Spherical harmonic analysis of the sound radiation from omnidirectional loudspeaker arrays

Omnidirectional sound sources are widely used in room acoustics. These devices are made up of loudspeakers mounted on a spherical or polyhedral cabinet, where the dodecahedral shape prevails. Although such electroacoustic sources have been made readily available to acousticians by many manufacturers, an in-depth investigation of their vibroacoustic behavior has not been provided yet. In order to fulfill this lack, this paper presents a theoretical study of the sound radiation from omnidirectional loudspeaker arrays, which is carried out by using a mathematical model based on the spherical harmonic analysis. Eight different loudspeaker arrangements on the sphere are considered: the well-known five Platonic solid layouts and three extremal system layouts. The latter possess useful properties for spherical loudspeaker arrays used as directivity controlled sound sources, so that these layouts are included here in order to investigate whether or not they could be of interest as omnidirectional sources as well. It is shown through a comparative analysis that the dodecahedral array leads to the lowest error in producing an omnidirectional sound field and to the highest acoustic power, which corroborates the prevalence of such a layout. In addition, if a source with less than 12 loudspeakers is required, it is shown that tetrahedra or hexahedra can be used alternatively, whereas the extremal system layouts are not interesting choices for omnidirectional loudspeaker arrays.     

Automatic 3D scanning surface generation for microphone array acoustic imaging

This work presents a new technique for automatically generating the 3D scanning surface for acoustic imaging using microphone arrays. Acoustic images, or maps, of sound coming from spatially distributed sources, may be generated from microphone array data using algorithms such as beamforming. Traditional 2D acoustic maps can contain errors in the near-field if the object being imaged has a 3D shape. It has been shown that using the 3D surface geometry of an object as a scanning surface for beamforming can provide more accurate results. The methods used previously to generate this 3D scanning surface have either required existing CAD (Computer-Aided Design) models of the object being acoustically imaged or have required separate equipment which is generally bulky and expensive. The new method uses one or more cameras in the array, a data projector, and structured light code to automatically generate the 3D scanning surface. This has the advantage that it is inexpensive, can be incorporated as an add-onto existing microphone arrays, has short scan time, and is capable of being extended to imaging dynamic scenes. This technique is tested using beamforming and CLEAN-SC (CLEAN based on spatial Source Coherence) algorithms for a spherical array and an Underbrink multi-arm spiral array. For sound sources located about 1.2 m from the array, the mean position errors obtained are 6 mm. This is a quarter of the diameter of the mini-speakers being used as a sound sources.     

Asymmetric angular dependence of Jc in coated conductors prior to and after fast neutron irradiation

Angular dependent measurements of J_c were performed on a commercial coated conductor (SuperPower) consisting of a 1 μm thick YBCO layer grown on a MgO IBAD buffer layer. An asymmetric behavior of the angular dependence of J_c (J_c(Ф)) was found with a changing distance between the two peaks at different temperatures and applied magnetic fields. One peak always occurs when the field is oriented parallel to the tape surface, the other smaller peak is located in the perpendicular orientation at high fields, but slightly shifted (by up to 10°) at 77 K and low magnetic fields. This peak shift, the overall J_c(Ф) asymmetry and the influence of fast neutron irradiation on J_c(Ф) are discussed. The spherical defects, introduced by collisions of fast neutrons with the lattice atoms, are randomly distributed, add to the as-grown defect structure and change the critical current anisotropy by altering both peaks.     

Vacancy-vacancy and vacancy-electron interactions in TiOx

Quantitative measurements have been made of the diffuse scattering from TiO_x, single crystals with both quenched specimens and specimens at high temperatures above the ordering reaction. These measurements indicate that there are strong vacancy-vacancy interactions in the fifth and sixth neighbor shell around a vacancy (J₅ and J₆). Also there is evidence for scattering due to the Fermi surface which arises from vacancy-electron interactions (J′), not phonon-electron interactions. The energies of these interactions have been measured: J₅ ? 4 meV, J₆ ? 9 meV. J′ ? 70 meV. These interactions exhibit correlation lengths of the order of 8 nm.     

三维空间非同步测量高分辨率声成像方法

为解决三维空间中声源成像分辨率低的问题,提出一种基于非同步测量的三维空间声成像方法.该方法首先通过移动球形传声器阵列扫描空间分布的声源,然后利用非同步测量技术近似得到大孔径、高密度的传声器阵列测量结果,最后通过传统波束形成算法成像.仿真及实验结果表明,该方法与单次测量下的波束形成方法相比,聚焦性能更好,空间分辨率高,可...     

