Omnidirectional Response Research Articles

Auralization of virtual concerts: A subjective evaluation comparing binaural and ambisonic rendering

Auralization renderings have reached a sufficient level of maturity that simulated auralizations can be comparable to measured ones. These auralizations can be rendered over a variety of sound systems, potentially combined with a visual model through VR interfaces. This study presents a perceptual evaluation of auralizations of a small ensemble virtual concert rendering, comparing a tracked binaural rendering to 2nd order Ambisonic rendering over a 32 loudspeaker array. The geometrical acoustic model of several actual performance spaces were created and then calibrated using in situ omni-directional room impulse response measurements. The performance stimuli consisted of 3 extracts of jazz anechoic recordings comprising trios and quartet ensembles, augmented by three-dimensional visual point-clouds of the musicians playing on stage. Participants of the listening test included a range of listening expertise level (acousticians, architects, students). Several room acoustical parameters were evaluated between rendering systems, seating positions, and rooms.

Large area laser scanning optical resolution photoacoustic microscopy using a fibre optic sensor.

A laser scanning optical resolution photoacoustic microscopy (LS OR-PAM) system based on a stationary fibre optic sensor is described. The sensor comprises an optically resonant interferometric polymer cavity formed on the tip of a rounded single mode optical fibre. It provides low noise equivalent pressure (NEP = 68.7 Pa over a 20 MHz measurement bandwidth), a broad bandwidth that extends to 80 MHz and a near omnidirectional response. The latter is a significant advantage, as it allows large areas (>1cm2) to be imaged without the need for translational mechanical scanning offering the potential for fast image acquisition. The system provides a lateral resolution of 8 µm, an axial resolution of 21 µm, and a field of view up to 10 mm × 10 mm. To demonstrate the system, in vivo 3D structural images of the microvasculature of a mouse ear were obtained, showing single capillaries overlaying larger vessels as well as functional images revealing blood oxygen saturation.

A low-g MEMS inertial switch with a novel radial electrode for uniform omnidirectional sensitivity

A low-g MEMS inertial switch with uniform omnidirectional sensitivity is designed and fabricated for automotive airbags in this paper. The design objective acceleration of the switch is about 38g. By adopting the combination of Archimedes’ spirals and a single circular proof mass as the mass-spring system, the switch can sense the applied accelerations from any radial directions in XOY plane. To obtain a good omnidirectional performance, a novel radial electrode with a spherical contact surface and novel serpentine springs is designed, and limitation blocks are beside it. The omnidirectional response of the switch is simulated and analyzed by ANSYS software. The simulated result indicates that the novel radial electrodes are conducive to achieve the uniform thresholds in all sensitive directions, and the switch can reach a reliable contact because of the novel serpentine spring. The switch is fabricated based on non-silicon surface micromachining technology. To decrease the deviation of threshold acceleration induced by fabrication errors of crucial dimensions, a new method of “thickness compensation” is proposed. The fabricated prototypes were tested by the centrifuge device and dropping hammer. The switch can be triggered under an acceleration of about 40g from any directions in XOY plane and has an excellent anti-overload performance.

Thermal imitators with single directional invisibility

Thermal metamaterials have been intensively studied during the past years to achieve the long-standing dream of invisibility, illusion, and other inconceivable thermal phenomena. However, many thermal metamaterials can only exhibit omnidirectional thermal response, which take on the distinct feature of geometrical isotropy. In this work, we theoretically design and experimentally fabricate a pair of thermal imitators by applying geometrical anisotropy provided by elliptical/ellipsoidal particles and layered structures. This pair of thermal imitators possesses thermal invisibility in one direction, while having thermal opacity in other directions. This work may open a gate in designing direction-dependent thermal metamaterials.

Directionality and gain of small acoustic velocity horns.

Acoustic Velocity Horns (AVHs) are acoustically small funnels open to incident acoustic waves from mouth and throat (for single horns) or both mouths for double horns. Unlike traditional pressure horns terminated at the throat, AVHs yield appreciable amplification of the particle velocity across a wide frequency range starting from extremely low infrasound frequencies. Such horns can be utilized to enhance the performance of conventional vector and acoustic intensity sensors. The present paper includes derivation of directional properties and gains for acoustic velocity as well as pressure for horns of various configurations: single and symmetrical double-horns, and symmetrical horns with inserts of various profiles. The study reveals that the conical double-horns provide the highest velocity gain as compared to horns with an exponential profile or with inserts. The maximum gain cannot exceed a horn's mouth-to-throat radii ratio. In addition to the velocity gain, AVHs offer dipole directionality for the particle velocity and omnidirectional response with no gain for acoustic pressure. These findings were experimentally validated using a water-submerged conical double-horn.

