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。     

利用气象要素估算海洋大气近地层光学湍流

基于近地层相似理论以及极端层结下对该理论的修正,建立了海洋大气近地层光学湍流估算模型,并进行了数值分析。结果表明:当气温与海温相差较大时,温度起伏对光学湍流贡献最大;当气温与海温相差较小时,海面湿度越小,光学湍流越强;在较低湿度条件下,尤其在红外窗区,湿度扰动对光学湍流贡献起支配作用。当气温小于海温时,光学湍流先随海面风速增大而增强,后随风速增大而减弱;当气温大于海温时,光学湍流总体上是伴随海面风速增大而减弱。当气温小于海温或与海温接近时,与可见光和近红外波段相比,红外窗区光学湍流显著偏强;当气温远大于海温时,两波段光学湍流接近。在中性和近中性层结下,大气折射率结构参量C2n依赖于垂直高度的负幂指数接近-2/3,伴随海面大气层结稳定度或不稳定度增强,C2n依赖于垂直高度的负幂指数的绝对值分别逐渐减小或增大,并分别趋近于稳定极限下的0或不稳定极限下的-4/3。     

空间有源消声的声能量流研究

本文从理论和实验两个方面对平面噪声场中单极子次级源、偶极子次级源最小辐射声功率时的空间产能量流作了研究，从而直观、清晰地描述了一种消声机理：有源声吸收。并指出：尽管此时次级声源是“能量吸收”结构，但依然存在空间能量转移现象，而且并非各种类型的次级源在各个方向都吸收声能量，次级源各个方向吸收的能量大小也是不同的。实验结果与理论分析相吻合。     

空气中声速自动测量系统

针对超声波测声速实验中存在的测量结果误差大的问题,基于虚拟仪器的设计原理,依托单片机设计了空气中声速测量电子装置,在该系统中温度/湿度传感器、气压传感器接收端进行信号采样,单片机依据声速和气压、温度、湿度、水的饱和蒸气压相关物理量间的关系进行数据运算后输出声速值,该装置所测得的声速值与理论声速值误差在1%以内.     

西北太平洋副热带模态水区域声传播特性*

西北太平洋副热带模态水(STMW)是在西北太平洋夏季出现的温跃层中温度、盐度和密度具有垂向均一性的水团。由于西北太平洋副热带模态水(STMW)的存在,深海声速剖面呈现出双跃层结构,对深海远程声传播产生较大的影响。本文对比分析了西北太平洋副热带模态水(STMW)区域夏冬两季典型声速剖面环境下的声传播规律。分析结果表明,夏季声速剖面环境下,声源位于浅层声道宽度内时,声传播为浅层声道的类深海声道传播与会聚区传播的复合形式。掠射角较小的声线被限制于浅层声道中,增加了会聚区内的到达结构,并且增强了在影区的声能量,在第二影区内的传播损失比冬季声速剖面环境下最多低近60dB。本文推导了浅层声道的截止频率的近似表达式,分析结果表明,截止频率主要由浅层声道的宽度和正梯度段的声速差值决定,夏季STMW区域浅层声道截止频率主要集中在100Hz左右。     

基于平面声源进行结构声辐射有源控制的实验研究

采用分布式平面声源作为次级声源,对振动钢板的声辐射进行了抵消实验,验证了以往研究中的一系列关键理论。实验研究结果表明:一个平面声源可以控制钢板奇-奇模态的声辐射,两个平面源可以控制结构偶-奇或奇-偶模态的声辐射,同时也可以控制结构奇-奇模态的声辐射;平面声源的面积和布放位置对降噪效果有重要影响,采用单个平面声源控制时,平面声源面积越大,控制效果越好;基于近场声压的误差传感策略是有效可行的,实际中,将近场测量面的声功率作为有源控制的目标函数与总声功率作为目标函数是一致的;控制后远场声压和声强都得到有效降低,部分区域的声能向声源流动,近场声压及声强分布也发生显著变化。     

低频三阶超心形指向性声源设计

为提高圆周阵列主极大的指向性增益,提出基于圆周等间距六元密排阵的低频三阶超心形指向性声源,利用点源方法对其波束特性进行研究与分析,得到了指向性声源的优化方案。以具有弱互辐射效应的单边辐射弯张换能器作为圆周密排阵阵元,利用有限元方法分析了三阶超心形指向性声源的声辐射特性,相比单极子声源,其主极大方向指向性增益为8.0 dB,可达到提高增益的目的。研制了实验样机,经水池实验获得该声源的工作特性:谐振频率为0.92 kHz,对应发送电压响应级为139.0 dB,-3 dB波束宽度为56.0°,各旁瓣小于-15.0 dB。有限元计算与实测结果验证了优化方案的可行性,实现了低频三阶超心形指向性发射。     

