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.

在利用超声波确定火灾的存在性的火灾警报系统中，声波生成部件和声波检测部件被配置为检测沿着具有不同路径长度的两个传播路径而行进的声波。所述系统包括：声压比计算部件，用于计算第一检测声压和第二检测声压之间的声压比，其中第一检测声压是经过第一传播路径传播的声波的声压，第二检测声压是经过第二传播路径传播的声波的声压；以及烟浓度估计部件。所述烟浓度估计器获得由所述声压比计算部件计算的声压比与预定的标准声压比之间的变化率，并根据表示所述变化率与烟浓度之间的关系的预设的关系表达式，由所述变化率确定烟浓度。当所述烟浓度超过预定的阈值时确定火灾已发生。