Minimally radiating sources for personal audio

In order to reduce annoyance from the audio output of personal devices, it is necessary to maintain the sound level at the user position while minimizing the levels elsewhere. If the dark zone, within which the sound is to be minimized, extends over the whole far field of the source, the problem reduces to that of minimizing the radiated sound power while maintaining the pressure level at the user position. It is shown analytically that the optimum two-source array then has a hypercardioid directivity and gives about 7 dB reduction in radiated sound power, compared with a monopole producing the same on-axis pressure. The performance of other linear arrays is studied using monopole simulations for the motivating example of a mobile phone. The trade-off is investigated between the performance in reducing radiated noise, and the electrical power required to drive the array for different numbers of elements. It is shown for both simulations and experiments conducted on a small array of loudspeakers under anechoic conditions, that both two and three element arrays provide a reasonable compromise between these competing requirements. The implementation of the two-source array in a coupled enclosure is also shown to reduce the electrical power requirements.     

Particle motion measured at an operational wind turbine in relation to hearing sensitivity in fish

The effect of sound pressure on the hearing of fish has been extensively investigated in laboratory studies as well as in field trials in contrast to particle motion where few studies have been carried out. To improve this dearth of knowledge, an instrument for measuring particle motion was developed and used in a field trial. The particle motion is measured using a neutrally buoyant sphere, which co-oscillates with the fluid motion. The unit was deployed in close vicinity to a wind turbine foundation at Utgrunden wind farm in the Baltic Sea. Measurements of particle motion were undertaken at different distances from the turbine as well as at varying wind speeds. Levels of particle motion were compared to audiograms for cod (Gadus morhua L.) and plaice (Pleuronectes platessa L.).     

A directional acoustic array using silicon micromachined piezoresistive microphones

The need for noise source localization and characterization has driven the development of advanced sound field measurement techniques using microphone arrays. Unfortunately, the cost and complexity of these systems currently limit their widespread use. Directional acoustic arrays are commonly used in wind tunnel studies of aeroacoustic sources and may consist of hundreds of condenser microphones. A microelectromechanical system (MEMS)-based directional acoustic array system is presented to demonstrate key technologies to reduce the cost, increase the mobility, and improve the data processing efficiency versus conventional systems. The system uses 16 hybrid-packaged MEMS silicon piezoresistive microphones that are mounted to a printed circuit board. In addition, a high-speed signal processing system was employed to generate the array response in near real time. Dynamic calibrations of the microphone sensor modules indicate an average sensitivity of 831 microV/Pa with matched magnitude (+/-0.6 dB) and phase (+/-1 degree) responses between devices. The array system was characterized in an anechoic chamber using a monopole source as a function of frequency, sound pressure level, and source location. The performance of the MEMS-based array is comparable to conventional array systems and also benefits from significant cost savings.     

Influence of viscosity on aerodynamic sound emission in free space

The theory of aerodynamic sound is reformulated with account taken of the influence of viscosity in the source flow on the sound emission in free space. This is based on the Ffowcs Williams form of the Lighthill equation. The source flow is assumed to be determined by a localized vorticity field and characterized by low Mach numbers and high Reynolds numbers. It is found that the acoustic pressure emitted by the viscous vortex motion is composed of a quadrupole and two kinds of monopole. The Reynolds stress in the source flow is decomposed into isotropic and non-isotropic parts. The non-isotropic part leads to the quadrupole wave derived by Möhring, while the isotropic part is related to one of the monopole-like waves radiated when the total kinetic energy changes. The other monopole wave is associated with entropy production by viscous dissipation of the kinetic energy. All three components are influenced by viscosity.     

