DOI: 10.5880/fidgeo.2020.028
title: Sound emission from shock-tube experiments for volcanological investigations

description: Explosive volcanic eruptions generate sound mostly in the infrasound (&lt;20 Hz), but also in the acoustic (&gt;20 and &lt; 20k Hz) frequency range. Sound from volcanoes is recorded and used to describe quantitatively properties of the eruptive column, e.g. mass flux, and therefore it has monitoring purposes. However, a physical understanding of the underlying processes, their efficiency, and – maybe most importantly – the acting parameters like gas overpressure, absolute gas/magma volume, fragmentation depth and geometry of the plumbing system are unknown. To shed light over the relationship between sound emissions and source conditions, we performed shock-tube experiments generating gas-only jets in an anechoic chamber testing the following conditions: - 3, 4, 50, 75, 80 and 130 bar reservoir overpressure; - 2.14 and 8.57 L/D non-dimensional reservoir volumes, where L is the length of the shock-tube reservoir and D the diameter; - cylinder and two funnels with 15- and 30-degree flaring walls nozzle geometries. The jets’ sound emissions were recorded with a near and far-field array composed of a total of 16 microphones. This archive consists of the raw sound emission recording for the experiments performed. Thus, for each experiment, the user can access a single experiment. Each file is in CSV format. File names are self-explanatory following the format: Acoustic_[vent shape]_[pressure ratio]_[non-dimensional mass supply]_[YYYYMMDDTHHMMSS].csv. As an example, a user who wishes to access the data corresponding to an experiment performed at 50 bar, L/D 8, and a cylindrical nozzle will have to look for the file Acoustic_cyl_50bar_LD8_20160830T094809.csv. For detailed description, please refer to the associated data description pdf.
description: Shock-tube system The shock-tube apparatus used to generate the gas-only jets was originally developed by Alidibirov and Dingwell [1996]. More recently, it has been used by Cigala et al. [2017] to investigate the dynamics of particle-laden starting jets at volcanic conditions. The shock-tube is made of Nimonic 105 alloy, it allows for high pressure (up to 100 MPa) and temperature (up to 850 °C) experiments. For the collection of the data presented here, the shock-tube has been deployed at ambient temperature and for reservoir overpressures of 3, 4, 50, 80, and 130 bar. The shock-tube reservoir has an internal diameter of 28 mm (D) and is 60 and 240 mm high (L), for a total of 0.032 and 0.127 l pressurize volume, respectively. The original length of the tube, 240 mm, is reduced to 60 mm thanks to a full cylinder of Teflon of 180 mm fixed at the bottom of the shock-tube. The pressurized reservoir is sealed from ambient conditions via one (or more, depending on the desired final overpressure) tailored metal disk and incrementally pressurized with argon gas at ambient temperature. For pressure higher than 100 bar, two or more metal disks are recommended to enhance repeatability. Testing both one and two metal disks setting rules out the sound produced by their opening in the data recorded. When the disk(s) bursts at the desired overpressure, four triangle-shaped segments bent upwards generating some internal diameter variation (between 26 and 28 mm). A pressure sensor, at the metal disk level, records the pressure drop in the reservoir and the onset of the ejection. Three nozzle geometries have been deployed: a cylindrical and two funnel-shaped nozzles. The geometries are parametrized by the exit-to-throat area ratio (Ae/AD) = (1, 2.37, 4). Each set of experimental conditions has been repeated at least two times. Acoustic data acquisition system: Near- and far-field arrays of 16 microphones were used to record the sound emission of the jets. In the near-field, 2 arrays of 4 microphones each were set at a distance of about 0.35m from the shock-tube axis. In the far-field, 4 arrays of 2 microphones each, were set at a distance of about 2.5m. The exact spatial distribution is shown in Figure 1 and the coordinates of each microphone are listed in Table 1. The microphones are manufactured by PCB, they are half-inch, free-field microphones with a frequency range of 3.75 – 20,000 Hz. The frequency response of the microphones is shown in Figure 2. The data acquisition system was manufactured by Data Translation. The analog input resolution is 24 bit with a maximum sampling rate of 102 kHz. BNC cables were used to transfer the data from the microphone to the data acquisition system. The measurements have been triggered manually, taking into consideration the release of the pressure in the shock-tube. Anechoic chamber: The experiments were performed in the anechoic chamber facility of the Technische Universität Berlin. The chamber has a free volume of 1070 m3 and a walkable area of 126 m2. The certified lower frequency limit of the anechoic chamber is 63 Hz.