The Construction of Acoustic Waveforms from Plane Wave Components to Enhance Energy Transmission into Solid Media

The transmission of acoustic energy into solid materials is of interest in a wide range of applications, including ultrasonic imaging and nondestructive testing. However, the large impedance mismatch at the solid interface generally limits the transmission of incident acoustic energy. With the goal of improving the fraction of the energy transmitted into solid materials, the use of various bounded spatial profiles, including commonly-employed forms, such as Gaussian distributions, as well as newly-constructed profiles, has been investigated. The spatial profile is specified as the pressure amplitude distribution of the incident wave. Bounded acoustic beams are represented here as sums of harmonic plane waves, and results obtained for the optimal parameters of incident plane wave components are used to inform the construction of bounded wave profiles. The effect of the form of the spatial profile is investigated, with the total energy carried by the incident wave held constant as the profile is varied, and the relationship with the plane wave components which superimpose to form the bounded wave is discussed. Direct comparisons are made for the efficiency of the energy transmission of different profiles. The results reveal that, by tuning the form of the profile, substantial improvements in the total energy transmission can be achieved as compared to Gaussian and square waveforms

Enhanced Acoustic Transmission Into Dissipative Solid Materials Through The Use Of Inhomogeneous Plane Waves

A number of applications, for instance ultrasonic imaging and nondestructive testing, involve the transmission of acoustic energy across fluid–solid interfaces into dissipative solids. However, such transmission is generally hindered by the large impedance mismatch at the interface. In order to address this problem, inhomogeneous plane waves were investigated in this work for the purpose of improving the acoustic energy transmission. To this end, under the assumption of linear hysteretic damping, models for fluid–structure interaction were developed that allow for both homogeneous and inhomogeneous incident waves. For low-loss solids, the results reveal that, at the Rayleigh angle, a unique value of the wave inhomogeneity can be found which minimizes the reflection coefficient, and consequently maximizes the transmission. The results also reveal that with sufficient dissipation levels in the solid material, homogeneous incident waves yield lower reflection values than inhomogeneous waves, due to the large degrees of inhomogeneity inherent in the transmitted waves. Analytical conditions have also been derived which predict the dependence of the optimal incident wave type on the dissipation level and wave speeds in the solid medium. Finally, implications related to the use of acoustic beams of limited spatial extent are discussed

Use of Evanescent Plane Waves for Low-Frequency Energy Transmission Across Material Interfaces

The transmission of sound across high-impedance difference interfaces, such as an air-water interface, is of significant interest for a number of applications. Sonic booms, for instance, may affect marine life, if incident on the ocean surface, or impact the integrity of existing structures, if incident on the ground surface. Reflection and refraction at the material interface, and the critical angle criteria, generally limit energy transmission into higher-impedance materials. However, in contrast with classical propagating waves, spatially decaying incident waves may transmit energy beyond the critical angle. The inclusion of a decaying component in the incident trace wavenumber yields a nonzero propagating component of the transmitted surface normal wavenumber, so energy propagates below the interface for all oblique incident angles. With the goal of investigating energy transmission using incident evanescent waves, a model for transmission across fluid-fluid and fluid-solid interfaces has been developed. Numerical results are shown for the air-water interface and for common air-solid interfaces. The effects of the incident wave parameters and interface material properties are also considered. For the air-solid interfaces, conditions can be found such that no reflected wave is generated, due to impedance matching among the incident and transmitted waves, which yields significant transmission increases over classical incident waves

On The Use Of Evanescent Plane Waves For Low-Frequency Energy Transmission Across Material Interfaces

The transmission of airborne sound into high-impedance media is of interest in several applications. For example, sonic booms in the atmosphere may impact marine life when incident on the ocean surface, or affect the integrity of existing structures when incident on the ground. Transmission across high impedance-difference interfaces is generally limited by reflection and refraction at the surface, and by the critical angle criterion. However, spatially decaying incident waves, i.e., inhomogeneous or evanescent plane waves, may transmit energy above the critical angle, unlike homogeneous plane waves. The introduction of a decaying component to the incident trace wavenumber creates a nonzero propagating component of the transmitted normal wavenumber, so energy can be transmitted across the interface. A model of evanescent plane waves and their transmission across fluid-fluid and fluid-solid interfaces is developed here. Results are presented for both air-water and air-solid interfaces. The effects of the incident wave parameters (including the frequency, decay rate, and incidence angle) and the interfacial properties are investigated. Conditions for which there is no reflection at the air-solid interface, due to impedance matching between the incident and transmitted waves, are also considered and are found to yield substantial transmission increases over homogeneous incident waves

Low-Frequency Energy Transmission across Material Interfaces using Incident Evanescent Waves

