Measuring directionality of small-scale explosions

Volcanic explosions produce infrasound that contains clues to the strength of the explosion and the amount of debris ejected.  Scale-model explosions can be used to learn more about how to interpret volcano infrasound.  Our goal was to explode oxy­acetylene balloons placed in specially designed craters. The influence of the crater design on the sound levels was investigated. Several components were required for a successful test. The meteorological conditions were monitored. Acoustic recordings required a careful and accurate setup of research grade microphones and high­-speed data acquisition system. The microphones were placed in a circular array, with 20 m radius, concentric with the crater. Explosive balloons were filled with stoichiometric mixtures that provided acoustic shocks with frequencies down into the infrasound spectrum. Two successful measurements were obtained for four crater morphologies. In analysis of peak sound level and sound exposure level polar plots, we were able to see that directionality of the acoustic shock was achieved in one crater.