Automatic fire alarm system - uses measure of local velocity of sound as indication of temp. with triggering when threshold is exceeded

摘要

A fire alarm system provides an alarm triggering when a threshold value of temp. and rate of change of temp. is exceeded. A sine wave generated (2) provides an acoustic signal from a loudspeaker (3) that is transmitted across the room to a receiving microphone (5). A phase comparator (6) allows the transmission time to be measured and this is proportional to the local velocity of sound. The signal is subtracted (8) from a reference voltage (7) to operate a trigger (9) to produce a measure of actual temp. Both the temp. and rate of change of temp. (10) are compared with reference values (11, 12). When both are exceeded (13), the alarm is activated.