FIRE ALARM SYSTEM

摘要

In a fire alarm system for judging the occurrence of a fire by ultrasound, a sound wave generating means and a sound wave detecting means are disposed so as to detect sound waves traveling through two propagation paths with different path lengths. The system comprises a sound pressure ratio calculating means for calculating a sound pressure ratio that is the ratio between a first detected sound pressure that is the sound pressure of the sound wave propagating through the first propagation path and a second detected sound pressure that is the sound pressure of the sound wave propagating through the second propagation path and a smoke density estimating means. The smoke density estimating means obtains the rate of change between the sound pressure ratio calculated by the sound pressure ratio calculating means and a predetermined standard sound pressure ratio and determines the smoke density from the rate of change in accordance with a preset relational expression representing the relation between the rate of change and the smoke density. The fire is judged to have occurred when the smoke density exceeds a predetermined threshold value.