Transmission of airborne sound into higher-impedance materials is of interest in a range of applications. Sonic booms, for example, may adversely affect marine life, if incident on the ocean surface, or may produce underground pressure waves potentially capable of impacting the integrity of existing structures, if incident on the ground surface. Energy transmission into higher-impedance materials is generally limited by significant reflection and refraction at the material interface, and by the critical angle criteria. However, unlike classical waves, spatially-decaying, or evanescent, incident waves can transmit energy at angles beyond the critical angle. When a decaying component is introduced into the incident trace wavenumber, the interaction at the interface produces a nonzero propagating component of the transmitted surface normal wavenumber, so energy is transmitted across the interface for all oblique incident angles. With the aim of investigating energy transmission using incident evanescent waves, a model for pressure and intensity transmission across the fluid-fluid and fluid-solid interfaces has been developed. Numerical results are given for common interfaces that include the air-water interface and typical air-solid interfaces, where the effects of the incident wave parameters and interface material properties are considered as well. For the air-solid interfaces, conditions can be tuned such that no reflected wave is generated at the interface, owing to impedance matching between the incident and transmitted waves, which yields considerable transmission increases over classical incident waves

Stress and Energy Transmission by Inhomogeneous Plane Waves into Dissipative Media

The characteristics of sound transmission into real, or dissipative, media differ from those of transmission into lossless media. In particular, when a plane wave in a fluid is incident upon a real, dissipative elastic material, the transmitted waves are in general inhomogeneous, even when the incident wave is itself homogeneous and incident at a sub-critical angle; and more significantly, energy transmission occurs even above the critical angle. In addition, for any real incidence angle, the parameters of an incident inhomogeneous wave may be tuned so that there is no reflection from the surface of a viscoelastic solid. That phenomenon may be exploited in applications requiring energy transmission into solids. In this work, the transmission of incident inhomogeneous, as well as homogeneous, acoustic waves into solid materials is characterized; a hysteretic damping model is assumed. Numerical results are presented for the transmitted stress and energy distributions for typical solid materials, including polymer-based solids. The conditions for total transmission, i.e., no reflection at the interface, are explored, where the propagation angle, degree of inhomogeneity, and frequency of the incident wave are varied for a given material. These investigations show substantial transmission gains in the vicinity of the zero of the reflection coefficient, compared to homogeneous incident waves

Correlation of Tire Intensity Levels and Passby Sound Pressure Levels

The object of the work reported here was to relate the acoustic intensity level measured near the contact patch of a driven tire on a passenger vehicle with the passby noise levels measured at a sideline microphone during coast and cruise conditions. Based on those measurements it was then possible to estimate the tire noise contribution to the passby level measured when the vehicle under test was accelerating. As part of this testing program, data was collected using five vehicles at fourteen passby sites in the United States: in excess of 800 data sets were obtained

Generation of Inhomogeneous Acoustic Waves Using an Array of Loudspeakers

In previous studies it has been shown that pressure fields created by inhomogeneous sound waves (waves which decay in a direction perpendicular to their propagation direction) are able to transmit energy into objects more effectively than ones created by conventional sound waves. This behavior may be useful in the detection of hidden explosive threats. To explore this, a device capable of constructing inhomogeneous waves is being developed. The proposed device is an acoustic array consisting of several high-frequency speakers. The speakers are independently driven to construct a desired inhomogeneous pressure field on a target surface. Inhomogeneous pressure fields were reconstructed across a span of decay parameters and standoff distances. Results show low root-mean-square errors at realistic levels of power consumption. These results imply that the device can recreate desired inhomogeneous pressure fields with high enough accuracy and low enough power consumption to test the energy transmission properties of inhomogeneous waves on mock explosives, which may be useful in applications related to improvised explosive device detection and defeat

“Go get a job right after you take a bath”: Occupy Wall Street as Matter Out of Place

Anthropological studies of purity reveal how notions of cleanliness influence political and social life. During its 2011 Zuccotti Park occupation in Lower Manhattan, Occupy Wall Street (OWS) contested spatial and symbolic manifestations of neoliberalism by re-inserting Otherness into sanitized and privatized space. But the demonstration provoked reactions from politicians and news media that entwined discourses of cleanliness and productivity (such as Newt Gingrich’s riposte to the protestors: “Go get a job right after you take a bath”). This ethnographic study argues that such representations had spatial and political effects. In particular, our account illuminates the plural agency of Occupiers, where resistance to depictions of dirt and idleness existed alongside the use of such discourses to discipline each other. We trace a discursive legacy of these events as notions of productivity and cleanliness have circulated within activist responses to 2012’s Superstorm Sandy and the 2014 Flood Wall Street mobilization.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1111/anti.1222

Experimental Measurements of Binder Wave Speeds using Wavenumber Decomposition

Prior work has provided few wave speed measurements for the binder materials commonly used with plastic- bonded energetics. Furthermore, those measurements that have been reported are largely based upon rudimentary, \u27pitch and catch\u27 methodologies, which involve sending a pulse from one transducer to another transducer at a set distance apart and measuring the time of flight. Given this, a more rigorous method for determining longitudinal and shear wave speeds in this important class of materials was desired. In this work, material wave speeds are recovered by measuring the vibrational response of a 2D line across the surface of a beam in response to a mechanical excitation and analyzing the data in the frequency-wavenumber domain