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SPECTRAL INVESTIGATION OF INFRASONIC SIGNALS RECORDED ON THE ROMANIAN BLACK SEA COAST
Abstract
The atmospheric sound waves with frequencies lower than 20Hz are named infrasound waves. Among the multitude of infrasonic signals that have been recorded and detected during time, some of them have been associated with different natural or anthropogenic sources and the source of many other remaining hidden and unknown. At the beginning of year 2013, a permanent infrasound monitoring system composed by a MB AZEL2007 microbarometer, the digitizer and the transmitting equipment, was installed by NIEP, at Mangalia, Romania, on the Black Sea coast line (at 50m from the water front) to test the infrasonic method in correlation with local, regional, and global sources producing acoustic waves with frequencies lower than 20Hz. Five years of recordings show at lower frequencies (less than 2Hz) a predictable behavior of infrasonic activity on the seashore, suggesting the presence of highly coherent infrasound waves in atmosphere. At higher frequencies (2Hz-8Hz) there appeared a type of waves that was never recorded before by our infrasonic sensors in other locations far away from the sea shore. The interesting aspect on the spectrogram resides in well defined, visible superior harmonics of the dominant signal, which develops between 2 and 8Hz. These harmonics are clear and create the specific spider legs aspect present in spectrograms. The precise source of these high amplitude signals at higher frequencies is under investigation and a couple of suppositions have been made. One of them refers to the composition of atmosphere that might be possible to act as a band pass filter which selects certain preferred frequencies from the background infrasonic noise induced by water's surface. The propagation parameters of this filter may be altered by aerosols concentration, humidity, temperature and, perhaps, by other factors. The corroboration of the infrasound recordings and spectrograms with local meteorological data show a good correlation between the spectrogram and the daily variation of the humidity parameter.
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References6
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