Interferometric imaging of acoustical phenomena using high-speed polarization camera and 4-step parallel phase-shifting technique

Acousto-optic tomography (AOT), a technology that reconstructs two- or three-dimensional sound fields from optically measured sound-field projections, has been widely studied as an efficient and high-spatial-resolution method for sound field observations. Recently, physical-model-based approaches have made significant progress, with higher accuracy and fewer sampling requirements than conventional methods. Nevertheless, it remains a challenge to reconstruct three-dimensional outgoing sound waves in the volume surrounding a sound source due to constraints on existing methods both in mathematical formulation and measurement systems. In this paper, we propose spherical harmonic-domain AOT, a three-dimensional reconstruction of sound fields by combining instantaneous sound-field imaging using parallel phase-shifting interferometry with a physics-based reconstruction using spherical wave function expansions. We formulated two problems: one for the sound field radiated by a transducer and the other for the sound field scattered by an object. Through experiments, we demonstrated the successful reconstruction of the two types of three-dimensional sound fields: one radiated from two ultrasonic transducers and the other scattered by an ellipsoid.

Noise reduction and control research are actively conducted as increasing noise problems compel the stringent noise requirement. Active noise control (ANC) is constructively used in various applications to reduce low-frequency noise. In previous studies, ANC systems were designed based on experiments, requiring extensive effort for effective implementation. In this paper, a real-time ANC simulation in a computational aeroacoustics framework based on the virtual-controller method is presented. The aims are to investigate sound field changes following ANC system operation and gain more insight into ANC system design through a computational approach. Using a virtual-controller ANC simulation, the approximate shape of the acoustic path filter and changes in the sound field when ANC is either "on" or "off" at the target domain can be obtained, enabling practical and detailed analyses. Then, the computational results of the duct and open space cases are predicted and compared with the experimental results to validate the prediction capability of the proposed method. In addition, the ANC system design parameters and their effects on sound fields with unintended phenomena can be predicted. Through case studies, the ability to design, optimize, and predict the performance of the ANC system using the computational method is also demonstrated.

Fiber-tip degradation, damage, or burn back is a common problem during the ureteroscopic laser lithotripsy procedure to treat urolithiasis. Fiber-tip burn back results in reduced transmission of laser energy, which greatly reduces the efficiency of stone comminution. In some cases, the fiber-tip degradation is so severe that the damaged fiber-tip will absorb most of the laser energy, which can cause the tip portion to be overheated and melt the cladding or jacket layers of the fiber. Though it is known that the higher the energy density (which is the ratio of the laser energy fluence over the cross section area of the fiber core), the faster the fiber-tip degradation, the damage mechanism of the fibertip is still unclear. In this study, fiber-tip degradation was investigated by visualization of shockwave, cavitation/bubble dynamics, and calculus debris ejection with a high-speed camera and the Schlieren method. A commercialized, pulsed Ho:YAG laser at 2.12 um, 273/365/550-um core fibers, and calculus phantoms (Plaster of Paris, 10x10x10 mm cube) were utilized to mimic the laser lithotripsy procedure. Laser energy induced shockwave, cavitation/bubble dynamics, and stone debris ejection were recorded by a high-speed camera with a frame rate of 10,000 to 930,000 fps. The results suggested that using a high-speed camera and the Schlieren method to visualize the shockwave provided valuable information about time-dependent acoustic energy propagation and its interaction with cavitation and calculus. Detailed investigation on acoustic energy beam shaping by fiber-tip modification and interaction between shockwave, cavitation/bubble dynamics, and calculus debris ejection will be conducted as a future study.

