Methods of acoustic gas flux inversion-Investigation into the initial amplitude of bubble excitation.

Passive acoustic inversion techniques for measuring gas flux into the water column have the potential to be a powerful tool for the long-term monitoring and quantification of natural marine seeps and anthropogenic emissions. Prior inversion techniques have had limited precision due to lack of constraints on the initial amplitude of a bubble's excitation following its release into the water column ( R). R is determined by observing the acoustic signal of bubbles released from sediment in a controlled experiment and its use is demonstrated by quantifying the flux from a volcanic CO2 seep offshore Panarea (Italy), improving the precision by 78%.

A proof-of-concept study of the removal of early and late phase biofilm from skin wound models using a liquid acoustic stream.

Chronic wounds fail to progress through the normal stages of healing, with the largest remediable cause of chronicity being presence of a multi-species biofilm. Removal of biofilm from the wound environment is central to wound care. A device for mechanically removing biofilms from wounds has been devised. The removal is caused by small-scale liquid currents and shear, generated by acoustically activated microscopic air bubbles. These bubbles and acoustic waves are delivered onto the wound by a gentle liquid stream, allowing cleaning in situ and removal of debris in the run-off liquid. We have investigated if this liquid acoustic wound stream (LAWS) can remove bacterial biofilm from soft biological wound models and studied the effect of LAWS on the cellular tissues of the substrate. LAWS will efficiently remove early Pseudomonas aeruginosa biofilm from an artificial wound in a pig's trotter, 24 hours-mature biofilm of P. aeruginosa from a pre-wounded human full thickness skin model (EpiDerm FT), and 3-day mature biofilm of P. aeruginosa or Staphylococcus aureus from a porcine skin explant. Histological examinations of uninfected EpiDerm models that had been treated by LAWS and then stained with Haematoxylin and Eosin, demonstrated no damage to the human tissue, and wound diameter was smaller in the treated skin models compared with untreated samples. Immunofluorescence staining for cytokeratin 14 showed that keratinocytes had migrated further across the wound in the uninfected samples treated by LAWS. We discuss the implications for wound healing and propose further laboratory and clinical studies to demonstrate the removal of biofilm from patients with chronic leg ulcers and the impact on healing.

The response of common carp (Cyprinus carpio) to insonified bubble curtains.

Acoustic bubble curtains have been marketed as relatively low cost and easily maintained behavioural deterrents for fisheries management. Their energy efficiency can be improved by reducing air flow and exploiting bubble resonance. In a series of three flume experiments, we: (1) investigated the reactions of carp to a low air flow bubble curtain, (2) compared the effectiveness of resonant versus non-resonant insonified bubble curtains (for the same volume flux of gas injected through the nozzles) to deter passage, and determined the stimuli responsible for eliciting deterrence, and (3) included the effect of visual cues generated by the bubble curtain. This study showed that bubble curtains with a higher proportion of resonant bubbles deterred carp relatively better. Passage rejection was likely influenced by multiple cues at distances within a body length of the fish- specifically the rate of change in both particle motion and flow velocity caused by rising bubbles. All acoustic bubble curtains were less effective in the presence of daylight, suggesting that vision plays an important role at mediating carp reactions. We discuss the importance of ascertaining the bubble size distribution, in addition to the gas flow rate and aperture size, when characterising acoustically active bubble curtains.

Acoustic propagation in gassy intertidal marine sediments: An experimental study.