Spatial intensity profiling of elastic and inelastic scattering in isotropic and anisotropic liquids by immersion of a spherical silicon photocell

The transverse spatial intensity distribution of elastic and inelastic light scattering in passive and active as well as weak and strong scattering liquid media has been studied by using Sphelar One p-n junction silicon spherical photocells. We immersed a Sphelar One in these scattering solutions and measured the photoconductive response in reverse biased photodiode (PD) configuration. The passive weak scattering medium was pure ethanol (EtOH), whereas the passive strong scattering medium was 5CB nematic liquid crystal (NLC). Solutions of 0.1mM Rhodamine 640 perchlorate laser dye in EtOH and in 5CB NLC were used as active scattering media. The response of Sphelar One was strongly enhanced in 5CB NLC compared to EtOH, as well as in active solutions compared to passive solutions. The morphology of the Sphelar One is already advantageous over conventional one-sided planar PDs inside liquid solutions. This omnidirectional response of the Sphelar One can further be enhanced by optimizing the properties of the surrounding passive elastic and active inelastic scatterers.

In-plane omnidirectional magnetic field sensor based on Giant Magneto Impedance (GMI)

In this work the design and characterization of an omnidirectional in-plane magnetic field sensor are presented. The sensor is based on the Giant Magneto Impedance (GMI) effect in glass-coated amorphous microwires of composition (Fe6Co94)72.5Si12.5B15. For the first time, a circular loop made with a microwire is used for giving omnidirectional response. In order to estimate the GMI response of the circular loop we have used a theoretical model of GMI, determining the GMI response as the sum of longitudinal sections with different angles of incidence. As a consequence of the circular loop, the GMI ratio of the sensor is reduced to 15% instead of 100% for the axial GMI response of a microwire. The sensor response has been experimentally verified and the GMI response of the circular loop has been studied as function of the magnetic field, driven current, and frequency. First, we have measured the GMI response of a longitudinal microwire for different angles of incidence, covering the full range between the tangential and perpendicular directions to the microwire axis. Then, using these results, we have experimentally verified the decomposition of a microwire with circular shape as longitudinal segments with different angles of incidence. Finally, we have designed a signal conditioning circuit for the omnidirectional magnetic field sensor. The response of the sensor has been studied as a function of the amplitude of the incident magnetic field.

An Optically-Triggered Switchable Mid-Infrared Perfect Absorber Based on Phase-Change Material of Vanadium Dioxide

Switchable nanoscale devices can be implemented in heterostructures that integrate plasmonic nanostructures with functional active materials and hence hold great potential for nanoscale-integrated circuits. The phase-change material of vanadium dioxide (VO2) has reversibly switchable optical/electrical properties and huge contrast in its refractive index in the infrared spectral range between insulator and metallic states. In this work, we numerically demonstrate all-optical manipulation of switchable absorption effect using the heterostructure incorporating the plasmonic resonance of Au nanoantennas with vanadium dioxide. Compared with the planar control device (without Au nanoantennas), the proposed design exhibits a pronounced resonant field enhancement as well as polarization-insensitive and omnidirectional absorption response. Meanwhile, the proposed device shows a large switching contrast (from ~ 99.9 to ~ 10% in absorption efficiency) at the mid-infrared wavelength of 3609 nm. Interestingly, the resonance of the proposed device can be continuously tuned by varying the side length of the antennas or governing the metallization level of vanadium dioxide layer. The photothermal mechanism is further investigated by numerical model calculations, indicating that the resonant, antenna-mediated local heating occurs on a sub-nanosecond time scale of 0.26 ns under a quite low incident intensity of 1.9 × 106 W/m2, which is about 12.5 times reduced with respect to that of control device. Therefore, the hybrid strategy of plasmonic antennas and vanadium dioxide provides a conceptual framework of switchable metamaterials for actively steering in ultrafast, energy-efficient electronic and photonic devices.

