Many centuries ago glass blowers observed that sound could be generated when blowing through a hot bulb from the cold end of a narrow tube. This phenomenon is a result of thermoacoustic oscillations: a pressure wave propagating in a compressible fluid (e.g. air) can sustain or amplify itself when being provided heat. To date, thermoacoustic engines and refrigerators have had remarkable impacts on many industrial applications.
After many centuries of thermoacoustic science in fluids, it seems natural to wonder if such a mechanism could also exist in solids. Is it reasonable to conceive thermoacoustics of solids? Can a metal bar start vibrating when provided heat?
The study of the effects of heat on the dynamics of solids has a long and distinguished history. The theory of thermoelasticity, which explains the mutual interaction between elastic and thermal waves, has been an active field of research since the 1950s. However, the classical theory of thermoelasticity does not address instability phenomena that can arise when considering the motion of a solid in the presence of a thermal gradient. In an analogous way to fluids, a solid element contracts when it cools down and expands when it is heated up. If the solid contracts less when cooled and expands more when heated, the resulting motion will grow with time. In other terms, self-sustained vibratory response of a solid could be achieved due to the application of heat. Such a phenomenon would represent the exact counterpart in solids of the well-known thermoacoustic effect in fluids.
By using theoretical models and numerical simulations, our study indicates that a small mechanical perturbation in a thin metal rod can give rise to sustained vibrations if a small segment of the rod is subject to a controlled temperature gradient. The existence of this physical phenomenon in solids is quite remarkable, so one might ask why it was not observed before despite the science of thermoacoustics have been known for centuries.
“Figure 1. The sketch of the solid-state thermoacoustic device and the plot of the self-amplifying vibratory response.”
It appears that, under the same conditions of mechanical excitation and temperature, a solid tends to be more “stable” than a fluid. The combination of smaller pressure oscillations and higher dissipative effects (due to structural damping) in solids tends to suppress the dynamic instability that is at the origin of the thermoacoustic response. Our study shows that, with a proper design of the thermoacoustic device, these adverse conditions can be overcome and a self-sustained response can be obtained. The interface conditions are also more complicated to achieve in a solid device and dictates a more elaborate design.
Nonetheless, this study shows clear theoretical evidence of the existence of the thermoacoustic oscillations in solids and suggests that applications of solid-state engines and refrigerators could be in reach within the next few years.
2pNS8 – Noise Dependent Coherence-Super Gaussian based Dual Microphone Speech Enhancement for Hearing Aid Application using Smartphone Nikhil Shankar
Underwater sound from recreational swimmers, divers, surfers, and kayakers
Christine Erbe – Curtin University, email@example.com
Miles Parsons – Curtin University and Australian Institute of Marine Science, firstname.lastname@example.org
Alec Duncan – Curtin University, A.J.Duncan@curtin.edu.au
Klaus Lucke – Curtin University and JASCO Applied Sciences, Klaus.email@example.com
Alexander Gavrilov – Curtin University, A.Gavrilov@curtin.edu.au
Kim Allen – THHINK Autonomous Systems, firstname.lastname@example.org
Centre for Marine Science & Technology, Curtin University, Bentley, 6102 Western Australia, AUSTRALIA|
Popular version of paper 1aAO5
Presented Monday morning, May 7, 2018, 11:10-11:25 a.m., GREENWAY A
175th ASA Meeting, Minneapolis, MN
Video 1: Underwater video and sound recording of different water sports activities.
Underwater sound contains a lot of information about the source that produces it. Ships, for example, have a characteristic sound signature underwater, by which the type of vessel, its speed, and its route can easily be determined. In some cases, individual vessels can be identified by their sound and information about the type of propulsion, operational mode, and load can be deduced and maintenance issues (e.g., relating to the propeller) can be picked out. Similarly, just by listening, we can study marine life from whales to fishes and shrimp; we can track their movements; monitor their behavior; and in the case of some species of dolphins, even say which family and individuals are there. Sound is an important commodity for marine life; marine mammals as well as fishes, for example, communicate through sound, sense their environment, navigate, and forage—all mediated by sound.
Given the important role sound plays in the life functions of marine fauna, the potential interference by man-made noise has received growing interest. Noise may disrupt animal behavior, affect their hearing abilities, mask communication, cause stress, and in extreme cases cause physical and physiological damage that can ultimately be fatal. The research and management focus has—quite sensibly—been on the strongest sources, such as geophysical surveys or coastal and marine construction. Non-motorised activities are expected quieter and have hardly been studied.
Within the framework of an underwater acoustic project, we had the opportunity to record ourselves and friends performing a number of recreational water sports activities in a quiet Olympic pool, with all surrounding machinery (including cleaning pumps) switched off [1,2]. Specifically, different people were filmed and acoustically recorded while swimming breaststroke, backstroke, freestyle, and butterfly; snorkeling with and without fins; paddling a surfboard with alternating single or double arms; scuba diving; kayaking; and jumping into the pool. Sound pressure and water particle velocity were measured.
Activities that occurred at the surface, involved repeatedly piercing the surface and hence created bubble clouds were the strongest sound generators. Received levels were 110-131 dB re 1 µPa (10-16,000 Hz) for all of the activities at the closest point of approach (1 m). Levels were lower than those found in environmental noise regulations, but were clearly above ambient noise levels recorded off beaches and hence predicted audible by marine fauna over tens to hundreds of meters.