The need to predict acoustic propagation through marine sediments that contain gas bubbles has become increasingly important for civil engineering and climate studies. There are relatively few in situ acoustic wave propagation studies of muddy intertidal sediments, in which bubbles of biogenic gas (generally methane, a potent greenhouse gas) are commonly found. We used a single experimental rig to conduct two in situ intertidal acoustical experiments to improve understanding of acoustic remote sensing of gassy sediments, eventually including gas bubble size distributions. In the first experiment, we measured sediment sound speed and attenuation between four aligned hydrophones for a quasi-plane wave propagating along the array. The second experiment involved a focused insonified sediment volume created by two transducers emitting coincident sound beams at different frequencies that generated bubble-mediated acoustic signals at combination frequencies. The results from sediment core analyses, and comparison of in situ acoustic velocity and attenuation values with those of water-saturated sediments, together provide ample evidence for the presence of in situ gas bubbles in the insonified volumes of sediments. These datasets are suitable for linear and non-linear inversion studies that estimate in situ greenhouse gas bubble populations, needed for future acoustical remote sensing applications.

Group behavior and tolerance of Eurasian minnow (Phoxinus phoxinus) in response to tones of differing pulse repetition rate.

Behavioral guidance systems are commonly used in freshwater fish conservation. The biological relevance of sound to fish and recorded responses to human-generated noise supports the viability of the use of acoustics as an effective stimulus in such technologies. Relatively little information exists on the long-term responses and recovery of fish to repeated acoustic exposures. In a controlled laboratory study, the response and tolerance of Eurasian minnow (Phoxinus phoxinus) shoals to tonal signals (150 Hz of 1 s pulse duration) differing only in temporal characteristics ("continuous," "slow," "intermediate," or "fast" pulse repetition rate) were investigated. In comparison to independent control groups, fish increased their mean group swimming speed, decreased inter-individual distance, and became more aligned in response to the onset of all four acoustic treatments. The magnitude of response, and time taken to develop a tolerance to a treatment differed according to pulse repetition rate. Groups were found to have the greatest and longest lasting response to tone sequences tested in this study when they were pulsed at an intermediate rate of 0.2 s-1. This study illustrates the importance of understanding the response of fish to acoustic signals, and will assist toward the development of longer-term effective acoustic guidance systems.

Underwater radiated noise from hydrofoils in coastal water.

Underwater noise from commercial shipping throughout the oceans has been increasing over the past decades and the environmental impact of this noise remains an area of great uncertainty. This has led to the measurement of noise from commercial vessels in order to understand the impacts that these vessels may engender. Hydrofoils are used by ferries in various locations around the world and locally may be a significant contributing factor of the soundscape. However, the investigation on underwater radiated noise from the activity of hydrofoils in the field has not been widely conducted. This article is an attempt to characterize the noise from hydrofoils in the field. Detailed measurements in the coastal water close to the Panarea port, Italy are reported. The investigation describes the broadband frequency spectrum with the main energy approximately centered on 30-130 Hz but covering frequencies up to tens of kHz. A key result was that the spectrum of the noise varied between the three stages (displacement, transition, and foiling) of the hydrofoils heading into or out of the port.

Three-dimensional finite element simulation of acoustic propagation in spiral bubble net of humpback whale.

In 2004, Leighton hypothesized that the acoustic calls emitted by humpback whales when feeding using bubble nets, may enhance the effectiveness of the net in confining prey (such as herring) by forming a "wall of sound" with a quiet zone within. Modelling of the acoustics of this phenomenon was previously restricted to 2D; this paper conducts a 3D model of the propagation of signals resembling those emitted by humpback whales when bubble netting, projected into an upward spiral bubble net which data to date suggest is the accurate form for the bubble net in 3D space. In this study, the feeding calls were analyzed in the time-frequency domain to extract acoustic information sufficient to allow modeling of the resulting spatial distribution of acoustic pressure and particle velocity, and how they vary over the duration of the call. Sound propagation in the bubble net was described by using a linear steady-state formulation for an effective medium of bubbly water. Using the predicted attenuation, phase velocity and density in bubbly water, a 3D finite element model was constructed to numerically simulate the upward-spiral bubble net which consists of a mixture of bubbles that exhibit a range of radii. The acoustic pressure field and particle motion field were both calculated within the bubble net. The simulation results show that the energy of the whale feeding call could be effectively focused in the bubble net, generating intensive sound pressure and particle motion fields in the bubbly arm of the net, but with some "quiet" regions closer to the center of the net, as Leighton hypothesized. Furthermore, when the hearing ability of herring is taken into consideration, the results suggest that this acoustic focusing effect could be a plausible factor in trapping them in the bubble net. It also allows speculation on the possible enhancements that the time-varying nature of the call during feeding could give to the whale in this mechanism for the bubble net feeding by humpback whales.