Bioacoustics monitoring has become increasingly popular for studying the behavior and ecology of vocalizing birds. This study aims to verify the practical effectiveness of localization technology for auditory monitoring of endangered Eurasian bittern (Botaurus stellaris) which inhabits wetlands in remote areas with thick vegetation. Their crepuscular and highly secretive nature, except during the breeding season when they vocalize advertisement calls, make them difficult to monitor. Because of the increasing rates of habitat loss, surveying accurate numbers and their habitat needs are both important conservation tasks. We investigated the feasibility of localizing their booming calls, at a low frequency range between 100–200 Hz, using microphone arrays and robot audition HARK (Honda Research Institute, Audition for Robots with Kyoto University). We first simulated sound source localization of actual bittern calls for microphone arrays of radii 10 cm, 50 cm, 1 m, and 10 m, under different noise levels. Second, we monitored bitterns in an actual field environment using small microphone arrays (height = 12 cm; width = 8 cm), in the Sarobetsu Mire, Hokkaido Island, Japan. The simulation results showed that the spectral detectability was higher for larger microphone arrays, whereas the temporal detectability was higher for smaller microphone arrays. We identified that false detection in smaller microphone arrays, which was coincidentally generated in the calculation proximate to the transfer function for the opposite side. Despite technical limitations, we successfully localized booming calls of at least two males in a reverberant wetland, surrounded by thick vegetation and riparian trees. This study is the first case of localizing such rare birds using small-sized microphone arrays in the field, thereby presenting how this technology could contribute to auditory surveys of population numbers, behaviors, and microhabitat selection, all of which are difficult to investigate using other observation methods. This methodology is not only useful for the better understanding of bitterns, but it can also be extended to investigate other rare nocturnal birds with low-frequency vocalizations, without direct ringing or tagging. Our results also suggest a future necessity for a robust localization system to avoid reverberation and echoing in the field, resulting in the false detection of the target birds.

Thanks to parallel phase-shifting digital holography using a high-speed camera, we succeeded in phase-shifting interferometry at the rate of up to 262,500 frames/s. Motion pictures of three-dimensional images of fast phenomena are demonstrated.

While travelling through railways, passengers are often subjected to a train’s interior noise. Compared to an open area, such noise increases when a train passes through a tunnel because the interior of the tunnel generally comprises a concrete surface with a very low rate of sound absorption, and the noise from railway vehicles undergoes multiple reflections inside the tunnel. To reduce a train’s interior noise while passing through a tunnel, it is necessary to analytically and experimentally study the noise reduction effect caused by incorporating high strength porous aggregate sound absorbing materials that satisfy the material performance standards required by railway facilities. Acoustic simulation of the sound field within a tunnel has been previously analyzed for predicting the noise reduction caused by porous aggregate sound absorbing materials, before and after installation. Furthermore, the sound absorption performance in the space between the installation section and the non-installation section has been compared by measuring the reverberation time after incorporating sound absorbing materials in an actual test tunnel. In this study, a method for efficient arrangement of sound absorbing materials is proposed based on the sound field analysis results of railway tunnels. The proposed study can thus provide fundamental data for further research on aspects such as sound insulation design for railway vehicle body and railway infrastructure. Key words: Railway Tunnel, Train Interior Noise, Sound Absorbing Material, Acoustic Analysis, Reverberation Time

Recent development of optical interferometry enables us to measure sound without placing any device inside the sound field. In particular, parallel phase-shifting interferometry (PPSI) has realized advanced measurement of refractive index of air. Its novel application investigated very recently is simultaneous visualization of flow and sound, which had been difficult until PPSI enabled high-speed and accurate measurement several years ago. However, for understanding aerodynamic sound, separation of air flow and sound is necessary since they are mixed up in the observed video. In this paper, guided-spatio-temporal filtering is proposed to separate sound from the optically measured images. Guided filtering is combined with a physical-model-based spatio-temporal filterbank for extracting sound-related information without the undesired effect caused by the image boundary or occluding objects. Such image boundary and occluding objects are typical difficulty arose in signal processing of an optically measured sound filed.

Near-field compensated higher order Ambisonics (NFC-HOA) and wave field synthesis (WFS) constitute the two best-known analytic sound field synthesis methods. While WFS is typically used for the synthesis of virtual sound scenes, NFC-HOA is typically employed in order to synthesize sound fields that have been captured with appropriate microphone arrays. Such recorded sound fields are essentially represented by the coefficients of the underlying surface spherical harmonics expansion. A sound field described by such coefficients cannot be straightforwardly synthesized in WFS. This is a consequence of the fact that, unlike in NFC-HOA, it is critical in WFS to carefully select those loudspeakers that contribute to the synthesis of a given sound source in a sound field under consideration. In order to enable such a secondary source selection, it is proposed to employ the well-known concept of decomposing the sound field under consideration into a continuum of plane waves, for which the secondary source selection is straightforward. The plane wave representation is projected onto the horizontal plane and a closed form expression of the secondary source driving signals for horizontal WFS systems of arbitrary convex shape is derived.