Present and Futuristic Trends in Weapon System

The basic principle of war is based on protection, mobility and firepower. The present scenario of omni-directional and multidisciplinary threats and decreasing response time has made intelligence inevitable for the system to survive and win the war. The intelligence of the system calls for interface of refined logic with systems without compromising any parameters of the gun. However, much of the efforts and funds are directed towards developing the newer technologies to enhance the performance of weapon system in terms of accuracy, consistency, range (stand-off), lethality and transportability. The efforts are also directed towards making smart weapons controlled distantly and cost effective. The future battlefields are going to be dominated by the technological advancements taking place and thus, the technological superiority (i.e. systems with high degree of intelligence) over the adversaries will be the deciding factor to win the war.

Binaural auralization of proposed room modifications based on measured omnidirectional room impulse responses

The auralization of rooms with dynamic binaural synthesis using binaural room impulse responses (BRIRs) is an established approach in virtual audio. The BRIRs can either be obtained by simulations or by measurements. Up to now changed acoustical properties, as they occur when a room is altered in a renovation, cannot easily be considered in a measurement-based approach. This paper presents a new method to auralize modifications of existing rooms. The authors already have shown in a previous publication that such an auralization can be done by appropriately shaping the reverberation tail of an impulse response. Furthermore, the authors have presented an approach to synthesize BRIRs based on one omnidirectional room impulse response (RIR). In this paper, both methods are combined: A single measured omnidirectional RIR is enhanced and adapted to create a binaural representation of a modified room. A listening experiment has been performed to evaluate the procedure and to investigate differences between synthesized and measured BRIRs. The advantages of this method are obvious: Planned room modifications can be made audible without complex measurements or simulations; just one omnidirectional RIR is required to provide a binaural representation of the desired acoustic treatment.

Omnidirectional measurement of room impulse response using a small bookshelf loudspeaker as a sound source

A dodecahedral loudspeaker is commonly used in room acoustical measurements as an omnidirectional sound source. However, inverse filtering to obtain an ideal room impulse response (RIR) is not easy because of its irregular frequency and temporal characteristics. When this RIR is convolved with music signals, the reproduced sound does not possess adequate fidelity to evaluate the acoustical quality of the concert hall. Hence a new measurement method of the RIR by an omnidirectional source is proposed: RIR’s are measured by a small bookshelf loudspeaker rotating around its acoustic center in multiple directions, and the RIR’s are averaged to obtain omnidirectional response. Physical characteristics of the RIR determined with this procedure are given and the room acoustical parameters are compared with those by the dodecahedral method. Finally, application to subjective experiments is described.

Deconvolution based photoacoustic reconstruction with sparsity regularization.

In most photoacoustic tomography (PAT) reconstruction approaches, it is assumed that the receiving transducers have omnidirectional response and can fully surround the region of interest. These assumptions are not satisfied in practice. To deal with these limitations, we present a novel deconvolution based photoacoustic reconstruction with sparsity regularization (DPARS) technique. The DPARS algorithm is a semi-analytical reconstruction approach in which the projections of the absorber distribution derived from a deconvolution-based method are computed and used to generate a large linear system of equations. In these projections, computed over limited viewing angles, the directivity effect of the transducer is taken into account. The distribution of absorbers is computed using a sparse representation of absorber coefficients obtained from the discrete cosine transform. This sparse representation helps improve the numerical conditioning of the system of equations and reduces the computation time of the deconvolution-based approach by one order of magnitude relative to Tikhonov regularization. The algorithm has been tested in simulations, and using two-dimensional and three-dimensional experimental data obtained with a conventional ultrasound transducer. The results show that DPARS, when evaluated using contrast-to-noise ratio and root-mean-square errors, outperforms the conventional delay-and-sum (DAS) reconstruction method.