The acoustics of the caves

This work shows the acoustic measurements realized at three underground places: the caves of Pertosa or of the Angel, the caves of Castelcivita and the caves of Castellana. These places are used for some years to represent music and theater performances, whose success is also due to the mystery and the charm that distinguish them, based on the natural setting of the caves. The places of the caverns where the shows are played were characterized analyzing the monaural acoustic parameters (T₃₀, EDT, C₈₀, D₅₀ and Rasti) obtained with the impulse response resulting from blowing up some balloons inside them. The values of monaural acoustic parameters measured in the caves can be compared with the parameters of the concert halls currently in use.     

Modelling magnetic polarisation J50 by different methods

Two different methods for modelling the angular behaviour of magnetic polarisation at 5000 A/m (J₅₀) of electrical steels were evaluated and compared. Both methods are based upon crystallographic texture data. The texture of non-oriented electrical steels with silicon content ranging from 0.11 to 3%Si was determined by X-ray diffraction. In the first method, J₅₀ was correlated to the calculated value of the average anisotropy energy in each direction, using texture data. In the second method, the first three coefficients of the spherical harmonic series of the ODF and two experimental points were used to estimate the angular variation of J₅₀. The first method allows the estimation of J₅₀ for samples with different textures and Si contents using only the texture data, with no need of magnetic measurement, and this is advantageous, because texture data can be acquired with less than 2 g of material. The second method may give better adjust in some situations but besides the texture data, it requests magnetic measurements in at least two directions, for example, rolling and transverse directions.     

The microwave spectrum of trifluoroacetonitrile in excited vibrational states

The microwave transitions J → J + 1 have been observed in the symmetric top molecule trifluoroacetonitrile, CF₃CN, for molecules excited into several of the low-lying vibrational states. Measurements made in the degenerate states v₇ = 1 v₈ = 1 have been fitted to the formula derived by Grenier-Besson and Amat (1) for transitions J → J + 1 in degenerate vibrational states of molecules with C_3v symmetry. The measurements for the state v₈ = 1 have been extended to 150 GHz where it is shown that the above formula becomes inadequate to describe accurately the observed spectrum.     

Kalman filter-based microphone array signal processing using the equivalent source model

This paper demonstrates that microphone array signal processing can be implemented by using adaptive model-based filtering approaches. Nearfield and farfield sound propagation models are formulated into state-space forms in light of the Equivalent Source Method (ESM). In the model, the unknown source amplitudes of the virtual sources are adaptively estimated by using Kalman filters (KFs). The nearfield array aimed at noise source identification is based on a Multiple-Input–Multiple-Output (MIMO) state-space model with minimal realization, whereas the farfield array technique aimed at speech quality enhancement is based on a Single-Input–Multiple-Output (SIMO) state-space model. Performance of the nearfield array is evaluated in terms of relative error of the velocity reconstructed on the actual source surface. Numerical simulations for the nearfield array were conducted with a baffled planar piston source. From the error metric, the proposed KF algorithm proved effective in identifying noise sources. Objective simulations and subjective experiments are undertaken to validate the proposed farfield arrays in comparison with two conventional methods. The results of objective tests indicated that the farfield arrays significantly enhanced the speech quality and word recognition rate. The results of subjective tests post-processed with the analysis of variance (ANOVA) and a post-hoc Fisher's least significant difference (LSD) test have shown great promise in the KF-based microphone array signal processing techniques.     

A technique for the in situ phase calibration of in-duct axial microphone arrays

In aeroengine noise experiments in-duct microphone arrays are often used to make detailed measurements of the sound field transmitted along the duct. The individual microphones in the array must be calibrated with respect to magnitude, and often more critically with respect to phase. Calibration is difficult to perform in situ due to the presence of the duct. This paper presents a technique to allow in situ phase calibration of axial microphone arrays. It relies on the observation that the measured cross-spectral pressure matrix at the array has a Hermitian Töplitz form in the case where the propagating duct modes are mutually incoherent. Using this property a system of equations can be written which, when solved, allows the phase calibration factors to be obtained. The technique is verified experimentally using a no-flow laboratory rig by comparing the phase calibration factors obtained with those measured in free-field conditions. The accuracy of the phase calibration factors obtained by the technique is limited by the degree of deviation of the measured cross-spectral matrix from Töplitz behaviour. In the experimental results shown this is less than 15° at duct frequencies below ka=25. The technique is a robust and rapid method for calibrating in-duct axial microphone arrays.