The measurement procedure recently developed in the investigations of vortex ring noise and based on synchronous azimuthal decomposition of signal by hexagon array of microphones with subsequent ensemble averaging is used for the diagnostics of aeroacoustics sources in the situation of the cylinder rod (diameter 2, 3, and 10 mm) inserted in a turbulent flow. The turbulent jet with initial velocity 90 m/s issued from the cylindrical nozzle of diameter 20 mm. A microphone array for recording was placed in the far sound field of the sound sources. The frequency range 500-2000Hz is analyzed when the radiation region can be considered as a compact sound source. Two dipoles are recognized in the sound field – the first one is directed along the mean flow, the second one is orthogonal to the flow (drag and lift dipoles). The third dipole, which could be directed along the cylinder, is absent due to the absence of considerable pulsation force along the cylinder. The measurement procedure enables to recognize the rod angle of inclination in the plane normal to the mean flow axis. The quadrupole sources in the separation zone appear to be strongly connected with the dipole sources located on the cylinder. Their interference leads to surprising picture of the resulting sound field: equivalent dipole sources are located in the separation zone far downstream from the cylinder. This effect manifests a dual role of small quadrupole sources in the low frequency range: (i) quadrupoles located closely to the cylindrical surface create the dipole sources in the cylinder (which radiate the main noise) and (ii) quadrupoles displace the dipole source far downstream due to the interference.

高速度カメラは80年以上の歴史を経て，現在までに数多くの撮影手法が提案された。例えば，顕微鏡観察手法，光増倍観察手法，分光温度計測手法が挙げられる。一方で，それらの撮影手法を光学特性で分類すると，偏光を用いた手法が少ない。偏光計測分野では，回転検光子，光弾性変調器，および液晶を用いた手法により定量化が可能となり，膜厚計測，形状計測，および複屈折計測へ応用された。しかしながら，従来の複屈折計測手法では，偏光素子を時間変調するため，動的現象の計測ができない。異なる手法として，回転偏光板とイメージングを組み合わせた手法が提案されているが，偏光素子の回転を伴うため高速化が難しい。これらの背景を踏まえ，筆者達のグループは偏光高速度イメージング装置開発を行うことで，高速度イメージングにおける偏光分野への新規利用用途拡大を目指している。要素技術である偏光高速度イメージセンサは，マイクロ偏光素子アレイと高速度イメージセンサの並列読み出し画素を直結し，高速化を実現したものである1）。本論では，その要素技術について概要説明するとともに，新規開拓された用途を紹介するものである。

Ultrashort-pulsed laser illumination focused inside a diamond converts sp³-bonded diamond to sp²-bonded amorphous carbon in the vicinity of the focal point and changes the color to black. A wire-shaped modified region is fabricated by scanning the laser focus toward the laser source in the diamond. Volumetric expansion by converting diamond to amorphous carbon forms cracks around the modified region. In this study, diamond slicing was attempted by using cracks formed around the modified region. A near-infrared picosecond laser was focused inside a high-temperature, high-pressure diamond. The cracks fabricated under various laser conditions were observed. The plane crack was formed by lining up the wire-shaped modified regions next to each one. During the fabrication, a high-speed polarization camera was used to observe the stress distribution around the modified region and in the adjacent wire-shaped modified region. The crack propagation was estimated by observing the stress distribution in situ. The kerf loss in the slicing process was estimated by observing the cross section of the cracks from multiple directions. These results demonstrate that plane cracks suitable for slicing the diamond were fabricated. Diamond separation was performed by applying an external force to the plane cracks.

Motivated by the problem of spatial sampling of a sound field by a spherical array, Chap. 3 presented methods for sampling functions on a sphere, followed by methods for reconstructing a function from its samples. These could form the basis for computing the sound pressure on the surface of a sphere, given measurements by an array of microphones. However, in spherical microphone array processing one may also be interested in computing the sound field around the array by decomposing the sound field into plane-wave components, for example. In this case, placing pressure or omni-directional microphones on the surface of a single sphere in free-field may not allow accurate plane-wave decomposition , due to zeros of the spherical Bessel function . This problem is presented at the beginning of the chapter. One possible solution is to place microphones on the surface of a rigid sphere . This configuration offers a practical advantage—the rigid sphere provides an ideal housing for all microphone wiring and conditioning electronics. However, one drawback of the rigid sphere is that sound scattered from the sphere can be reflected back by surrounding objects, thereby modifying the sound field it measures. This is particularly important for arrays used for sound field analysis in room acoustics, for example, in which case placing microphones in a free field, in an open-sphere configuration, may be preferable. Open spherical array configurations that avoid the problem of the zeros of the spherical Bessel function are therefore presented next. The array configuration may also affect other aspects of array performance related to the frequency range of operation and to the sensitivity to sensor noise and to other uncertainties. A general framework for array design that considers a range of objectives is introduced, followed by example designs. The chapter concludes with a description of an open spherical array configuration in which the microphones are placed within the volume of a shell. Other array configurations, including the hemispherical array , another array comprised of concentric rigid and open spheres , and an array incorporating non-spherical sampling surfaces, are also discussed.

