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Measuring sound with light
Sound waves propagating through a medium cause a tiny change in the refractive index of the medium, which can be detected with a laser-doppler vibrometer. In that way, the waves can be made "visible". We would like to investigate this exciting measurement technique in our laboratory.
Keywords: geophysics; propagation of sound; experimentation; laboratory work; laser-doppler vibrometry
Microphones (or hydrophones) are the state-of-the-art for measuring acoustic wavefields in air (or water). However, they have at least two significant shortcomings: in order to obtain dense measurements of an acoustic wavefield, large quantities of microphones are required, which themselves scatter the acoustic wavefield. An alternative, non-invasive approach is based on the so-called piezo-optic effect: A pressure wave passing through a medium causes changes in the refractive index of light. While the changes are minute, previous studies have demonstrated that pressure fluctuations can nonetheless be detected using Laser-Doppler-Vibrometry (Fig. 1) as the laser light effectively integrates the changes. However, the majority of studies has focused on underwater experiments using He-Ne lasers in the ultrasound regime [e.g., to visualize and quantify the ultrasound field radiated from an underwater transducer (Chen et al. (2011)[1] or scattered by a hydrophone (Chen et al. (2011)][2].
[1]Non-perturbing measurement of sound pressure fields by means of laser Doppler vibrometer. Chen et al., IEEE International Ultrasonics Symposium, IUS, 1095–1098. https://doi.org/10.1109/ULTSYM.2011.0269.
[2]Quantitative reconstruction of a disturbed ultrasound pressure field in a conventional hydrophone measurement. Chen at al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 60(6), 1199–1206. https://doi.org/10.1109/TUFFC.2013.2682.
Microphones (or hydrophones) are the state-of-the-art for measuring acoustic wavefields in air (or water). However, they have at least two significant shortcomings: in order to obtain dense measurements of an acoustic wavefield, large quantities of microphones are required, which themselves scatter the acoustic wavefield. An alternative, non-invasive approach is based on the so-called piezo-optic effect: A pressure wave passing through a medium causes changes in the refractive index of light. While the changes are minute, previous studies have demonstrated that pressure fluctuations can nonetheless be detected using Laser-Doppler-Vibrometry (Fig. 1) as the laser light effectively integrates the changes. However, the majority of studies has focused on underwater experiments using He-Ne lasers in the ultrasound regime [e.g., to visualize and quantify the ultrasound field radiated from an underwater transducer (Chen et al. (2011)[1] or scattered by a hydrophone (Chen et al. (2011)][2].
[1]Non-perturbing measurement of sound pressure fields by means of laser Doppler vibrometer. Chen et al., IEEE International Ultrasonics Symposium, IUS, 1095–1098. https://doi.org/10.1109/ULTSYM.2011.0269.
[2]Quantitative reconstruction of a disturbed ultrasound pressure field in a conventional hydrophone measurement. Chen at al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 60(6), 1199–1206. https://doi.org/10.1109/TUFFC.2013.2682.
In this thesis project, we propose to explore the feasibility of performing dense, non-invasive measurements of airborne sound in the audible range by leveraging a robotized infrared Laser-Doppler-Vibrometer (LDV) available in the Centre for Immersive Wave Experimentation (CIWE) at ETH Zürich [3]. The findings will offer new insights into laser-based interrogation of sound fields and will be of great importance for the visualization, interpretation and validation of current and future experiments within CIWE and particularly the MATRIX project. If time allows, non-invasive measurement of sound fields (compressional waves) in transparent solids such as acrylic glass will also be investigated. Furthermore, we will explore the extent to which tomographic approaches are needed to map the integrated changes in the refractive index back to the underlying spatial variations in the sound field.
[3] https://eeg.ethz.ch/research/centre-immersive.html
In this thesis project, we propose to explore the feasibility of performing dense, non-invasive measurements of airborne sound in the audible range by leveraging a robotized infrared Laser-Doppler-Vibrometer (LDV) available in the Centre for Immersive Wave Experimentation (CIWE) at ETH Zürich [3]. The findings will offer new insights into laser-based interrogation of sound fields and will be of great importance for the visualization, interpretation and validation of current and future experiments within CIWE and particularly the MATRIX project. If time allows, non-invasive measurement of sound fields (compressional waves) in transparent solids such as acrylic glass will also be investigated. Furthermore, we will explore the extent to which tomographic approaches are needed to map the integrated changes in the refractive index back to the underlying spatial variations in the sound field.
If you are interested in this opportunity, please get in touch with us:
Theodor Becker (theodor.becker@erdw.ethz.ch)
Dirk-Jan van Manen (dirkjan.vanmanen@erdw.ethz.ch)
If you are interested in this opportunity, please get in touch with us: Theodor Becker (theodor.becker@erdw.ethz.ch) Dirk-Jan van Manen (dirkjan.vanmanen@erdw.ethz.ch)