Ultrasound in air-Guidelines, applications, public exposures, and claims of attacks in Cuba and China.

This editorial introduces a Special Issue of the Journal of the Acoustical Society of America, on "Ultrasound in Air." In this Special Issue, one paper covers ways of categorizing the ultrasonic regimes, and three papers cover human effects. One of those three, plus five others, constitute the six papers that report on the measured outputs of commercial devices. Two cover calibration, and the final three papers cover novel applications. This editorial outlines the context in which these papers provide individual studies, including the development of technology and guidelines for safe exposure, and ending with an analysis of what is currently known about claims of sonic attacks on embassy staff in Cuba and China.

Public exposure to ultrasound and very high-frequency sound in air.

Recent work showing the presence of a new generation of ultrasound (US) sources in public places has reopened the debate about whether there are adverse effects of US on humans, and has identified weaknesses in standards and exposure guidelines. Systems that rely on very high-frequency sound (VHFS) and US include public-address voice-alarm (PAVA) systems (whose operational status is often monitored using tones at ∼20 kHz) and pest deterrents. In this study, sound pressure levels (SPLs) produced by 16 sources that were either publically available or installed in busy public spaces were measured. These sources were identified through a citizen science project, wherein members of the public were asked to provide smartphone recordings of VHFS/US sources. With measurements made in realistic listening positions, pest deterrents were found that produced levels of up to 100 dB SPL at ∼20 kHz, and a hand dryer was found to produce 84 dB SPL at 40 kHz. PAVA systems were found to emit lower levels of up to 76 dB SPL at ∼20 kHz. Pest deterrents measured breach recommended safe listening limits for public exposure for people who are nearby even for relatively short periods.

Effects of very high-frequency sound and ultrasound on humans. Part II: A double-blind randomized provocation study of inaudible 20-kHz ultrasound.

Some people have reported symptoms such as nausea, dizziness, and headaches that they attribute to ultrasound (US) emitted by devices in public places. The primary aim of the present study was to investigate whether inaudible US can provoke adverse symptoms compared to a sham presentation, under double-blind conditions. A second aim was to investigate whether the expectation of US being present could provoke adverse symptoms (a nocebo response). The US stimulus was a 20 kHz tone presented continuously for 20 min set to at least 15 dB below the participants' detection threshold, giving a typical sound pressure level (SPL) of 84 dB. No evidence that US provoked symptoms was found, but there was evidence of small nocebo effects. A case study on an individual with high self-reported sensitivity to US gave similar results. The present study did not reproduce the severe symptoms reported previously by some members of the public; this may be due to the SPL or duration of the stimulus, or strength of the nocebo stimulus. These findings cannot be used to predict outcomes from exposures to sounds that are audible to the individual in question, or to sounds with higher SPLs, longer durations, or different frequency content.

Effects of very high-frequency sound and ultrasound on humans. Part I: Adverse symptoms after exposure to audible very-high frequency sound.

Various adverse symptoms resulting from exposure to very high-frequency sound (VHFS) and ultrasound (US) have previously been reported. This study aimed to establish whether these symptoms are experienced under controlled laboratory conditions and are specific to VHFS/US. To do this, participants were exposed to VHFS/US (at frequencies between 13.5 and 20 kHz and sound pressure levels between 82 and 92 dB) and to a 1 kHz reference stimulus, both at 25 dB above their hearing threshold. The VHFS/US and reference stimuli were presented 4 times, each time for 3 min, during which participants performed a sustained attention task, rated their symptom severity, and had their galvanic skin response (GSR) measured to assess their level of anxiety. Prior to exposure, participants were assigned either to a symptomatic or an asymptomatic group, based on their prior history of symptoms that they attributed to VHFS/US. In both groups, overall discomfort ratings were higher in the VHFS/US condition than the reference condition. In the symptomatic group only, difficulty concentrating and annoyance were also rated higher in the VHFS/US than the reference condition. No difference between the two stimulus conditions was seen in performance on the attention task or on average GSRs for either group.