Shock waves, which are generated by pulse discharges in an excimer laser, cause arcing and nonhomogeneous excitation in the laser cavity. Experiments and numerical calculations are conducted to clarify the generation and propagation of shock waves in the cavity of an excimer laser. The shock Waves are visualized by using a CCD color Schlieren method. Numerical calculations using Yee’s symmetric TVD scheme are carried out for conditions which correspond to the experiments. The initial conditions of the calculations are determined from measured results with the laser Schlieren method, and from Schlieren photographs. Color Schlieren photographs are constructed from the numerical results. The numerical Schlieren photographs are found to be in good agreements with the experimental Schlieren photographs.

The main objective of a multi-channel sound reproduction system is to give an optimal acoustical sensation to the listener. These systems are designed to produce sounds that are as natural as possible, so that the listener does not realize that they are generated by a loudspeakers system. For this purpose, the knowledge of only audio temporal information is not sufficient: also spatial information is needed. Furthermore, since the invention of stereophony, it is well known that at least two loudspeakers are needed in order to generate a virtual source that is not spatially localized at the speaker position. However, the stereo reproduction is very limited because the optimal source localization is focused on one point, called sweet spot. Starting from this assumption, in the recent literature, research efforts have focused on reproduction techniques that use an extremely high number of loudspeakers in order to reproduce not only a simple audio source but a complete sound field. Various techniques, able to record and to reproduce the entire sound field, have been proposed in the literature (Berkhout et al., 1993; Daniel et al., 2003; Fazi et al., 2008). One of the most studied techniques is Wave Field Synthesis (WFS) that is directly based on the Huygens’ principle. It has been introduced in the late ’80s by Berkhout, who showed that audio reproduction can be linked to audio holography concepts (Berkhout, 1988). WFS is based on Kirchhoff-Helmholtz integral which permits the calculation of the pressure field inside a volume by knowing pressure and normal particle velocity on the enclosing surface. The underlying idea of WFS is to generate a sound field inside a volume bounded by an array of loudspeakers. Actually, the surface is reduced to a 2D curve positioned on the ear plane. The number of loudspeakers on this curve depends on the desired localization quality. Similarly, Wave Field Analysis (WFA) implements a sound field recording technique based on microphone arrays (Hulsebos et al., 2002). Therefore, this approach allows to record the entire sound field in the recording room (WFA) and subsequently to reproduce it in the listening room (WFS) more or less accurately depending on loudspeakers/microphones number. As it happens in traditional reproduction systems, digital algorithms based on adaptation processing have to be applied inside the WFS/WFA in order for these techniques to be used in real applications (music, cinema, theatre, etc.). Examples of adaptive algorithms that are very useful in real applications are Acoustic Echo Cancellation (AEC), Active Noise Control (ANC), room compensation, etc. (Haykin, 1996). A straightforward implementation of these algorithms in WFA/WFS systems is not feasible due to the dramatically high 25

A unified framework is proposed for analysis and synthesis of spatial sound fields. In the sound field analysis (SFA) phase, an unbaffled 24-element circular microphone array (CMA) is utilized to “encode” the sound field based on the plane-wave decomposition, whereas in the sound field synthesis (SFS) phase a 32-element rectangular loudspeaker array is employed to “decode” the target sound field using pressure matching technique. Depending on the sparsity of the sound sources, the SFA stage can be implemented in two ways. For the sparse-source scenario, a two-stage algorithm is utilized to estimate the source bearings using the minimum power distortionless response (MPDR) and the associated amplitude coefficients of plane waves using the Tikhonov regularization (TIKR) algorithm. Alternatively, a one-stage algorithm based on compressive sensing (CS) algorithm can be used. For the nonsparse-source scenario, a one-stage TIKR is utilized to solve for the amplitude coefficients for plane-wave components uniformly distributed in the angular domain. The SFA technique for the non-sparse source scenario is also useful in establishing the room response model, as required in the pressure matching step of the SFS phase. The integrated acoustic array system is validated with localization and listening tests.