{"id":{"repo_id":"dundee","oai_identifier":"oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92"},"canonical_url":"https://search.dev.ndltd.org/etd/dundee/oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92","repository":{"repo_id":"dundee","name":"University of Dundee","base_url":"https://discovery.dundee.ac.uk/ws/oai"},"display":{"title":"The cavitation subharmonic signal : mechanistic source and optimised detection","abstract":"The cavitation subharmonic signal, emitted at frequency values sub-multiple to that of the acoustic driving, is held to be exclusive to the occurrence of driven bubbles within a host medium. Recently, detection of the subharmonic signal has seen a resurgence of interest, particularly for the prospect of cavitation-mediated therapy during the application of focused ultrasound to tissue. Remarkably, bubble-based mechanisms for the origin of the subharmonic signal - which can account for the range of experimental configurations from which it has been detected - have remained elusive since the signal was first identified, by Esche in 1952.<br/><br/>This thesis describes cavitation observations in water, driven by propagating focused ultrasound fields typical of those used for medical therapy, using ultra high-speed shadowgraphic imaging at frame rates well in excess of the fundamental driving frequency. Moreover, single nanosecond laser pulses at energies below the plasma-forming threshold, are used to nucleate acoustic cavitation such that activity may be observed from the outset.<br/><br/>Clouds of densely packed and strongly interacting bubbles are seen to rapidly develop from the nucleation event. Within a few acoustic cycles, the cloud adopts a breathing mode response, with component bubbles collectively oscillating, approximately in-phase. The frequency of cloud oscillation matches that of the fundamental driving, however, at intervals dependent on the pressure amplitude of the driving, the cloud undergoes strong collapses, coincident to emitting a shockwave. In parallel to the high-speed imaging, a number of hydrophone detectors are used to collect the acoustic emissions, and confirm that periodic shockwaves mediate the subharmonic signals.<br/><br/>Acoustic detection of broadband, impulsive pressure transients is particularly susceptible to convolution with the frequency response of the detector. Accordingly, a PVdF needle hydrophone was calibrated for magnitude and phase from 125 kHz – 20 MHz, at the National Physical Laboratory. Detector deconvolution is demonstrated for shockwaves emitted during the formation of large plasma-mediated bubbles, each generated with a laser pulse of energy above the threshold. Similarly, the needle hydrophone is deconvolved from the emissions collected from acoustic cavitation clouds, indicating peak-positive pressure amplitudes for periodic shockwaves in the order of 10 kPa, at the distance detected. The development of a single element passive cavitation detector, dedicated to the detection of low-amplitude shockwaves with high sensitivity, is subsequently described. Detector construction, specifically the selection of matching and backing layers, is guided via a Finite Element model of the device, adapted to support simulated shockwave propagation. Detector performance is characterised with plasma bubble shockwaves, and evaluated for the detection of the subharmonic signal from a cavitation cloud, against a commercially available device.","abstract_html":"The cavitation subharmonic signal, emitted at frequency values sub-multiple to that of the acoustic driving, is held to be exclusive to the occurrence of driven bubbles within a host medium. Recently, detection of the subharmonic signal has seen a resurgence of interest, particularly for the prospect of cavitation-mediated therapy during the application of focused ultrasound to tissue. Remarkably, bubble-based mechanisms for the origin of the subharmonic signal - which can account for the range of experimental configurations from which it has been detected - have remained elusive since the signal was first identified, by Esche in 1952.&lt;br/&gt;&lt;br/&gt;This thesis describes cavitation observations in water, driven by propagating focused ultrasound fields typical of those used for medical therapy, using ultra high-speed shadowgraphic imaging at frame rates well in excess of the fundamental driving frequency. Moreover, single nanosecond laser pulses at energies below the plasma-forming threshold, are used to nucleate acoustic cavitation such that activity may be observed from the outset.&lt;br/&gt;&lt;br/&gt;Clouds of densely packed and strongly interacting bubbles are seen to rapidly develop from the nucleation event. Within a few acoustic cycles, the cloud adopts a breathing mode response, with component bubbles collectively oscillating, approximately in-phase. The frequency of cloud oscillation matches that of the fundamental driving, however, at intervals dependent on the pressure amplitude of the driving, the cloud undergoes strong collapses, coincident to emitting a shockwave. In parallel to the high-speed imaging, a number of hydrophone detectors are used to collect the acoustic emissions, and confirm that periodic shockwaves mediate the subharmonic signals.