Measurements of ultrasonic deterrents and an acoustically branded hairdryer: Ambiguities in guideline compliance.

Acoustic radiation from three commercial pest deterrents and two hair dryers were measured in an anechoic chamber. The deterrents were chosen because the frequency range at which they emit the most energy is either in the very high-frequency sound band (11.2-17.8 kHz) or the ultrasound band (greater than 17.8 kHz). These are sources that may be heard by a subset of the general population, with the young typically having better high frequency sensitivity. A hairdryer reported to increase the frequency of the motor noise above the audible hearing range was compared with a standard hairdryer. The outputs of the deterrents are compared against six international regulations and guidelines for audible and ultrasound exposure. Multiple ambiguities in the application of these guidelines are discussed. These ambiguities could lead to a device being considered as in compliance despite unconventionally high levels. Even if a device measured here meets a guideline, actual exposures can exceed those taken here and may therefore breach guidelines if the listener is closer to the device or reflections increase the exposure level.

Acoustic wave propagation in gassy porous marine sediments: The rheological and the elastic effects.

The preceding paper in this series [Mantouka, Dogan, White, and Leighton, J. Acoust. Soc. Am. 140, 274-282 (2016)] presented a nonlinear model for acoustic propagation in gassy marine sediments, the baseline for which was established by Leighton [Geo. Res. Lett. 34, L17607 (2007)]. The current paper aims further advancement on those two studies by demonstrating the particular effects of the sediment rheology, the dispersion and dissipation of the first compressional wave, and the higher order re-scattering from other bubbles. Sediment rheology is included through the sediment porosity and the definition of the contact interfaces of bubbles with the solid grains and the pore water. The intrinsic attenuation and the dispersion of the compressional wave are incorporated using the effective fluid density model [Williams, J. Acoust. Soc. Am. 110, 2276-2281 (2001)] for the far field (fully water-saturated sediment). The multiple scattering from other bubbles is included using the method of Kargl [J. Acoust. Soc. Am. 11, 168-173 (2002)]. The overall nonlinear formulation is then reduced to the linear limit in order to compare with the linear theory of Anderson and Hampton [J. Acoust. Soc. Am. 67, 1890-1903 (1980)], and the results for the damping coefficients, the sound speed, and the attenuation are presented.

Does Masking Matter? Shipping Noise and Fish Vocalizations.

Shipping creates large near-field background noises at levels similar to or higher than fish vocalizations and in the same critical bandwidths. This noise has the potential to "mask" biologically important signals and prevent fish from hearing them; any interference with the detection and recognition of sounds may impact fish survival. The Lombard effect, whereby vocalizations are altered to reduce or exclude masking effects, is an adaptation that has been observed in mammals and birds. Research is needed to establish whether the Lombard effect occurs in fish to gain a better understanding of the implications of noise pollution on fish populations.

Sonar equations for planetary exploration.

The set of formulations commonly known as "the sonar equations" have for many decades been used to quantify the performance of sonar systems in terms of their ability to detect and localize objects submerged in seawater. The efficacy of the sonar equations, with individual terms evaluated in decibels, is well established in Earth's oceans. The sonar equations have been used in the past for missions to other planets and moons in the solar system, for which they are shown to be less suitable. While it would be preferable to undertake high-fidelity acoustical calculations to support planning, execution, and interpretation of acoustic data from planetary probes, to avoid possible errors for planned missions to such extraterrestrial bodies in future, doing so requires awareness of the pitfalls pointed out in this paper. There is a need to reexamine the assumptions, practices, and calibrations that work well for Earth to ensure that the sonar equations can be accurately applied in combination with the decibel to extraterrestrial scenarios. Examples are given for icy oceans such as exist on Europa and Ganymede, Titan's hydrocarbon lakes, and for the gaseous atmospheres of (for example) Jupiter and Venus.

