基于声场重建的材料吸声系数测量方法

针对现有方法对材料吸声系数进行现场测量时存在低频测量误差大的问题,本文提出了一种利用扬声器线阵列对材料吸声系数进行现场测量的新方法。该方法使用基于能量比值约束的最小二乘法在待测材料表面进行平面波声场重建并结合双传声器传递函数法对材料的吸声系数进行测量。数值仿真表明在100～1600 Hz频率范围内,新方法在未加约束时能够对材料的吸声系数进行准确测量。在半消声室中利用新方法测量了三聚氰胺泡沫的吸声系数,分析了能量比值约束值对测量结果的影响,并和阻抗管以及其它两种现场测量方法的测量结果进行了对比。结果表明该方法能够对吸声材料在160～1600 Hz频段内的吸声系数进行准确测量,并且相较于现存的现场测量方法,新方法具有更低的测量频率下限。     

On the scattering of sound by a magnetic field in a MHD fluid

We investigate the effect of a localized magnetic field on the propagation of sound in an infinite fluid described by the magnetohydrodynamic equations (MHD). An externally imposed magnetic field will scatter an acoustic wave, and the scattered wave is related to the spatial structure of the magnetic field. Measuring it is thus a non-intrusive probe for the magnetic field. Simple examples likely to be encountered in practice are worked out, and estimates are given that suggest the practical feasability of this diagnostic tool in current MHD experiments.Received: 1 May 2003, Published online: 15 October 2003PACS: 42.25.Fx Diffraction and scattering - 47.65.+a Magnetohydrodynamics and electrohydrodynamics - 52.35.Dm Sound waves     

On the low-frequency sound scattering in a moving microinhomogeneous medium

The Rayleigh law that governs low-frequency sound attenuation due to the scattering by inhomogeneities in a microinhomogeneous medium is generalized to the case of particles moving in a flow or falling under gravity. Corrections to the scattering’s cross section that adjust the Rayleigh law to the case of a potential flow around inhomogeneities are calculated. It is shown that, when microinhomogeneities are moving in a viscous medium, the characteristics of discrete scatterers may considerably deviate from the Rayleigh law. Based on the data on the velocity and size distribution of falling drops of water in air, refinements are proposed for the laws of low-frequency sound scattering by rain.     

On the scattering of sound by a vortex ring

An analytical treatment is given of the scattering of sound by a circular vortex ring translating at low Mach number in an unbounded stationary medium. The theoretical results complement the recent numerical investigation of this problem by Kambe and Mya Oo [1].     

一种现场测量材料吸声特性的新方法

现有的材料吸声系数测量方法主要有混响室法和驻波管法,都属于实验室测量方法,不适合现场测量。使用普通扬声器的反射法可以对材料的吸声特性进行现场测量,但是对材料尺寸和测试环境有较高的要求。本文利用参量阵非线性自解调可听声的高指向性和在阵长距离内的平面波特性,结合传递函数法,测量材料的吸声系数,并与传统驻波管测量结果进行了对比。结果表明在普通房间条件下,不需要驻波管,混响室等实验环境,即可对小尺寸的材料进行吸声系数的现场快速测量,具有较大的实用性。     

Development of a simplified field method of measuring sound insulation

A simplified field method of measuring the sound insulation of partitions within the framework of the British building regulations has been proposed and its development is described in this paper. The selection of the source type and the weighting function for the measurement of the overall sound pressure levels and the effects of loudspeaker and room response are discussed.A simple method of normalising for receiving room absorption is proposed and evaluated. Some results of experiments, carried out both in the laboratory and in field conditions, are reported. The results of this study show that the simplified field method has considerable potential for bringing about wide-scale enforcement of the building codes, which have so far been ineffective.     