&lt;br/&gt;&lt;br/&gt;Acoustic detection of broadband, impulsive pressure transients is particularly susceptible to convolution with the frequency response of the detector. Accordingly, a PVdF needle hydrophone was calibrated for magnitude and phase from 125 kHz – 20 MHz, at the National Physical Laboratory. Detector deconvolution is demonstrated for shockwaves emitted during the formation of large plasma-mediated bubbles, each generated with a laser pulse of energy above the threshold. Similarly, the needle hydrophone is deconvolved from the emissions collected from acoustic cavitation clouds, indicating peak-positive pressure amplitudes for periodic shockwaves in the order of 10 kPa, at the distance detected. The development of a single element passive cavitation detector, dedicated to the detection of low-amplitude shockwaves with high sensitivity, is subsequently described. Detector construction, specifically the selection of matching and backing layers, is guided via a Finite Element model of the device, adapted to support simulated shockwave propagation. Detector performance is characterised with plasma bubble shockwaves, and evaluated for the detection of the subharmonic signal from a cavitation cloud, against a commercially available device.","abstract_has_math":false,"creators":["Johnston, Keith"],"institution":"University of Dundee","degree_name":"Doctor of Philosophy","degree_level":"Doctoral Thesis","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Cuschieri, Alfred"],"committee_chairs":[],"committee_members":[],"year":2016,"date_issued":"2016","date_published":"2016","updated_at":"2026-07-24T02:08:19Z","subjects":[],"languages":["eng"],"rights":[],"rights_urls":[],"identifier_entries":[{"key":"dc:identifier","label":"Identifier","values":["oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92"],"render_values":[{"text":"oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92","href":null,"code":true}]}]},"links":{"outbound_url":"https://discovery.dundee.ac.uk/en/studentTheses/34568741-12be-4a90-8bbf-82f5900a4b92","outbound_label":"Repository record","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Cuschieri, Alfred"]},{"key":"dc:contributor.sponsor","label":"Sponsor","values":["European Research Council"]},{"key":"dc:creator","label":"Author","values":["Johnston, Keith"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2016"]},{"key":"dc:date.issued","label":"Date","values":["2016"]},{"key":"dc:publisher.department","label":"Dc Publisher Department","values":["Imaging and Technology"]},{"key":"dc:publisher.institution","label":"Dc Publisher Institution","values":["University of Dundee"]},{"key":"dc:relation.isreferencedby","label":"Dc Relation Isreferencedby","values":["https://discovery.dundee.ac.uk/en/studentTheses/34568741-12be-4a90-8bbf-82f5900a4b92"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["Doctoral Thesis"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["Doctor of Philosophy"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["eng"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92","https://discovery.dundee.ac.uk/en/studentTheses/34568741-12be-4a90-8bbf-82f5900a4b92"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://discovery.dundee.ac.uk/files/36510123/PhD_thesis_Keith_Johnston.pdf"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["The cavitation subharmonic signal, emitted at frequency values sub-multiple to that of the acoustic driving, is held to be exclusive to the occurrence of driven bubbles within a host medium. Recently, detection of the subharmonic signal has seen a resurgence of interest, particularly for the prospect of cavitation-mediated therapy during the application of focused ultrasound to tissue. Remarkably, bubble-based mechanisms for the origin of the subharmonic signal - which can account for the range of experimental configurations from which it has been detected - have remained elusive since the signal was first identified, by Esche in 1952.<br/><br/>This thesis describes cavitation observations in water, driven by propagating focused ultrasound fields typical of those used for medical therapy, using ultra high-speed shadowgraphic imaging at frame rates well in excess of the fundamental driving frequency. Moreover, single nanosecond laser pulses at energies below the plasma-forming threshold, are used to nucleate acoustic cavitation such that activity may be observed from the outset.<br/><br/>Clouds of densely packed and strongly interacting bubbles are seen to rapidly develop from the nucleation event. Within a few acoustic cycles, the cloud adopts a breathing mode response, with component bubbles collectively oscillating, approximately in-phase. The frequency of cloud oscillation matches that of the fundamental driving, however, at intervals dependent on the pressure amplitude of the driving, the cloud undergoes strong collapses, coincident to emitting a shockwave. In parallel to the high-speed imaging, a number of hydrophone detectors are used to collect the acoustic emissions, and confirm that periodic shockwaves mediate the subharmonic signals.