大气温度分布特性及对折射率结构常数的影响

影响大气湍流运动强弱和时空结构的因子比较复杂,大气中平均流场和大气温度的分布都是不均匀的,特别是沿垂直方向的大气温度分布,决定着垂直方向的热力不稳定性和湍流的强弱。观测事实表明,随着季节和天气条件的不同,大气温度垂直分布有很大的变化。通过对我国安徽合肥地区整层(0～20km)大气温度的观测资料的分析,得到了大气温度的垂直分布廓线和统计特性模式误差廓线;大气折射率结构常数Cn^2是表示大气光学湍流强度的一个重要参量,但大气折射率的测量较为困难,因此通常先测量温度起伏量,再用平均的温度和气压来计算得到Cn^2。通过对温度和气压模式误差的分析,可以计算得到Cn^2结果的误差,重点分析大气温度分布特性及由此带来的模式误差,并讨论其对计算Cn^2的影响。     

三通管路管壁声传递损失测试方法

针对汽车进气系统三通管路的特点,提出了多通管路的管壁传递损失测试方法。并以某车型的双涡轮增压发动机进气三通管道为例,采用该方法评价其用塑料代替铝后的声学性能,主要以声传递损失来评价涡轮增压器噪声通过三通连接管路管壁的辐射和透射特性。测试过程中,三通管道的两个连接涡轮增压器端口分别用声源两次发声,靠近进气歧管端口采用两种不同反射末端,然后在每段管路布置两个压力场扬声器进行测试,并基于平面波分离入射波和反射波,同时在三通管道外用声功率半球面十点分布法自由场扬声器测试,经过3次测量来计算管道管壁的声传递损失。由于声传递损失是管道本身特性决定,所以该测试方法能够准确找出塑料件和金属件在不同频率的声学特性差异。而后,在声传递损失测试结果的基础上,结合近场声全息方法和波束形成原理进行声源识别,可知该三通管路材质改为塑料后主要噪声来自焊缝薄弱处的中高频透射声和管壁结构的低频辐射声。     

基于圆柱形非均匀迷宫结构的动态可调定向声辐射

构造了一种圆柱形四通道非均匀迷宫结构,利用该结构设计的圆环型超构材料能够实现动态可调的定向声辐射.所构造的圆柱形非均匀迷宫结构具有偶极子共振特性,在偶极子共振频率附近,声波能够从两个占比较大的扇形通道开口向外辐射,此时的圆柱形非均匀迷宫结构可近似地看作一个偶极子源.当圆柱形非均匀迷宫结构围绕圆心进行旋转时,所形成的偶极子源的位置和向外辐射声波的方向也随之发生改变.将点声源放置在由18个非均匀迷宫结构组成的圆环型超构材料的中心,调节圆柱形非均匀迷宫结构的旋转角度,使各微结构处于导通或截止状态,从而控制点声源在各个方向上的传播特性,实现具有动态可调特性的定向声辐射.此外,研究了圆柱形非均匀迷宫结构旋转角度对透射声波的影响,探究了微结构的开关效应,为构造简易的声定向辐射设备提供了新思路.     

Sound radiation into air by a point source moving underwater

Based on the discovery that the majority of radiated energy of a stationary sound source in shallow water is into the air at infrasonic frequencies, the sound transmission into air from a point source moving underwater is investigated in this letter. It is found that a moving sound source can radiate more acoustic energy into the air than a stationary one and the amount of energy radiated into the air increases with the speed of the moving source. Simulations show that the sound transmission into air is dominated by the inhomogeneous waves generated by the moving source.     

Vortex sound due to a flexible boundary backed by a cavity in a low mach number mean flow

Low frequency sound radiated due to the unsteady motion of an inviscid vortex in the proximity of a flexible membrane backed by an airtight cavity on an otherwise rigid plane is investigated theoretically. Results show that both monopole and dipole are created but the latter is important only when the vortex is traversing over the membrane. The monopole results from the membrane vibration and the dipole from the transverse motion of the vortex. It is also found that these sound fields tend to counteract each other. The increase in the mean flow speed in general results in a stronger acoustic power radiation, but sound attenuation may be possible if the membrane-cavity system is weak compared with the mean flow momentum.     

