An Experimental Study of Particle‐Laden Jet Noise as an Analogue to Tephra‐Laden Volcanic Eruptions

Abstract

Explosive volcanic eruptions produce vast amounts of acoustic energy, generally in the infrasound range. For sustained eruptions, infrasound is often dominated by jet noise, a form of noise similar to the sound from human‐made jet engines. Jet noise largely results from the combination of two sound‐producing components: fine‐scale turbulence (FST) and large‐scale turbulence (LST), with distinctive frequency spectra. FST and LST spectra successfully reproduce single‐phase jet noise data (gas‐only), but little research has been directed toward multi‐phase jets, like volcanic jets (gas with solid particles). Here, we report novel experiments investigating the noise produced by particle‐laden jets. We produced both pure‐air and particle‐laden jets (nozzle diameters: 3.5–8 mm) and recorded the resulting sound at observation angles 42–159° (where 180° is the jet's propagation direction). Fitting FST and LST spectra to the particle‐laden noise spectra was less accurate than in the pure‐air case. The most prominent difference is the absence of LST‐dominated noise at high observation angles. We hypothesize that the particles break down large‐scale structures in the jet, thus favoring FST noise generation. Slower jet exit velocities due to the presence of particles may also explain some of the discrepancies observed and will require careful constraining in the future. The effect of particles on jet noise was most easily measured at high observation angles, which are also the most challenging to monitor at active volcanoes. The experiments presented here mark a first step toward integrating experimental, numerical, and observational approaches, to unravel the acoustic signatures of volcanic eruptions.