounded in the presence of soot. In recent years, new sounding technologies have been introduced for smoke alarms in order to improve their effectiveness in alerting people to fire emergencies who may not respond reliably to traditional smoke alarm tones and patterns. Many researchers have studied the effectiveness of different alarm signals and have found that low-frequency tones and voice messages show increased effectiveness in alerting people, particularly at-risk individuals such as children and the hearing impaired. The new alarms producing low-frequency tones and voice messages generate a far different acoustic field than traditional smoke alarms and require additional understanding of the acoustic agglomeration and sonic soot deposition patterns. A low-frequency alarm sounder, a voice-incorporated alarm using one sounder to generate both the smoke alarm pattern and a voice message, and a voice-incorporated alarm using two sounders to generate the smoke alarm pattern and voice message separately were exposed while sounding to five different smoke sources representative of residential fires. When examined for the presence of enhanced soot deposition, the new alarm sounders solely producing a voice message did not exhibit sonic soot deposition but sounders that produced both the traditional smoke alarm pattern and voice message did exhibit sonic soot deposition. This indicates that the deposition is due to the traditional T-3 alarm sound rather than the voice message. Additionally, low-frequency sounders exhibited sonic soot deposition but only in the presence of the heavy, resin-like tarry deposition from the smoldering polyurethane foam source. As has been found in other studies, this study confirms that the presence of enhanced soot deposition or tarry residue deposition proximate to the horn of a smoke alarm is a reliable indicator that the alarm sounded in the presence of deposition. However, the absence of enhanced soot deposition proximate to the horn of the alarm is not necessarily a reliable indicator that the alarm did not sound in the presence of soot.
