Vegetation effects on impulsive events in the acoustic signature of fires

Abstract

Acoustic impulse events have long been used as diagnostics for discrete phenomena in the natural world, including the detection of meteor impacts and volcanic eruptions. Wildland fires display an array of such acoustic impulse events in the form of crackling noises. Exploratory research into the properties of these impulse events revealed information regarding the specific properties of plant material. Unique acoustic frequency bands in the upper end of the sonic spectrum correlated to changes in vegetation properties. The signature of acoustic impulse events as they relate to plant species and plant water stress, were investigated in controlled laboratory combustion experiments. Correlation in the frequency range of 6.0-15.0 kHz was found for both species and water stress, indicating the possibility that a digital filter may be capable of identifying vegetation properties during wildland fire events.

Quantifying Firebrand Production and Transport Using the Acoustic Analysis of In-Fire Cameras

“…Alongside this fire-induced wind was episodic sounds similar to the firebrand impact, but occurring exclusively after the passage of the fire front when the fire had passed and no further high-intensity fire was upwind. These events and those observed during the active flaming observations at each camera likely correspond to the combustion-derived Acoustic Impulsive Events (AIE) [29]. Such AIE as recorded by [29] had an a sound pressure of 2 Pa recorded at 0.25 m distance, equivalent to 60 dB at 0.25 m measurement distance.…”

“…These events and those observed during the active flaming observations at each camera likely correspond to the combustion-derived Acoustic Impulsive Events (AIE) [29]. Such AIE as recorded by [29] had an a sound pressure of 2 Pa recorded at 0.25 m distance, equivalent to 60 dB at 0.25 m measurement distance. Similar AIE produced at the main flaming fire front but recorded 25 m away, coincident with the shortest disance of maximum firebrand production for most cameras (Fig.…”

“…Logically, firebrand production and impact to the camera body is extremely unlikely after the passage of the main fire front, due to the prevailing wind direction during the fire. However, acoustic detections and sound power levels were retained from post-fire acoustic detections in order to help distinguish firebrand impact acoustic events from acoustic impulse events derived from the combustion process itself [29].…”

“…While the audio track has been recorded and archived as incidental information, the information contained in the audio such as the sound of firebrands impacting the camera box has not previously been quantified. Acoustic impulsive events, the autogenic production of sound from the combustion process itself, has been documented in laboratory ignitions of conifer seedlings [29], though for microphones placed approximately 0.25 m from a lowintensity flame. In contrast, firebrands that drive medium-distance structure ignition, spot fire production, and enhancing fire spread travel on the order of 30 m [22] to upwards of a kilometre [24].…”

Embracing Complexity to Advance the Science of Wildland Fire Behavior

“…A current significant challenge in advancing fire behavior science is the difficulty in collecting quantifiable, in-situ measures of fire behavior. We argue that by broadening the scope of fire behavior research, new technologies (e.g., acoustic emissions as a combustion remote sensing technique [46]) and tools will emerge organically References [35,[47][48][49]. The three brief scenarios were intended to highlight the expanding opportunities currently available in fire behavior research by embracing the inherent variation of wildland fire behavior as its fundamental attribute.…”