Acoustic Transparency Research Articles

Unique challenges for hearing protection in military noise environments.

Military noise environments, and, in particular, the noise environments faced by dismounted soldiers on the battlefield, are characterized by wide variations in ambient level. Situations can quickly and unexpectedly change from quiet conditions where the sound of a snapping twig might alert the listener to hostile enemy activity to extreme noise conditions where firefights, explosions, or loud machinery create levels of noise that can cause hearing loss in a matter of minutes or seconds. This poses a unique challenge for the designers of military hearing protection, who must produce systems that provide enough comfort and acoustic transparency to convince users to consistently wear them in quiet conditions to ensure that they will be in place if an unexpected blast or other potentially damaging noise exposure occurs. In this talk, we discuss some of the issues that can influence the willingness of listeners to wear hearing protection for extended periods in quiet environments, including acoustic factors that have been shown to influence situational awareness while wearing hearing protection. Additionally, lessons on the factors impacting the comfort of hearing devices drawn from studies of how consistently hearing aids are worn by hearing impaired listeners will be discussed.

Acoustic self-induced transparency for transverse waves in a system with resonant and quasi-resonant transitions

A theoretical analysis of acoustic self-induced transparency is presented for transverse elastic waves propagating perpendicular to an applied magnetic field through a crystal with spin-3/2 paramagnetic impurities. The interaction between an acoustic pulse and magnetic field is described by Maxwell-Bloch-type equations for a system with transitions inhomogeneously broadened because of a quadrupole Stark shift. If the pulse carrier frequency is resonant with one transition and quasi-resonant with another transition, then the evolution of a one-dimensional pulse is described by an integrable Konno-Kameyama-Sanuki (KKS) equation. The underlying physics of its soliton solution and the corresponding behavior of the medium are analyzed. Self-focusing and self-trapping conditions are found for a pulse of finite transverse size. In the latter regime, the pulse stretches along the propagation direction, transforming into a “hollow bullet,” while its transverse size remains constant.

Analysis of Gene Transfer Rate with Immobilized Retroviral Vectors

Efficient delivery of transgenes into the cell nucleus by retroviral vectors in a static culture system is limited by the intrinsic features of incompetent retroviruses (i.e., thermodynamically unstable envelope proteins and low titers). Although several physicochemical approaches (e.g., adding polycationic polymer and applying magnetic force) have been reported to augment the retroviral gene transfer rate, none are suitable for scaling up to a setting for clinical use. The study of using acoustic fields with the form of standing waves has recently been reported to be a feasible way to enhance retroviral gene delivery efficiency in large-scale settings. The concept of using ultrasound standing-wave fields to increase retrovirus-mediated gene transfer is based on quickly established cell bands on acoustic nodal planes as nucleating sites to capture unstable colloidlike retroviruses. In this study, instead of having retroviral nanoparticles circulated between nodal planes, we proposed to immobilize retroviruses onto acoustic transparent films arranged in an acoustic chamber. Then, cells inoculated into the acoustic chamber can be driven by the primary radiation forces to the retrovirus-coated films that are constructed on the nodal planes. To obtain the optimal time of immobilizing retroviruses onto the acoustic transparent film prior to the inception of acoustic fields, we developed a retroviral diffusion-reaction model to describe such a static retroviral system. Analysis of viral transport model has its merit to guide experimental design for attaining high gene transfer efficiency.

Acoustic transparency in two-dimensional sonic crystals

Acoustic transparency is studied in two-dimensional sonic crystals consisting of hexagonal distributions of cylinders with continuously varying properties. The transparency condition is achieved by selectively closing the acoustic bandgaps, which are governed by the structure factor of the cylindrical scatterers. It is shown here that cylindrical scatterers with the proposed continuously varying properties are physically realizable by using metafluids based on sonic crystals. The feasibility of this proposal is analyzed by a numerical experiment based on multiple scattering theory.

Acoustical Coupling of Lizard Eardrums

Lizard ears are clear examples of two-input pressure-difference receivers, with up to 40-dB differences in eardrum vibration amplitude in response to ipsi- and contralateral stimulus directions. The directionality is created by acoustical coupling of the eardrums and interaction of the direct and indirect sound components on the eardrum. The ensuing pressure-difference characteristics generate the highest directionality of any similar-sized terrestrial vertebrate ear. The aim of the present study was to measure the gain of the direct and indirect sound components in three lizard species: Anolis sagrei and Basiliscus vittatus (iguanids) and Hemidactylus frenatus (gekkonid) by laser vibrometry, using either free-field sound or a headphone and coupler for stimulation. The directivity of the ear of these lizards is pronounced in the frequency range from 2 to 5 kHz. The directivity is ovoidal, asymmetrical across the midline, but largely symmetrical across the interaural axis (i.e., front-back). Occlusion of the contralateral ear abolishes the directionality. We stimulated the two eardrums with a coupler close to the eardrum to measure the gain of the sound pathways. Within the frequency range of maximal directionality, the interaural transmission gain (compared to sound arriving directly) is close to or even exceeds unity, indicating a pronounced acoustical transparency of the lizard head and resonances in the interaural cavities. Our results show that the directionality of the lizard ear is caused by the acoustic interaction of the two eardrums. The results can be largely explained by a simple acoustical model based on an electrical analog circuit.

