Acoustic observations of a prescribed burn

Abstract

Audible sound is known to originate from wildland fires. We show audible sound is accompanied by infrasound emissions, originating from the active combustion zone in a prescribed fire. We collected data from one of multiple prescribed burns conducted at Eglin Airforce Base, Florida in March of 2023. Single-unit infrasonic sensors and a six-element array were deployed outside the burn unit to capture acoustic signals. The array was configured as a hexagon with aperture of ∼10 m. Dual-band radiometers were deployed inside the unit to track the fire’s evolution. A broadband signal is observed by all infrasound sensors with variable intensity and frequency content between 2 Hz and up to 90 Hz. The sensor closest to the perimeter of the unit detected frequencies up to 90 Hz for a few minutes. Array analysis shows a coherent broadband signal with frequencies between 2 and 40 Hz emanating from the burn unit that we argue is sound from the active combustion zone. The estimated azimuths follow the ignition pattern with delays of several minutes between the ignition and detection of sound. We discuss the potential for using acoustic measurements to study the spatial and temporal evolution of fire and extract spectral features of vegetation type-specific heat release rates. Extracting fire characteristics and dynamics using sound may complement other more established measurements by providing continuous data remotely that are not constrained to line-of-sight or visibility conditions that can be affected by smoke or topography.