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Abstract
The physical properties and propagation characteristics of infrasound and subsonic disturbances in the atmosphere determine the technique and instrumentation of their measurements. Infrasounds are characterized by their propagation velocities close to acoustic velocity but frequencies less than that of audible sound, while the subsonic disturbances refer to disturbances having both frequency and propagation velocity below those of audible sound. The sources of both types of disturbances include many geophysical phenomenon as well as powerful explosions, supersonic vehicles etc. A microbarograph recording system has been described to record pressure variations caused by these disturbances. The system includes the micrograph transducer, its calibration assembly and associated electronics. The transducer uses sensitive bellows to sense the atmospheric pressure and a variable reluctance transducer converts the movements of the bellows into electrical signal. The frequency response of the system is determined by various acoustic elements of the transducer. Few typical recordings of this system are also presented and discussed.