Acoustic transparency by switching of phononic band gaps

Acoustic transparency is here studied in two‐dimensional phononic crystals made of hexagonal arrangements of layered cylinders. These layered cylinders, which have their acoustical properties radially dependent, can be achieved by using the recently proposed acoustical metamaterials [D. Torrent and J. Sanchez‐Dehesa, New J. Phys., 9, 323 (2007)]. The transparency condition is here achieved by switching of the phononic band gaps, which is governed by the scattering form factor of the acoustic parameters (bulk modulus and sound speed). Various dependence have been studied. The feasibility of this proposal is demonstrated by multiple scattering simulations of the proposed metamaterials [Work supported by NSF of USA and MEC of Spain.]

Modeling of influencing parameters in active noise control on an enclosure wall

This paper investigates, by means of a numerical model, the possibility of using an active noise barrier to virtually reduce the acoustic transparency of a partition wall inside an enclosure. The room is modeled with the image method as a rectangular enclosure with a stationary point source; the active barrier is set up by an array of loudspeakers and error microphones and is meant to minimize the squared sound pressure on a wall with the use of a decentralized control. Simulations investigate the effects of the enclosure characteristics and of the barrier geometric parameters on the sound pressure attenuation on the controlled partition, on the whole enclosure potential energy and on the diagonal control stability. Performances are analyzed in a frequency range of 25–300 Hz at discrete 25 Hz steps. Influencing parameters and their effects on the system performances are identified with a statistical inference procedure. Simulation results have shown that it is possible to averagely reduce the sound pressure on the controlled partition. In the investigated configuration, the surface attenuation and the diagonal control stability are mainly driven by the distance between the loudspeakers and the error microphones and by the loudspeakers directivity; minor effects are due to the distance between the error microphones and the wall, by the wall reflectivity and by the active barrier grid meshing. Room dimensions and source position have negligible effects. Experimental results point out the validity of the model and the efficiency of the barrier in the reduction of the wall acoustic transparency.

Effects of soliton dynamics of acoustic pulses in a paramagnetic crystal

Nonlinear dynamics of longitudinal-transverse acoustic pulses in the deformed cubic crystal containing a paramagnetic impurity with effective spin S = 1 is theoretically investigated. Soliton solutions of systems of equations describing the propagation of extremely short and ultrashort pulses at an arbitrary angle to the direction of external deformation parallel to the crystal symmetry axis are obtained. Classification of longitudinal-transverse soliton types and resonant acoustic transparency regimes is given.

Acoustic wave transparency for a multilayered sphere with acoustic metamaterials

By analogy with the electromagnetic wave, the acoustic transparency phenomenon is analyzed for a multilayered sphere with acoustic metamaterials. The neutral-inclusion concept is used to predict the transparency conditions in the quasistatic case, which are further confirmed by a full-wave analysis. The mechanism of the transparency is based on lowering the total-scattering cross section of the composite sphere. It is found that, to improve the transparency, the angle-dependent scattering cross section must also be minimized for all directions.

A heat-driven thermoacoustic cryocooler capable of reaching below liquid hydrogen temperature

A “double-gas acoustic amplifier” is introduced to couple a thermoacoustic heat engine and a two-stage pulse tube cooler in this paper. Compared with previous acoustic amplifiers, this new acoustic amplifier maintains the function of amplification for pressure amplitude. In particular, the novel acoustic amplifier with a reservoir makes it possible to install an acoustic transparent but gas blocking elastic membrane between the engine and the cooler. Thus, the engine can use nitrogen as the working gas to work at low frequency; and meanwhile, the cooler can still use helium as the working gas to maintain its high performance. With this new amplifier, the cooling temperature of a two-stage pulse tube cooler driven by an energy-focused thermoacoustic engine reached 18.7 K.

Are binaural cues available for sound source localization in teleost fishs?

When reading the review literature regarding directional hearing or sound source localization by fishs, one usually sees a list of reasons why fish cannot use binaural cues. The assumptions are that teleost fishs cannot detect left versus right differences in arrival times (due to the speed of sound in water), amplitude differences (due to the closeness of the ears and the acoustic transparency of the tissues), nor phase differences (due to the long wavelengths of the frequencies most fish can hear best). However, there is some behavioral evidence that fishs can distinguish the direction of a sound source, if not the actual location. We will present evidence that there are potential binaural cues that might be available given left versus right differences in auditory input to the brain that are the result of both anatomical organization and physiological mechanisms. In addition, sharpened directional responses are present in the medulla and the midbrain, which are likely to be due to binaural computations. [Work funded by the National Institutes of Health, NIDCD.]