Near-field signal acquisition for smartglasses using two acoustic vector-sensors

Smartglasses, in addition to their visual-output capabilities, often contain acoustic sensors for receiving the user’s voice. However, operation in noisy environments may lead to significant degradation of the received signal. To address this issue, we propose employing an acoustic sensor array which is mounted on the eyeglasses frames. The signals from the array are processed by an algorithm with the purpose of acquiring the desired near-field speech signal produced by the wearer while suppressing noise signals originating from the environment. The array is comprised of two acoustic vector-sensors (AVSs) which are located at the fore of the glasses’ temples. Each AVS consists of four collocated subsensors: one pressure sensor (with an omnidirectional response) and three particle-velocity sensors (with dipole responses) oriented in mutually orthogonal directions. The array configuration is designed to boost the input power of the desired signal, and to ensure that the characteristics of the noise at the different channels are sufficiently diverse (lending towards more effective noise suppression). Since changes in the array’s position correspond to the desired speaker’s movement, the relative source-receiver position remains unchanged; hence, the need to track fluctuations of the steering vector is avoided. Conversely, the spatial statistics of the noise are subject to rapid and abrupt changes due to sudden movement and rotation of the user’s head. Consequently, the algorithm must be capable of rapid adaptation toward such changes. We propose an algorithm which incorporates detection of the desired speech in the time-frequency domain, and employs this information to adaptively update estimates of the noise statistics. The speech detection plays a key role in ensuring the quality of the output signal. We conduct controlled measurements of the array in noisy scenarios. The proposed algorithm preforms favorably with respect to conventional algorithms.

Efficient, broadband and wide-angle hot-electron transduction using metal-semiconductor hyperbolic metamaterials

Hot-electron devices are emerging as promising candidates for the transduction of optical radiation into electrical current, as they enable photodetection and solar/infrared energy harvesting at sub-bandgap wavelengths. Nevertheless, poor photoconversion quantum yields and low bandwidth pose fundamental challenges to fascinating applications of hot-electron optoelectronics. Based on a novel hyperbolic metamaterial (HMM) structure, we theoretically propose a vertically-integrated hot-electron device that can efficiently couple plasmonic excitations into electron flows, with an external quantum efficiency approaching the physical limit. Further, this metamaterial-based device can have a broadband and omnidirectional response at infrared and visible wavelengths. We believe that these findings may shed some light on designing practical devices for energy-efficient photodetection and energy harvesting beyond the bandgap spectral limit.

Two-microphone spatial filtering improves speech reception for cochlear-implant users in reverberant conditions with multiple noise sources.

This study evaluates a spatial-filtering algorithm as a method to improve speech reception for cochlear-implant (CI) users in reverberant environments with multiple noise sources. The algorithm was designed to filter sounds using phase differences between two microphones situated 1 cm apart in a behind-the-ear hearing-aid capsule. Speech reception thresholds (SRTs) were measured using a Coordinate Response Measure for six CI users in 27 listening conditions including each combination of reverberation level (T60 = 0, 270, and 540 ms), number of noise sources (1, 4, and 11), and signal-processing algorithm (omnidirectional response, dipole-directional response, and spatial-filtering algorithm). Noise sources were time-reversed speech segments randomly drawn from the Institute of Electrical and Electronics Engineers sentence recordings. Target speech and noise sources were processed using a room simulation method allowing precise control over reverberation times and sound-source locations. The spatial-filtering algorithm was found to provide improvements in SRTs on the order of 6.5 to 11.0 dB across listening conditions compared with the omnidirectional response. This result indicates that such phase-based spatial filtering can improve speech reception for CI users even in highly reverberant conditions with multiple noise sources.

Two-microphone spatial filtering provides speech reception benefits for cochlear implant users in difficult acoustic environments.

This article introduces and provides an assessment of a spatial-filtering algorithm based on two closely-spaced (∼1 cm) microphones in a behind-the-ear shell. The evaluated spatial-filtering algorithm used fast (∼10 ms) temporal-spectral analysis to determine the location of incoming sounds and to enhance sounds arriving from straight ahead of the listener. Speech reception thresholds (SRTs) were measured for eight cochlear implant (CI) users using consonant and vowel materials under three processing conditions: An omni-directional response, a dipole-directional response, and the spatial-filtering algorithm. The background noise condition used three simultaneous time-reversed speech signals as interferers located at 90°, 180°, and 270°. Results indicated that the spatial-filtering algorithm can provide speech reception benefits of 5.8 to 10.7 dB SRT compared to an omni-directional response in a reverberant room with multiple noise sources. Given the observed SRT benefits, coupled with an efficient design, the proposed algorithm is promising as a CI noise-reduction solution.

3D Impulse Response Measurements of Spaces Using an Inexpensive Microphone Array

The acoustical characteristics of a room are traditionally determined using omnidirectional impulse response measurements, yielding information about sound reflections in terms of magnitude and time, but not direction. However, the direction of reflections is often important, and thus the need for a practical, low cost measurement system for determining this. In this paper we present the performance of a low cost measurement system utilising an inexpensive microphone array, namely the Core Sound TetraMic, for the determination of 3D room impulse responses. These can then be visualised, for example, as a “hedgehog pattern”. Experiments undertaken in an anechoic chamber indicate that the accuracy of directional estimation of this system is in the region of ±7.5°.