近场影响消除法测量声吸收系数

本文采用数值模拟方法理论研究了活塞式辐射器的辐射近场和采用“大”尺寸接收器接收对声场的平滑作用以及近场(及其过渡区域)对测量声吸收系数的影响.计算表明,在近场及其附近,用通常的方法测量声吸收系数将产生很大的误差.通过分析误差产生的原因,本文提出了近场影响消除法测量声吸收系数,并对a/λ=3、9,b=0、0.5a、a(a,b分别是辐射器和接收器半径)几种情况进行了数值模拟计算,结果表明,采用这种方法不仅能在近场区域进行测量,而且具有较高的测量精度。     

Evaluation of the scattering coefficient for high-frequency sound scattered from the sea surface

The effect of the deviation of the real rough sea surface from the Gaussian isotropic surface on the scattering coefficient of high-frequency sound is analyzed. The analysis relies on the data on the sea surface slopes, obtained from field experiments in the Black Sea with the use of a two-dimensional laser slopemeter. It is shown that the effect of the anisotropy of the surface wave field on the scattering coefficient is small when the angle of incidence is small, but this effect rapidly increases as the angle of incidence grows. The deviation of the real statistical moments of the sea surface slopes from those corresponding to the Gaussian distribution results in a ±20% error in the theoretical values of the scattering coefficient.     

On the sound field of a resilient disk in free space

Radiation characteristics are calculated for a circular planar sound source in free space with a uniform surface pressure distribution, which can be regarded as a freely suspended membrane with zero mass and stiffness. This idealized dipole source is shown to have closed form solutions for its far-field pressure response and radiation admittance. The latter is found to have a simple mathematical relationship with the radiation impedance of a rigid piston in an infinite baffle. Also, a single expansion is derived for the near-field pressure field, which degenerates to a closed form solution on the axis of symmetry. From the normal gradient of the surface pressure, the surface velocity is calculated. The near-field expression is then generalized to an arbitrary surface pressure distribution. It is shown how this can be used as a simplified solution for a rigid disk in free space or a more realistic sound source such as pre-tensioned membrane in free space with non-zero mass and a clamped rim.     

On the neutron diffraction in crystals in the field of a sound wave

Diffraction of neutrons in crystals under influence of a sound wave is considered. The probability of scattering of neutrons at the elastic interaction with the crystal is calculated. On the contrary, scattering of neutrons by an acoustical phonon has inelastic character. The possibility to control the Debye-Waller factor is shown.     

On the statement of the problem of sound scattering by a cylindrical vortex

The well-known problem of sound wave scattering by a Rankine vortex is investigated. Although the problem has been studied for years, none of the solutions reported in the literature can be considered completely correct. It is demonstrated that the main difficulty consists in the absence of a mathematically well-posed statement of the problem for a plane wave (which is used in most of the approaches) because of the slow decrease in the mean flow velocity at infinity. This gives rise to multiple solutions, including those singular on a line behind the vortex, and each of them claims to be correct. It is shown that, in spite of the decrease in the mean flow velocity, the problem does not possess any remote region at infinity, tending to which it is possible to preset the plane wave condition for the incident field. Therefore, for the external field, the remote point source condition is proposed. This approach makes it possible to state a mathematically well-posed problem, to reveal the origin of the aforementioned ambiguity, and to compare previous approaches used for solving the problem under consideration.     

On the sound field of a vault with two open ends

The sound field in a vault with two open ends is studied and its sound pressure distribution is compared with that in an enclosure and a long space. The accuracy of estimation on this kind of space by the classic Sabine Formula, the modified Sabine Formula and the ray tracing method are compared through numerical simulations and experiments. It is found that the sound field in this type of non-close space is not uniform, SPL declines faster near the openings, and RT values have a large span when the source is not at the center. Non-exponential decay is also found in this open vault, and the reason and precondition of its occurrence are discussed.     

Methods for measuring bistatic characteristics of sound scattering by the ocean bottom and surface

It is very interesting to measure the bistatic characteristics of sound scattering by the ocean bottom and surface for the development of present-day hydrolocation net-centric schemes. Possible methods for measuring scattering bistatic characteristics are evaluated. The angular characteristics of reverberation related to illumination of a water area by active linearly frequency-modulated signals are studied using linear passive arrays to test a measuring scheme in a shallow water area with a depth of ~20 m in the 1–3 kHz frequency range. The bistatic sound-scattering characteristics in the water area are calculated based on the measurements. The obtained characteristics are compared with the known data on the backscattering of sound.     