线阵波束形成声强缩放方法估算声源的辐射声功率

为了解决波束形成声源识别过程中声源辐射声功率定量计算的问题,给出了阵型简洁、便于组合的线阵声强缩放模型。通过推导线阵的声强缩放系数,建立起线阵波束输出结果与声源辐射声功率之间的换算关系。无论是线阵还是平面阵的声强缩放方法,对于偏离阵列中心位置较远处的声源进行辐射声功率估算时都存在较为明显的误差。通过理论推导和仿真模拟计算,研究了同一单极子点声源在不同位置处的声功率估算偏差随频率、幅度的变化规律,发现该估算偏差只与声源偏离位置有关,而与声源自身的强度信息无关的结论,据此给出了相应的声功率估算修正方法。半消声室实验结果和声压法测量结果对比表明:修正后的线阵声强缩放方法用于中高频声源的辐射声功率计算时,单频声源的估算误差不超过1.0 dB,宽带声源的估算误差不超过1.8 dB。     

Geoacoustic Inversion Based on a Vector Hydrophone Array

We propose a geoacoustic inversion scheme employing a vector hydrophone array based on the fact that vector hydrophone can provide more acoustic field information than traditional pressure hydrophones. Firstly, the transmission loss of particle velocities is discussed. Secondly, the sediment sound speed is acquired by a matchedfield processing （MFP） procedure, which is the optimization in combination of the pressure field and vertical particle velocity field. Finally, the bottom attenuation is estimated from the transmission loss difference between the vertical particle velocity and the pressure. The inversion method based on the vector hydrophone array mainly has two advantages： One is that the MFP method based on vector field can decrease the uncertain estimation of the sediment sound speed. The other is that the objective function based on the transmission loss difference has good sensitivity to the sediment attenuation and the inverted sediment attenuation is independent of source level. The validity of the inverted parameters is examined by comparison of the numerical results with the experimental data.     

Sound field separation with a double layer velocity transducer array (L)

In near-field acoustic holography sound field separation techniques make it possible to distinguish between sound coming from the two sides of the array. This is useful in cases where the sources are not confined to only one side of the array, e.g., in the presence of additional sources or reflections from the other side. This paper examines a separation technique based on measurement of the particle velocity in two closely spaced parallel planes. The purpose of the technique is to recover the particle velocity radiated by a source in the presence of disturbing sound from the opposite side of the array. The technique has been examined and compared with direct velocity based reconstruction, as well as with a technique based on the measurement of the sound pressure and particle velocity. The double layer velocity method circumvents some of the drawbacks of the pressure-velocity based reconstruction, and it can successfully recover the normal velocity radiated by the source, even in the presence of strong disturbing sound.     

四壁倾斜水池点源低频声场特性的理论计算及实验测量

本文对哈船院柴油机水下排气噪声实验用四壁倾斜水池的低频声场特性进行了理论计算和实验测量。在理论上采用边界无法对水池点源声场进行数值模似,并将计算结果与球面波扩展规律进行比较。利用柴油机水下排气管口体积脉动声辐射作为声源在柴油机排气频率范围内和常用测点区域内对水池进行声场特性测量。这一工作对柴油机水下排气噪声的实验研究和理论预报有一定的指导作用。     

基于波束形成缩放声强的声源局部声功率计算

基于波束形成法识别噪声源时,为计算主要噪声源的辐射声功率,给出了基于平面波模型的声强缩放方法,模拟计算了单极子点声源局部声功率的计算误差,结果显示:当阵列平面与声源计算平面间距离等于阵列直径时,基于波束形成缩放声强计算的声功率误差仅略高于0.1 dB。为克服旁瓣干扰,给出了具有一定动态范围的声源计算平面积分法,模拟计算了单极子点声源的局部声功率,结果表明:该积分法的计算值与主瓣区域积分法的计算值近似相等,均约等于理论声功率。进一步,波束形成法与声强法的对比算例试验验证了基于波束形成缩放声强计算声源局部声功率方法的有效性。     

有限长导管声场预报的一种边界元方法

为了研究介质流动情况下有限长导管中点声源产生的声场,提出了一种以源势密度作为未知数的间接边界元方法。首先将整个求解空间划分为管内和管外两个相互独立的封闭域,然后利用压力和速度在开口处的连续条件和壁面边界条件将内外域的声场方程联立为一个矩阵方程,求解出源势密度后由源势密度计算出任意位置的声场。与Myers的基准数据比较表明本文所得计算结果与之相符。该方法简化了过去声场边界积分方程的形式和数值积分奇性的解决过程,稍加变换便可应用于任意形状导管和复杂声源。     

