Farfield coherent infrasound generation using an air-propane burner

An invaluable tool in the characterization of any receiver, propagation path, or detection system is a source with known and repeatable signal characteristics. This article presents the theoretical development and engineering design of a coherent (nonexplosive, periodic with controlled duration) infrasound source in the sub-hertz to several hertz band. Design of a sound source within this band is a difficult engineering challenge. The simple source equation, which will govern any portable human-fabricated infrasound source due to the long wavelengths, shows this fundamental difficulty. As frequency decreases, volume displacement must increase by the squared inverse factor of frequency in order to maintain an equal pressure amplitude at equal range. For this reason, the authors evaluate using the high energy density available in gas combustion to periodically displace large volumes of air within the open atmosphere. Prototype testing has verified the capability of generating continuous signals at a fundamental frequency of 0.25 Hz in the farfield - ranges in which pressure and particle velocity can be considered in-phase - where the product of the acoustic wavenumber and range is near 4.7. The generation of frequency content throughout the 0.25-4.0 Hz band with a reasonable signal-to-noise ratio was also demonstrated.