Infrasound is characterized by frequencies below 16 to 20 Hz and above air pressure variations related to changing weather conditions. Infrasound waves of very low frequency can travel long distances. Such waves are generated by variations of air volume, and thus have many different natural and artificial sources. Low frequency waves which originate from earthquakes, volcanic eruptions, meteorite impacts, lightning bolts, or explosions can travel several thousand kilometres through the atmosphere. First investigations regarding the dispersion of infrasound waves were conducted in Austria as early as 1912 and 1917. At the Conrad Observatory an infrasound test facility was constructed in 2009. Infrasound is recorded by so-called micro barometers. These instruments are sensible to pressure variations. Usually spatial filters are connected to micro barometers, i.e. the starlike configuration of tubes in

Fig. 1. Such a spatial configuration allows for direct averaging of pressure and wind induced variations.

The infrasound facility is maintained and used by the comprehensive nuclear-test-ban treaty organization (CTBTO). A primary task is to detect any nuclear explosion conducted on Earth – underground, underwater or in the atmosphere. Infrasound is one of the four monitoring technologies of the verification regime of the CTBTO/IMS, along with seismic, hydroacoustic, and radionuclide techniques.