Abstract

In a fire alarm or detector where radiation in the visible or infrared spectral range, is transmitted in a pulsed mode in a measuring chamber, there is provided an acoustical receiver which collects the air pressure vibrations arising upon absorption of the radiation pulses by smoke or aerosol particles and evaluates them in coincidence with the radiation pulses. Upon exceeding a certain intensity of the acoustical pulses there is triggered a fire alarm signal. In this way it is particularly possible to positively detect smoke which is intensively radiation absorbent and with increased sensitivity within a measuring chamber having small dimensions. A further development of the invention contemplates the provision of an additional scattered radiation receiver, preferably registering smoke having markedly scattering properties for the radiation. The acoustical and the scattered radiation-evaluation channel are connected in an OR-circuit with the alarm signal transmitter. It is also preferable to use radiation having a wavelength in the range of the resonance radiation of a carbon oxide.