The characterization and quantification of underwater sound from recreational water sports has applicability well beyond environmental management. For example, just by listening to the recordings, it is easy to identify who of the volunteers was in the pool and which activity (including which style of swimming, with or without fins, with single versus double arms, etc.) was performed. The better (i.e., faster and smoother) swimmers were the quieter swimmers. Underwater sound might be a useful tool to assess professional or competitive swimmer performance and can be used for security monitoring of pools.
 C. Erbe, M. Parsons, A. J. Duncan, K. Lucke, A. Gavrilov and K. Allen, “Underwater particle motion (acceleration, velocity and displacement) from recreational swimmers, divers, surfers and kayakers,” Acoustics Australia 45, 293-299 (2017). doi: 10.1007/s40857-017-0107-6
 C. Erbe, M. Parsons, A. J. Duncan and K. Allen, “Underwater acoustic signatures of recreational swimmers, divers, surfers and kayakers,” Acoustics Australia 44 (2), 333-341 (2016). doi: 10.1007/s40857-016-0062-7
Acoustic Cloaking Using the Principles of Active Noise Cancellation
Jordan Cheer – email@example.com
Institute of Sound and Vibration Research
University of Southampton
Popular version of paper 4pEA7, “Cancellation, reproduction and cloaking using sound field control”
Presented Thursday morning, December 1, 2016
172nd ASA Meeting, Honolulu
Loudspeakers are synonymous with audio reproduction and are widely used to play sounds people want to hear. Loudspeakers have also been used for the opposite purpose, to attenuate noise that people may not want to hear. Active noise cancellation technology is an example of this, which combines loudspeakers, microphones and digital signal processing to adaptively control unwanted noise sources .
More recently, the scientific community has focused attention on controlling and manipulating sound fields to acoustically cloak objects, with the aim of rendering objects acoustically invisible. A new class of engineered materials called metamaterials have already demonstrated this ability . However, acoustic cloaking has also been demonstrated using methods based on both sound field reproduction and active noise cancellation . Despite its demonstration there has been limited research exploring the physical links between acoustic cloaking, active noise cancellation and sound field reproduction. Therefore, we began exploring these links with the aim of developing active acoustic cloaking systems that build on the advanced knowledge of implementing both audio reproduction and active noise cancellation systems.
Acoustic cloaking attempts to control the sound scattered from a solid object. Using a numerical computer simulation, we therefore investigated the physical limits on active acoustic cloaking in the presence of a rigid scattering sphere. The scattering sphere, shown in Figure 1, was surrounded by an array of sources (loudspeakers) used to control the sound field, shown by the black dots surrounding the sphere in the figure. In the first instance we investigated the effect of the scattering sphere on a simple sound field.
Looking at a horizontal slice through the simulated sound field without a scattering object, shown in the second figure, modifications by the presence of the scattering sphere are obvious in comparison to the same slice when the object is present, seen in third figure. Scattering from the sphere distorts the sound field, rendering it acoustically visible.
Figure 1 – The geometry of the rigid scattering sphere and the array of sources, or loudspeakers used to control the sound field (black dots).
Figure 3 – The sound field produced when an acoustic plane wave is incident on the rigid scattering sphere.
To understand the physical limitations on controlling this sound field, and thus implementing an active acoustic cloak, we investigated the ability of the array of loudspeakers surrounding the scattering sphere to achieve acoustic cloaking . In comparison to active noise cancellation, rather than attempting to cancel the total sound field, we only attempted to control the scattered component of the sound field and thus render the sphere acoustically invisible.
With active acoustic cloaking, the sound field appears undisturbed, where the scattered component has been significantly attenuated and results in a field, shown in the fourth figure, that is indistinguishable from the object-less simulation of the Figure 2.
Figure 4 – The sound field produced when active acoustic cloaking is used to attempt to cancel the sound field scattered by a rigid scattering sphere and thus render the scattering sphere acoustically ‘invisible’.
Our results indicate active acoustic attenuation can be achieved using an array of loudspeakers surrounding a sphere that would otherwise scatter sound detectably. In this and related work, further investigations showed that the performance of active acoustic cloaking is most effective when the loudspeakers are in close proximity to the object being cloaked. This may lead to design concepts involving acoustic sources embedded in objects for acoustic cloaking or control of the scattered sound field.
Future work will attempt to demonstrate the performance of active acoustic cloaking experimentally and overcome significant challenges of not only controlling the scattered sound field, but detecting it using an array of microphones.
 P. Nelson and S. J. Elliott, Active Control of Sound, 436 (Academic Press, London) (1992).
 L. Zigoneanu, B.I. Popa, and S.A. Cummer, “Three-dimensional broadband omnidirectional acoustic ground cloak”. Nat. Mater, 13(4), 352-355, (2014).
 E. Friot and C. Bordier, “Real-time active suppression of scattered acoustic radiation”, J. Sound Vib., 278, 563–580 (2004).
 J. Cheer, “Active control of scattered acoustic fields: Cancellation, reproduction and cloaking”, J. Acoust. Soc. Am., 140 (3), 1502-1512 (2016).