Enhanced sound transmission from water to air at low frequencies

Excitation of acoustic radiation into the air from a low-frequency point source under water is investigated using plane wave expansion of the source spectrum and Rayleigh reflection/transmission coefficients. Expressions are derived for the acoustic power radiated into air and water as a function of source depth and given to lowest order in the air/water density ratio. Near zero source depth, the radiation into the water is quenched by the source's acoustic image, while the power radiated into air reaches about 1% of the power that would be radiated into unbounded water.     

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.     

An improved method for determination of radiated sound power and sound transmission from large sources

The proposed method is an improvement of the methods for measurement of sound power level, in which the real sound source is represented by an equivalent monopole. It is based on a new concept, the equivalent acoustical centre, which is introduced and defined in the paper. The main assets of the method in comparison with the existing monopole-methods are a higher accuracy, a possibility of measuring at considerably shorter distances from the noise source, and a certain freedom in the choise of the measuring points around the source. Moreover, since the position of the equivalent monopole is obtained, it can be used for more accurate calculations of the sound transmission to the surroundings of the source. The method is particularly suitable for determination of sound power levels and sound transmission of noise radiated from large areas, such as industrial complexes, installations of petrochemical industries and the like. It can be especially useful in disputable cases of industrial plants that produce noise close to the legally permissible limit. The new method was first reported at the meeting of the Dutch Acoustical Society which took place in Utrecht on 22 November 1978.     

偶极随钻声波测井声压和径向位移响应的差异*

声波测井仪接收到的电信号通常是多个压电片响应的叠加,它主要是由声压还是径向位移响应转化而来,或是两种响应兼有目前未有定论。该文通过实轴积分法和复变函数法计算并对比分析随钻声波测井的声压和径向位移场,发现这两种响应特性有着显著的差异。首先,软地层的偶极随钻测井时,声压信号包含钻铤波和舒尔特波两个波群,而径向位移信号仅有钻铤波波群;其次,单极声源情况下,声压和径向位移信号的钻铤波能量分别集中在钻铤内、外壁,而偶极情况恰好相反,可见,钻铤按照单极情况的分析结果进行刻槽后,高频时的拖尾现象会影响偶极信号中舒尔特波对横波速度的反演。因此,阐明两类信号的差异对横波速度的反演和钻铤波的压制都具有重要意义。     

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。      

Statistical techniques analysis of SST and SLP in the Persian Gulf

Thirty-four years of data (1967-2000) are used to investigate the variability pattern relevant to air-sea interaction in the Persian Gulf. The patterns are derived using statistical techniques, such as empirical orthogonal function (EOF) and singular value decomposition (SVD). Statistical analysis methods are applied to determine the coupled modes of variability of monthly sea surface temperature (SST) and sea level pressure (SLP). The significant of the air-sea interaction is found by a strong resemblance between EOF and SVD eigenvectors and expansion coefficients of SST and SLP. We find that the four leading EOF patterns of SST together account for 99.8% of the total monthly SST variance and 94.4% of the SLP variance. The zero contour in the first SST EOF identified the front which separates the Persian Gulf cyclonic gyres.The SVD modes provide more information on the coupling between the fields than the modes obtained by EOF methods. Lagged correlation analysis between SVD1(SLP) and SVD1(SST) indicates that the coupling is strongest when SLP leads SST by −12, −6, 6 and 12 months. Therefore, the first mode of the SVD analysis seems to depict an air-to-sea forcing, in which the sea response to the atmospheric changes appears with an semiannual and interannual time lag.The two leading SVD modes of variability of the coupled SST and SLP fields account for 99.6% of the total variance. The main patterns of both variables of variability of both variables independently provide considerable information on the coupling, but only one of the two variables dominates each of the two first coupled modes.The first coupled mode of variability between the SST and atmospheric pressure can be described as a strengthening and weakening of the cyclonic gyres, which seems to force fluctuations in a north-south dipole structure in the SST by Ekman upwelling which is a wind-related process. The atmospheric forcing of the SST changes is detectable in the sea with a lag of 1 and 6 months.     

Energy radiated by a point acoustic dipole that reverses its uniform velocity along its rectilinear path

This work extends a mathematical approach developed recently for monopoles to describe the sound energy radiated by a rectilinearly moving dipole that changes direction along its trajectory. Although the dipole travels with constant speed, it undergoes acceleration by reversing its direction during a finite time interval along its path. This work determines the joint angular and frequency distribution of the radiated energy, its angular distribution, and the total radiated energy output. Results for the radiated energy are systematized by expressing the radiation integrals in terms of hypergeometric functions. This procedure simplifies the evaluations, particularly at low Mach numbers, and permits the comparison of results to the earlier monopole case.     

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.