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Designing a coherent (non-explosive) sound source within the infrasound band (0.1 to 4 Hz in our case) is a difficult engineering challenge. This is due to the large air mass that must be moved to create useful signal levels. The fundamental simple source equation, which will govern almost any human-made infrasound source due to the long wavelengths, shows this fundamental difficulty. As frequency decreases, the volume velocity must increase by the inverse factor of frequency in order to maintain an equal pressure amplitude at equal range. In other words, 1000 times the cubic volume of air must be moved at 0.1 Hz as compared to 100 Hz, in order to maintain an equal output pressure amplitude! For this reason, a novel method has been proposed to skirt this engineering challenge; using the large energy density available in gas combustion for periodic thermal expansion and contraction of an air mass. This paper briefly discusses previous work by the authors (Smith and Gabrielson, J. Acoust. Soc. Am. 137, 2407, 2015 and Smith and Gabrielson, J. Acoust. Soc. Am. 143, 1808, 2018) and presents new measurements and thermodynamic model comparisons of infrasound generated utilizing a liquid-propane burner system from a hot air balloon.Designing a coherent (non-explosive) sound source within the infrasound band (0.1 to 4 Hz in our case) is a difficult engineering challenge. This is due to the large air mass that must be moved to create useful signal levels. The fundamental simple source equation, which will govern almost any human-made infrasound source due to the long wavelengths, shows this fundamental difficulty. As frequency decreases, the volume velocity must increase by the inverse factor of frequency in order to maintain an equal pressure amplitude at equal range. In other words, 1000 times the cubic volume of air must be moved at 0.1 Hz as compared to 100 Hz, in order to maintain an equal output pressure amplitude! For this reason, a novel method has been proposed to skirt this engineering challenge; using the large energy density available in gas combustion for periodic thermal expansion and contraction of an air mass. This paper briefly discusses previous work by the authors (Smith and Gabrielson, J. Acoust. Soc. Am. 137, 2407... |