An electrochemical and high-speed imaging study of micropore decontamination by acoustic bubble entrapment.

Electrochemical and high-speed imaging techniques are used to study the abilities of ultrasonically-activated bubbles to clean out micropores. Cylindrical pores with dimensions (diameter × depth) of 500 µm × 400 µm (aspect ratio 0.8), 125 µm × 350 µm (aspect ratio 2.8) and 50 µm × 200 µm (aspect ratio 4.0) are fabricated in glass substrates. Each pore is contaminated by filling it with an electrochemically inactive blocking organic material (thickened methyl salicylate) before the substrate is placed in a solution containing an electroactive species (Fe(CN)6(3-)). An electrode is fabricated at the base of each pore and the Faradaic current is used to monitor the decontamination as a function of time. For the largest pore, decontamination driven by ultrasound (generated by a horn type transducer) and bulk fluid flow are compared. It is shown that ultrasound is much more effective than flow alone, and that bulk fluid flow at the rates used cannot decontaminate the pore completely, but that ultrasound can. In the case of the 125 µm pore, high-speed imaging is used to elucidate the cleaning mechanisms involved in ultrasonic decontamination and reveals that acoustic bubble entrapment is a key feature. The smallest pore is used to explore the limits of decontamination and it is found that ultrasound is still effective at this size under the conditions employed.

Acoustic attenuation, phase and group velocities in liquid-filled pipes III: nonaxisymmetric propagation and circumferential modes in lossless conditions.

Equations for the nonaxisymmetric modes that are axially and circumferentially propagating in a liquid-filled tube with elastic walls surrounded by air/vacuum are presented using exact elasticity theory. Dispersion curves for the axially propagating modes are obtained and verified through comparison with measurements. The resulting theory is applied to the circumferential modes, and the pressures and the stresses in the liquid-filled pipe are calculated under external forced oscillation by an acoustic source. This provides the theoretical foundation for the narrow band acoustic bubble detector that was subsequently deployed at the Target Test Facility (TTF) of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL), TN.

Demonstration comparing sound wave attenuation inside pipes containing bubbly water and water droplet fog.

This paper describes a demonstration and explanation of sound absorption in water due to bubbles, and in air due to a fog of water droplets. It is suitable for 10-12 year olds, but the paper indicates where further exploration of the simplifications in the explanations provided for that age range would allow the demonstration to be used for undergraduate and Masters-level teaching. Applications to submarines, the space shuttle, and neutron generators are described. The demonstration is designed for transportation in a family-sized car.

Acoustic attenuation, phase and group velocities in liquid-filled pipes II: simulation for Spallation Neutron Sources and planetary exploration.

This paper uses a finite element method (FEM) to compare predictions of the attenuation and sound speeds of acoustic modes in a fluid-filled pipe with those of the analytical model presented in the first paper in this series. It explains why, when the predictions of the earlier paper were compared with experimental data from a water-filled PMMA pipe, the uncertainties and agreement for attenuation data were worse than those for sound speed data. Having validated the FEM approach in this way, the versatility of FEM is thereafter demonstrated by modeling two practical applications which are beyond the analysis of the earlier paper. These applications model propagation in the mercury-filled steel pipework of the Spallation Neutron Source at the Oak Ridge National Laboratory (Tennessee), and in a long-standing design for acoustic sensors for use on planetary probes. The results show that strong coupling between the fluid and the solid walls means that erroneous interpretations are made of the data if they assume that the sound speed and attenuation in the fluid in the pipe are the same as those that would be measured in an infinite volume of identical fluid, assumptions which are common when such data have previously been interpreted.