Patent ID: 8519854

Claim:
A fire alarm system comprising: a sound wave generator sending a sound wave; a sound wave detector being located so as to sandwich in a monitoring space between itself and the generator, the sound wave detector detecting the sound wave sent from the generator through the monitoring space; a controller being configured to control the generator and the sound wave detector; a processing circuit being configured to process an output detected by the sound wave detector, the processing circuit comprising a sound pressure detector for detecting a sound pressure based on the output, a smoke density estimator for estimating a density of smoke existing in the monitoring space and a fire alarm device being configured to transmit fire alarm when the smoke density estimated by the smoke density estimator exceeds a predetermined threshold; wherein the generator and the sound wave detector are located in the monitoring space so as to form a first and second propagation paths for propagating the sound wave from the generator to the sound wave detector, the first and second propagation paths having different lengths from each other, wherein the processing circuit comprises a calculation means for calculating a pressure ratio between a first sound pressure being a sound pressure of a sound wave propagated through the first propagation path and a second sound pressure being a sound pressure of a sound wave propagated through the second propagation path, wherein the smoke density estimator is configured to calculate a change ratio between the pressure ratio calculated by the calculation means and a predetermined average pressure ratio, and to determine the smoke density from the change ratio based on a predetermined relational expression, the relational expression describing the change ratio in relation to the smoke density, wherein the first and second propagation paths extend from the same generator to the same sound wave detector.