All-angle negative refraction and active flat lensing of ultraviolet light

Decades ago, Veselago predicted that a material with simultaneously negative electric and magnetic polarization responses would yield a 'left-handed' medium in which light propagates with opposite phase and energy velocities--a condition described by a negative refractive index. He proposed that a flat slab of left-handed material possessing an isotropic refractive index of -1 could act like an imaging lens in free space. Left-handed materials do not occur naturally, and it has only recently become possible to achieve a left-handed response using metamaterials, that is, electromagnetic structures engineered on subwavelength scales to elicit tailored polarization responses. So far, left-handed responses have typically been implemented using resonant metamaterials composed of periodic arrays of unit cells containing inductive-capacitive resonators and conductive wires. Negative refractive indices that are isotropic in two or three dimensions at microwave frequencies have been achieved in resonant metamaterials with centimetre-scale features. Scaling the left-handed response to higher frequencies, such as infrared or visible, has been done by shrinking critical dimensions to submicrometre scales by means of top-down nanofabrication. This miniaturization has, however, so far been achieved at the cost of reduced unit-cell symmetry, yielding a refractive index that is negative along only one axis. Moreover, lithographic scaling limits have so far precluded the fabrication of resonant metamaterials with left-handed responses at frequencies beyond the visible. Here we report the experimental implementation of a bulk metamaterial with a left-handed response to ultraviolet light. The structure, based on stacked plasmonic waveguides, yields an omnidirectional left-handed response for transverse magnetic polarization characterized by a negative refractive index. By engineering the structure to have a refractive index close to -1 over a broad angular range, we achieve Veselago flat lensing, in free space, of arbitrarily shaped, two-dimensional objects beyond the near field. We further demonstrate active, all-optical modulation of the image transferred by the flat lens.

NIST System for Measuring the Directivity Index of Hearing Aids under Simulated Real-Ear Conditions.

The directivity index is a parameter that is commonly used to characterize the performance of directional hearing aids, and is determined from the measured directional response. Since this response is different for a hearing aid worn on a person as compared to when it is in a free field, directivity index measurements of hearing aids are usually done under simulated real-ear conditions. Details are provided regarding the NIST system for measuring the hearing aid directivity index under these conditions and how this system is used to implement a standardized procedure for performing such measurements. This procedure involves a sampling method that utilizes sound source locations distributed in a semi-aligned zone array on an imaginary spherical surface surrounding a standardized acoustical test manikin. The capabilities of the system were demonstrated over the frequency range of one-third-octave bands with center frequencies from 200 Hz to 8000 Hz through NIST participation in an interlaboratory comparison. This comparison was conducted between eight different laboratories of members of Working Group S3/WG48, Hearing Aids, established by Accredited Standards Committee S3, Bioacoustics, which is administered by the Acoustical Society of America and accredited by the American National Standards Institute. Directivity measurements were made for a total of six programmed memories in two different hearing aids and for the unaided manikin with the manikin right pinna accompanying the aids. Omnidirectional, cardioid, and bidirectional response patterns were measured. Results are presented comparing the NIST data with the reference values calculated from the data reported by all participating laboratories.

Directional and frequency saccular sensitivity of the little skate, Raja erinacea.

Although a number of previous behavioral studies have demonstrated that elasmobranch fishes can detect and are attracted to low frequency sounds, few physiological studies have characterized the auditory response properties of the elasmobranch inner ear to such low frequency sounds. In this study, we examined the directional and frequency responses of the inner ear saccule in the little skate, Raja erinacea to low frequency stimuli. Evoked microphonic potentials were recorded from the middle region of the saccule while sound was generated using a shaker table designed to mimic the particle motion vector component of sound. 8 test frequencies (50, 64, 84, 100, 140, 185, 243, and 303 Hz) and 11 directions were used to characterize the displacement sensitivity, frequency response and directional response properties of the skate saccule. Saccular potentials were evoked and measured at twice the stimulus frequency in vivo using a wave analyzer while stimuli were generated via the shaker table system. The right and left saccules appeared to have an omnidirectional response based on initial measurements, and the frequency response of the skate saccule had lowest displacement thresholds from 100 to 185 Hz.