近场声全息技术应用有关物理问题研究

近场声全息(NAH)技术,用于大型结构或水声基阵性能参数测量、校准,要有一套高精度声场全息扫描测量接收基阵及同步测量和控制系统装置,以及适应的处理、分析、计算软件。研究实践表明:有关这套测控、分析系统硬、软件的设计指标、参数和使用要求,都涉及一些声学物理问题。为确保全息场量值测量可靠、场重建精度高,对声源的辐射声信号波数谱和近场声压分布,进行了必要的物理分析、计算。得到要保留的高波数分量、全息面的形式和尺度及其离声源的间距、全息面上测点间距等诸参数,与声源形状、尺度、频率等量之间定量关系并给出要求、判据的参考。     

Scattering of spatially inhomogeneous sound waves by a spherical particle

The problem of monopole, dipole, and rotational scattering of a spatially inhomogeneous time-harmonic sound field by an arbitrary spherical particle is solved for the cases of the medium being a viscous compressible liquid or an isotropic elastic medium. Equations for the spherical mean fields at the particle are obtained. These equations are used to derive the formulas for the scattered fields. Different limiting cases of particle behavior are considered. In particular, it is shown that the dipole scattering is determined by two components of particle oscillations, one of which corresponds to translational oscillatory motion and the other to oscillations of two antiphase monopoles. For these types of particle oscillations, a scattering matrix, which determines the scattering of an arbitrary field by a particle, is constructed. The matrix allows the formalization of the processes of multiple sound scattering by particles and is valid for any distances between the particles down to their contact.     

一种预测扩散声场在非规则形状刚性壁面附近干涉图样的方法

扩散声场会在反射边界附近形成干涉图样,研究方法包括平面波模型、简正模态分析、渐进模态分析等,但仅适用于尺度远大于声波波长的矩形声腔。提出一种预测扩散声场在非规则刚性壁面结构附近形成的干涉图样的数值方法,表明结构附近“受挡”声压的互谱矩阵取决于:(1)假定该结构在自由空间中振动辐射声音时其表面法向振速到表面及场点声压的边界元系数矩阵;(2)假定结构置于自由空间中且表面刚性时,点声源辐射声波入射到结构表面上产生的散射声场的边界元系数矩阵;(3)扩散声场均方声压。仿真表明,该途径预测的干涉图样与理论值完全吻合。该预测方法还可用于混响环境下声源附近直达声压均方值的空间分布估计,为混响环境下设备的声源定位提供帮助。     

Spin-12 particles in the field of monopoles

The equations of motion of a $\text{spin}\text{-}\text{1}\text{2}$ particle in the field of a point-like (Abelian and non-Abelian) monopole are solved; helicity states and scattering solutions are constructed. The motion in an extended monopole and dyon source is discussed: charge-exchange scattering amplitudes are computed in the distorted-wave Born approximation. By the introduction of suitable spinors, the separation of variables is accomplished for any value of isospin. The non-existence of fermion zero modes in the field of an SU(2) monopole when $\text{T > 1, J >}\text{1}\text{2}$ is shown.     

环境因素对海-气界面低频异常声透射的影响研究

针对海中声源在海-气界面低频异常声透射问题, 根据两层媒质声传输模型, 分析了大气声速和密度与气压、气温、湿度及海水中声速和密度与海温、盐度间的关系, 研究了低频声透射和传输受温度、气压、盐度、湿度等因素的影响, 分析了各因素对声透射和传输的影响程度. 结果表明: 1) 声透射到大气中的声功率与气温、湿度负相关, 与海温、盐度、气压正相关; 2) 单极子与水平偶极子声源辐射到海中的声功率与海温、盐度负相关, 而垂直偶极子声源辐射到海中的声功率与海温、盐度正相关; 3) 声透射指向性与海温正相关, 与气温负相关; 4) 低频声透射受温度影响最大, 其次是盐度, 受气压和湿度影响较小, 垂直偶极子声源的声透射受温度影响大于水平偶极子和单极子声源.     