Modes of nonlinear acoustic transparency in the strained paramagnetic crystal

The propagation of transverse-longitudinal acoustic pulses through a strained cubic crystal containing resonant paramagnetic impurities with effective spin S =1 is investigated. It is supposed that the pulses propagate under arbitrary angle with respect to the direction of the external static deformation parallel to the fourth-order symmetry axis. In this geometry, both the transverse and longitudinal components of the acoustic field have high-frequency and zero-frequency spectral components. We show that a pulse can propagate in modes different from the acoustic self-induced transparency. In particular, a pulse propagating in the mode of an acoustic self-induced supertransparency substantially changes the populations of the spin sublevels, but its group velocity remains almost equal to the linear velocity of the sound. If a pulse propagates in the acoustic extraordinary transparency mode, then its group velocity is substantially lower while the sublevel populations remain virtually invariant. Also, the modes of propagation under conditions of weakly excited spin transitions and large detuning of the pulse high-frequency components are identified.

Nonlinear acoustic transparency phenomena in strained paramagnetic crystals

Nonlinear dynamics of longitudinal acoustic pulses propagating in a strained paramagnetic cubic crystal at low temperature is analyzed. The direction of constant uniform strain is parallel to one of the fourth-order symmetry axes. Effective-spin 1 ions are considered as paramagnetic impurities with the strongest spin-lattice interaction. In this medium, normally degenerate magnetic sublevels are dynamically shifted by the quadrupole Stark effect, and the frequencies of the transitions induced by an acoustic pulse change accordingly. The self-consistent system of equations derived in this study without using the slowly varying envelope approximation describes pulse propagation at an arbitrary angle to the direction of the static strain. Exponentially and rationally decaying monopolar and breather-like solutions to the system are obtained. An analysis of the solutions reveals an asymmetry of the pulse polarity depending on the type of strain (extension or compression) and the pulse propagation direction. In particular, it is shown that acoustic transparency associated with monopolar strain pulses exhibits threshold behavior. The sign of the time area (zeroth harmonic) of breather-like strain pulses is such that the transition frequency averaged over an oscillation period dynamically decreases. This behavior determines the efficiency of generation of high-order harmonics of acoustic pulses in strained paramagnetic crystals.

Anomalous transparency of the water/air interface for low-frequency sound

Sound transmission through water/air interface is normally weak because of a stark density contrast between the two media. For a monopole sound source, at depth larger than a wavelength, the ratio R of the acoustic power transmitted into the air to the total emitted acoustic power is a small quantity of the order of the ratio of impedances of air and water. Acoustic transparency of the interface is shown to rise dramatically at low frequencies, where depth of the monopole source is small compared to the wavelength, with R increasing by up to 36 dB. Under certain conditions, almost all of the acoustic energy emitted by a low-frequency monopole source underwater is predicted to be radiated into the air rather than into the water. Physically, higher transparency at lower frequencies is due to increasing role of inhomogeneous waves emitted by the source and a destructive interference of fields in water due to the actual and image sources. The anomalous transparency of the water/air interface may have significant implications for generation of low-frequency acoustic noise in the air by bubbles collapsing underwater and for acoustic monitoring and detection of powerful underwater explosions for the purposes of the Comprehensive Nuclear Test Ban Treaty.

Nonlinear effects of resonance transparency for two-frequency acoustic pulses in a paramagnetic crystal

Two modes of nonlinear propagation of two-frequency acoustic pulses in a low-temperature crystal containing paramagnetic resonance impurities with an effective spin S = 1 in an external magnetic field and a field of the static strain are considered. It is shown that the spin-phonon transitions occurring within spin triplets according to the V scheme are responsible for two-frequency self-induced acoustic transparency. When the spin-phonon transitions follow the Λ scheme, there can arise an acoustic effect similar to electromagnetically induced transparency in a pulsed mode, which is accompanied by trapping of the population of the spin sublevels.

Nanostructured Ultrathin NbN Film as a Terahertz Hot-Electron Bolometer Mixer

ABSTRACTPlanar spiral antenna coupled and directly lens coupled NbN HEB mixer structures are studied. An additional MgO buffer layer between the superconducting film and Si substrate is introduced. The buffer layer enables us to increase the gain bandwidth of a HEB mixer due to better acoustic transparency. The gain bandwidth is widened as NbN film thickness decreases and amounts to 5.2 GHz. The noise temperature of antenna coupled mixer is 1300 and 3100 K at 2.5 and 3.8 THz respectively. The structure and composition of NbN films is investigated by X-ray diffraction spectroscopy methods. Noise performance degradation at LO frequencies more than 3 THz is due to the use of a planar antenna and signal loss in contacts between the antenna and the sensitive NbN bridge. The mixer is reconfigured for operation at higher frequencies in a manner that receiver's noise temperature is only 2300 K (3 times of quantum limit) at LO frequency of 30 THz.

