In many cases whether a smoke alarm sounded can be determined by enhanced soot around the horn. Reference: 2017 NFPA 921, Sections 18.104.22.168-22.214.171.124.5.
NFPA 101 - 1976, required smoke alarms to be in every home
NFPA 74 - 1989, required interconnected smoke alarms for new construction [5,6]. If one alarms, they all alarm.
NFPA 72 - 1993, required hardwired smoke alarms in every bedroom or sleeping area [5,6].
NFPA 72 - 1996, required hardwired smoke alarms in new construction to have battery back-up .
NFPA 72 - 1996 July 1, required the smoke alarm signal to be 3 beeps, pause and repeat - aka the Temporal 3 Sound and the Audible Emergency Evacuation Signal, ANSI S3.41 .
NFPA 72 - 1999, required smoke to be replaced every 10 years.
UL 217 - 1999, required manufactures to print the date in plain English on the back of the smoke alarm.
NFPA 72 - 2007, required existing homes to have interconnected smoke alarms.
An ionization detector consists of two metal plates with a voltage across them, along with a radiation source of americium. When alpha particles from the americium collide with oxygen and nitrogen atoms in the air, electrons from the atoms are freed. The electron has a negative charge, and the atom with the missing electron, a positive charge. The positive charged atoms are called ions. The ions are attracted to the negative plate, and the free electrons are attracted to the positive plate (opposites attract). The flow of electrons and ions is an electrical current, which is sensed by a detector. Smoke particles attach to the ions and neutralize them (zero charge). This reduces the electrical current, which is sensed by the detected.
For Maximum Resolution, Click on the Body of the Picture.