Interior and exterior sound field control using general two-dimensional first-order sources

Reproduction of a given sound field interior to a circular loudspeaker array without producing an undesirable exterior sound field is an unsolved problem over a broadband of frequencies. At low frequencies, by implementing the Kirchhoff-Helmholtz integral using a circular discrete array of line-source loudspeakers, a sound field can be recreated within the array and produce no exterior sound field, provided that the loudspeakers have azimuthal polar responses with variable first-order responses which are a combination of a two-dimensional (2D) monopole and a radially oriented 2D dipole. This paper examines the performance of circular discrete arrays of line-source loudspeakers which also include a tangential dipole, providing general variable-directivity responses in azimuth. It is shown that at low frequencies, the tangential dipoles are not required, but that near and above the Nyquist frequency, the tangential dipoles can both improve the interior accuracy and reduce the exterior sound field. The additional dipoles extend the useful range of the array by around an octave.     

非线性对大气介质中阵列聚焦声场分布影响的研究

基于时域有限差分算法将大气中近似到二阶微小项的非线性声波波动方程进行离散化,得到了模拟采用的差分波动方程.在此基础上,数值模拟了初始声压强弱不同的5个点声源组成的线阵列垂直或斜向辐射的连续正弦波在大气中传播时二维声场的分布情况.将线性条件下的模拟结果与非线性条件下的模拟结果进行比较后发现:弱非线性会对声场的分布和阵列聚焦增益产生一定的影响,使声场分布波形比线性条件下的声场分布波形更加靠近阵列,聚焦效果变差;强非线性会使波形发生更严重畸变,这是由于产生了基频以外的其他频率声波引起的;非线性对斜向传播时声场分布的影响与垂直传播时的影响效果基本相同,但由于斜向辐射时的声波几何扩展造成的轴向声压衰减要大于垂直辐射时的轴向声压衰减,因此聚焦增益和强非线性的影响都将小于垂直辐射时的情况.     

浅海舰船噪声声强流的垂直方向性

浅海环境中,确定性声源的多途声信号干涉使得接收点处声强流的方向发生改变,不再与声源位置处的声强流方向一致。只测量声场的标量声强时,无法得到接收点处声强流的垂直方向性,而基于简正波矢量场建模和仿真,可获得理想条件下宽带点声源激发声场声强流的垂直方向性。本文采用单矢量水听器进行海上实验,获得了海洋环境噪声和干扰条件下舰船噪声声强流的垂直方向性。仿真和实验结果表明:远场条件下,浅海干涉现象引起接收点处声强流的方向(极角)随频率和距离变化,其时间-频率分布呈现与LOFAR谱干涉条纹相似的条纹,声强流的极角值主要分布在70?～110?范围内。     

套管井体胶结状态对井孔中声传播的影响

从波动方程出发,利用匹配边界条件,木文推导出套管井中由单极子和多极子声源激发的井内声场定解表达式及相应的导波色散方程.利用实轴积分法和快速傅里叶交换数值方法,具体计算了套管井中不同胶结情况时由单极子声源和偶极子声源激发的井轴上不同源距处的全波列波形。结合套管井中的轴对称导波模式,分析了胶结状况对套管井中单极子和偶极子声源激发的声场的影响.研究结果表明:仅利用普通单极子声源或偶极子声源进行套管井测井,难以区分出套管胶结良好、套管与水泥为滑移胶结和自由套管情况三种不同的胶结情况;只有把普通单极子声源测井与偶极子声源测井结合起来,才有可能准确评价套管的胶结情况。     

Experimental Estimation of the Emission Directivity of a Moving Surface Vessel in Shallow Water

The possibility of constructing an acoustic model of a surface ship’s noise emission in the far field using monopole-type emitters uniformly distributed along the hull is investigated. Experimental data obtained in shallow water are used to calculate the characteristics of equivalent monopole emission sources that form a total sound field similar to the sound field from a moving surface ship. The powers of each monopole and the cross-correlations between them are calculated. For the selected discrete components and linear model of an extended source, the directivity patterns are constructed, reduced to the free space. In the experiments and calculations, technical tools and algorithms were used that ensure high-precision positioning of the vessel with respect to the receiving elements of the array. An equivalent model of the waveguide transfer function in the operations area was preliminarily obtained by acoustic waveguide calibration using specially developed equipment, experimental techniques, and processing algorithms. This made it possible to use adequate seafloor models and the waveguide transfer function when calculating the equivalent sound field and directivity pattern. Good agreement is shown between the calculated and experimental data, both of the directivity pattern and field distribution along the transit characteristics. Practical recommendations are given for developing methods to measure the noise fields of surface vessels.