Acoustic radiation from arrays on coated prolate spheroids

An important aspect in the design of conformal sonar arrays is the effect of an outer coating on the array response. The design of the coating is subject to the competing requirements of acoustic transparency, toughness for impacts, and flow noise rejection. The present study considers an array of rectangular pistons, conformal to a rigid prolate spheroidal baffle, covered by an outer coating. The coating is modeled as a fluid, i.e., it is assumed to have a vanishingly small value of the shear modulus, typical of many elastomers. Unlike a coating on a spherical or cylindrical baffle, where a uniform thickness coating can be defined by simply increasing the value of the radial coordinate, the corresponding operation in prolate spheroidal coordinates results in a confocal layer, i.e., a layer that is not of constant thickness. In this paper a new method is described that allows for a uniform thickness coating in prolate spheroidal geometry. This method employs the spheroidal addition theorem to satisfy the boundary conditions. Comparisons of the beamformed array response for a confocal coating versus a uniform thickness coating are given for a sample array with various coating properties. [Work supported by the NUWC ILIR Program.]

Acoustic-transparency effects for longitudinal-transverse waves

The propagation of longitudinal-transverse acoustic pulses in a system of resonance paramagnetic impurities is theoretically investigated. It is shown that, when the transverse component of a pulse is dominant and propagates along an external magnetic field, a regime of acoustic self-induced transparency is implemented. In the opposite limit, a new soliton regime is implemented, accompanied by a decrease of the propagation rate similar to what takes place for self-induced transparency. In this case, however, the populations of the quantum Zeeman sublevels remain virtually unchanged, and the transverse component of the pulse experiences phase modulation. A close parallel is drawn with the corresponding optical effects of laser-pulse propagation. © 2004 Optical Society of America

Late Quaternary seismic stratigraphy and geological development of the south Vøring margin, Norwegian Sea

High-resolution seismic data and sediment cores show that an up to 280 m thick sedimentary sequence has been deposited on the south Vøring margin, off mid-Norway, the last ca 250 ka. The sedimentary succession has been divided into six seismic units, dominated by hemipelagic sediments. Five wedge-shaped massive sequences, of marine isotope stages 8, 6 and 2, interfinger the hemipelagic deposits on the upper slope. The wedge-shaped sequences represent glacigenic debris flows that have been fed by till transported to the shelf edge by grounded ice sheets during maximum glaciations. The hemipelagic units show well-defined depocentres, of various thicknesses, on the upper continental slope. Seismic facies interpretation indicates that the sediment distribution locally has been controlled by currents. Commonly, the hemipelagic units are characterised by parallel and continuous reflectors. However, the second youngest unit identified, deposited between 15.7 and 15.0 14C ka BP, is acoustic transparent. We suggest that this unit has been sourced by along-slope transported meltwater plume deposits, released during the initial stage of the last deglaciation of the Norwegian Channel. The hemipelagic sedimentation rates have varied considerably throughout the studied time period. Until ca 21 14C ka BP the rates did not exceed 1.4 m/kyr, whereas during the Last Glacial Maximum the rates increased and reached values of about 36 m/kyr before decreasing again at ca 15 14C ka BP. Observation of iceberg scourings, of MIS 8 age, about 800 m below the present day sea level, suggest that the south Vøring margin has subsided by a rate of 1.2 m/kyr in the Late Quaternary.

Magnetic resonance measurement of dynamics of cavitating fluid

The prevalent methods used in studies of cavitation are optical and acoustical. They are sensitive to changes in optical and/or acoustical transparency and are not applicable to studies of opaque media. Magnetic resonance (MR) methods, on the other hand, can be applied to arbitrarily opaque media, providing both dynamic and molecular information. A hindrance to implementation of MR methods in cavitation research is their relatively long measurement time: they cannot compete with optics or acoustics when a researcher needs a snapshot technique to study quickly changing processes on a microsecond scale. There are also some experimental problems with compatibility of MR scanners and acoustical equipment. However, if one is interested in longer time-scale processes, MR can provide data on velocity distribution, evaluate the turbulent processes in cavitating fluid, and measure correlations between the directions of flow on a microsecond scale. In this work, we show feasibility of an application of MR to studies of dynamics of cavitating fluid, with measurements of spatially resolved velocity spectra and other above-mentioned parameters.