<br/><br/>Acoustic detection of broadband, impulsive pressure transients is particularly susceptible to convolution with the frequency response of the detector. Accordingly, a PVdF needle hydrophone was calibrated for magnitude and phase from 125 kHz – 20 MHz, at the National Physical Laboratory. Detector deconvolution is demonstrated for shockwaves emitted during the formation of large plasma-mediated bubbles, each generated with a laser pulse of energy above the threshold. Similarly, the needle hydrophone is deconvolved from the emissions collected from acoustic cavitation clouds, indicating peak-positive pressure amplitudes for periodic shockwaves in the order of 10 kPa, at the distance detected. The development of a single element passive cavitation detector, dedicated to the detection of low-amplitude shockwaves with high sensitivity, is subsequently described. Detector construction, specifically the selection of matching and backing layers, is guided via a Finite Element model of the device, adapted to support simulated shockwave propagation. Detector performance is characterised with plasma bubble shockwaves, and evaluated for the detection of the subharmonic signal from a cavitation cloud, against a commercially available device."]},{"key":"dc:title","label":"Title","values":["The cavitation subharmonic signal : mechanistic source and optimised detection"]}]}],"canonical_facts":{"dc:contributor.advisor":["Cuschieri, Alfred"],"dc:contributor.sponsor":["European Research Council"],"dc:creator":["Johnston, Keith"],"dc:date":["2016"],"dc:date.issued":["2016"],"dc:description.abstract":["The cavitation subharmonic signal, emitted at frequency values sub-multiple to that of the acoustic driving, is held to be exclusive to the occurrence of driven bubbles within a host medium. Recently, detection of the subharmonic signal has seen a resurgence of interest, particularly for the prospect of cavitation-mediated therapy during the application of focused ultrasound to tissue. Remarkably, bubble-based mechanisms for the origin of the subharmonic signal - which can account for the range of experimental configurations from which it has been detected - have remained elusive since the signal was first identified, by Esche in 1952.<br/><br/>This thesis describes cavitation observations in water, driven by propagating focused ultrasound fields typical of those used for medical therapy, using ultra high-speed shadowgraphic imaging at frame rates well in excess of the fundamental driving frequency. Moreover, single nanosecond laser pulses at energies below the plasma-forming threshold, are used to nucleate acoustic cavitation such that activity may be observed from the outset.<br/><br/>Clouds of densely packed and strongly interacting bubbles are seen to rapidly develop from the nucleation event. Within a few acoustic cycles, the cloud adopts a breathing mode response, with component bubbles collectively oscillating, approximately in-phase. The frequency of cloud oscillation matches that of the fundamental driving, however, at intervals dependent on the pressure amplitude of the driving, the cloud undergoes strong collapses, coincident to emitting a shockwave. In parallel to the high-speed imaging, a number of hydrophone detectors are used to collect the acoustic emissions, and confirm that periodic shockwaves mediate the subharmonic signals.<br/><br/>Acoustic detection of broadband, impulsive pressure transients is particularly susceptible to convolution with the frequency response of the detector. Accordingly, a PVdF needle hydrophone was calibrated for magnitude and phase from 125 kHz – 20 MHz, at the National Physical Laboratory. Detector deconvolution is demonstrated for shockwaves emitted during the formation of large plasma-mediated bubbles, each generated with a laser pulse of energy above the threshold. Similarly, the needle hydrophone is deconvolved from the emissions collected from acoustic cavitation clouds, indicating peak-positive pressure amplitudes for periodic shockwaves in the order of 10 kPa, at the distance detected. The development of a single element passive cavitation detector, dedicated to the detection of low-amplitude shockwaves with high sensitivity, is subsequently described. Detector construction, specifically the selection of matching and backing layers, is guided via a Finite Element model of the device, adapted to support simulated shockwave propagation. Detector performance is characterised with plasma bubble shockwaves, and evaluated for the detection of the subharmonic signal from a cavitation cloud, against a commercially available device."],"dc:identifier":["oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92","https://discovery.dundee.ac.uk/en/studentTheses/34568741-12be-4a90-8bbf-82f5900a4b92"],"dc:identifier.uri":["https://discovery.dundee.ac.uk/files/36510123/PhD_thesis_Keith_Johnston.pdf"],"dc:language":["eng"],"dc:publisher.department":["Imaging and Technology"],"dc:publisher.institution":["University of Dundee"],"dc:relation.isreferencedby":["https://discovery.dundee.ac.uk/en/studentTheses/34568741-12be-4a90-8bbf-82f5900a4b92"],"dc:title":["The cavitation subharmonic signal : mechanistic source and optimised detection"],"dc:type":["Thesis"],"dc:type.qualificationlevel":["Doctoral Thesis"],"dc:type.qualificationname":["Doctor of Philosophy"]},"updated_at":"2026-07-24T02:08:19Z"}