On the sound field of a shallow spherical shell in an infinite baffle

A method is presented for calculating the far field sound radiation from a shallow spherical shell in an acoustic medium. The shell has a concentrated ring mass boundary condition at its perimeter representing a loudspeaker voice coil and is excited by a concentrated ring force exerted by the end of the voice coil. A Green's function is developed for a shallow spherical shell, which is based upon Reissner's solution to the shell wave equation [Q. Appl. Math. 13, 279-290 (1955)]. The shell is then coupled to the surrounding acoustic medium using an eigenfunction expansion, with unknown coefficients, for its deflection. The resulting surface pressure distribution is solved using the King integral together with the free space Green's function in cylindrical coordinates. In order to eliminate the need for numerical integration, the radiation (coupling) integrals are solved analytically to yield fast converging expansions. Hence, a set of simultaneous equations is obtained which is solved for the coefficients of the eigenfunction expansion. These coefficients are finally used in formulas for the far field sound radiation.     

Visualization of the sound scattering by a cylinder

The schlieren method is applied to visualizing the sound scattering ofa cylinder immersed in the liquids.Images of the echoes,caused by normal andoblique incidence from a cylinder,are obtained for two kinds of liquids:(1)theliquid is composed of a transparent liquid;and(2)the liquid is composed of twonon-mixable transparent liquids.It is found experimentally that the echo canbe caused by the discontinaites in the shadow zone.This fact is utilized to dem-onstrate the locus of the helical surface wave creeping along the surface of thecylinder.For such cases the three-dimensional image can be described by thismethod and has not to use the system with special appliances and complexalgorithm.     

柱体散射的声场显示

本文应用Schlieren显示方法,对有限长圆柱体在单一液体及有分层界面的两液层中声波垂直入射和斜入射的回波结构以及界面对回波结构的影响作了实验研究,实验用图象显示了影区内不连续处产生的回波,我们利用这一结果,成功地描绘了螺旋表面波的轨迹,从而实现了应用Schlieren法来阐明三维声场的途径,避免了需要用庞大设备和复杂运算的三维显示技术。     

有限液态介质样品声衰减的实验室测试新方法

本文介绍了一种在实验室条件下利用声脉冲测定有限容量液态介质样品声衰减系数的测试方法。该方法根据声纳方程原理导出了与测试系统参数和声场参数无关的计算式，从而避开了对测试系统参数和声场参数的测量，采用截取未被边界散射干扰的直达声脉冲的部分信号进行频谱分析，消除了在有限样品情况下必然存在的边界散射干扰，同时采用多次统计平均的做法抑制了噪声干扰。     

Methods of measuring frequency shifts in the interference structure of the sound field in oceanic waveguides

The efficiency of the correlation method is considered as applied to measuring frequency shifts of maxima in the interference structure of the sound speed under the influence of distortions of the sound-speed profile. The method is based on tracing the position of the maximum of the cross-correlation function corresponding to the spectrum of the transmitted signal in the frequency domain. The distortion is modeled by seasonal variations of the hydrological environment. The noise immunity of the method is analyzed. The correlation method is compared with other known methods of tracing frequency shifts of the interference maxima.     

Free field measurements of the absorption coefficient for nonlocally reacting sound absorbing porous layers

A simple asymptotic approximation with two parameters (the plane wave reflection coefficient and a correction factor) has been derived by Brekhovskikh and Godin [Acoustics of Layered Media II. Point Sources and Bounded Beams (Springer, New York, 1992)], for the spherical field reflected by nonlocally reacting surfaces. In the present work, an expression for the correction factor is obtained for the case of a homogeneous and isotropic porous layer. A free field method for evaluating reflection and absorption coefficients with this approximation is presented. The evaluation of the absorption coefficient at normal incidence is performed on a porous layer on a rigid backing, and compared to measurements performed using a two microphone technique and an impedance tube.     

On the vertical structure of the sound field in a canonical waveguide at long ranges

The geometrical-acoustics approach is used to calculate the vertical structure of the sound field in an oceanic waveguide. The profile of the sound speed is specified to be canonical and range-independent along a 1000-km propagation path. A monochromatic sound source lies on the waveguide axis. It is shown that, at long distances from the source, the sound field formed by the water-path rays is mainly concentrated in the caustics, the number of which is determined by the number of the overlapping ray cycles at a given distance. A method for estimating the amplitude of the sound field produced by individual rays is proposed. The amplitudes obtained are used to calculate the total sound field along the vertical. A possible cause of the chaotic distribution of ray coordinates is considered. This cause may consist in the arbitrary choice of the number of rays and their departure angles without taking into account the discrete character of one of the variables. This mechanism of ray chaos formation furnishes an explanation for the fact that the chaos obtained in calculations is mainly associated with the flat rays.