Reconstruction of the signal produced by a directional sound source from remote multi-microphone recordings

A mathematical method for reconstructing the signal produced by a directional sound source from knowledge of the same signal in the far field, i.e., microphone recordings, is developed. The key idea is to compute inverse filters that compensate for the directional filtering of the signal by the sound source directivity, using a least-square error optimization strategy. Previous work pointed out how the method strongly depends on arrival times of signal in the microphone recordings. Two strategies are used in this paper for calculating the time shifts that are afterward taken as inputs, together with source directivity, for the reconstruction. The method has been tested in a laboratory environment, where ground truth was available, with a Polaroid transducer as source. The reconstructions are similar with both strategies. The performance of the method also depends on source orientation.     

有限振幅声波间的非线性相互作用对声源远场指向性的影响

根据Fenlon理论推导得到了多频声源的辐射声压.基于单频声源谐波指向性的求解方法,得到了二阶近似下的双频声源辐射出的声波相互作用时的远场指向性.分别研究和讨论了在初始辐射声压和频率不同情况下,两列波的相互作用对其中一列波的一阶波和二阶波远场指向性的影响.结果表明,声波间的相互作用对声源远场指向性的影响根据各波之间的相对初始辐射声压和相对频率的不同而有所变化. 关键词： 有限振幅声波 非线性相互作用 远场指向性     

Incorporating source directionality into outdoor sound propagation calculations

Many outdoor sound sources, such as aircraft or ground vehicles, exhibit directional radiation patterns. However, long-range sound propagation algorithms are usually formulated for omnidirectional point sources. This paper describes two methods for incorporating directional sources into long-range sound propagation algorithms. The first is the equivalent source method (ESM), which determines a compact distribution of omnidirectional point sources reproducing a given directivity pattern in the far field. This method can be used with any propagation algorithm because it explicitly reconstructs a source function as a set of point sources with certain amplitudes and positions. The second is a directional starter method (DSM), which is developed specifically for the parabolic equation (PE) algorithms. This method derives narrow- or wide-angle directional starter fields, corresponding to a given source directivity pattern, without reconstructing the equivalent source distribution. Although the ESM can also be used for the PE, the DSM is simpler and can be more convenient, especially if the sound propagation is calculated only for one or a few azimuthal directions. While these two methods are found to produce generally distinct starter fields, they nonetheless yield identical directivity patterns.     

A simple impulse sound source for measurements in room acoustics

This paper deals with the construction and characteristics of a specially designed wooden clapper intended to operate as an impulse sound source for measurements in rooms. It is to be used as an alternative impulse sound source for experimental estimations of impulse responses of rooms located at remote and specific places where none of the standard sound sources - an omnidirectional loudspeaker system, etc. - can be used. The paper describes the spectral characteristics and directivity of the clapper impulse. Its features are compared with other impulse sources.     

Walk away method versus reverberation time method in determining the room constant

The room constant is a quantity relating to the sound absorption in an enclosure. It is necessary to room acoustics engineering calculations based on the perfectly diffuse sound field theory, both in noise control work and sound system design. Normally, in the validation of the above-mentioned theory, in situ evaluations of the room constant are performed on the basis of reverberation time measurements in the relevant space. Such methods fail to give accurate results when uncertainties regarding the volumes and areas effectively involved in the reverberation process arise. In this case the walk away method is more advantageous in that it only needs steady state sound pressure level measurements at various distances from a small sound source whose directivity factor is known.In this paper both methods are discussed and experimental results relative to four different rooms are compared.     

跑道线圈换能器在铁路重载货车探伤中的应用*

针对在线车轮超声探伤系统的工程应用,研究了跑道线圈电磁超声换能器由洛伦兹力换能机制在钢中产生的辐射声场。根据外磁场和线圈配置,将表面力源近似为非均匀的水平剪切力源,数值计算得到了此力源的辐射声场指向性图,并与实验结果进行比对,结果表明辐射的水平偏振横波具有中间强两侧弱的指向性,对理解跑道形电磁线圈超声换能器的辐射声场和在重载货车车轮辋裂缺陷探伤中的工程应用具有一定的指导意义。     

压力罐中水声材料声学特性的参量源测量法

设计了一种原波频率500 kHz、差频范围1~30 kHz的截断宽带参量阵,作为水声材料测量系统的声源。通过分析典型频率下的宽带参量源指向性理论计算和实际测量结果,发现两者结果的曲线基本吻合,证明计算模型是正确的。应用钟形短时脉冲实现水声材料声特性的宽带测量,有益于降低样品边缘衍射干扰。并建立了测量水声材料大面积板状样品声压反射系数、声压透射系数和吸声系数的压力罐测量系统,罐体内尺寸Φ4 m×12 m,最高静水压4.5 MPa,测量频率范围1~30 kHz。对标准样品(尺寸1m×1m)进行了测量实验,其测量结果和理论曲线有很好的吻合,参量源测量法得到了验证;之后,通过对一块橡胶板样品在不同静压力下的吸声性能进行了测量和有效评估,进一步确认了参量源测量法在压力罐这样有限水域中的潜在应用价值。     

线阵声强缩放方法在水下声源辐射功率估算中的应用

为了解决水下声源辐射声功率难以计算的问题,利用线阵声强缩放方法在波束形成声源识别的基础上,根据波束输出结果与声源辐射声功率之间的换算关系来获得相应的声功率。为了提高线阵声强缩放方法的水下声功率估算精度,给出了一定动态范围限制的主瓣区域积分方法,并通过仿真验证了该方法的有效性。在消声水池中开展了水下声功率估算的实验研究。在不同的测试距离下,对双声源条件下的单频以及宽带声源在阵列侧的辐射声功率进行了估算,以混响法的测量结果为参考值,研究了估算误差随声源频率、测试距离等影响因素的变化规律。实验结果表明,无论是单频还是宽带声源,声功率的最大估算误差不超过2.6 dB,在高频时不超过1.6 dB。验证了线阵声强缩放方法应用于水下声源辐射声功率估算的正确性与可行性。     

An alternative method to measure the on-axis difference-frequency sound in a parametric loudspeaker without using an acoustic filter

The self-demodulation characteristic of finite-amplitude ultrasonic sound waves can be applied with parametric loudspeaker to reproduce audible sound with highly directivity. But measuring the difference-frequency sound is still a problem due to the spurious sound generated as a result of nonlinearity caused by the product of the primary waves at the receiving system. In this paper, based on the phase-cancellation method and the Gaussian beam expansion technique, an alternative method is proposed to measure the on-axis difference-frequency sound accurately without using any traditional acoustic filter, where the spurious sound can be greatly reduced or even eliminated. The proposed method is more suitable for the case where the piston source in the parametric loudspeaker comprises multiple small piezoelectric transducers (PZTs) and each transducer element in the array may have different frequency response. The validity of the proposed method is confirmed both by simulations and experiments.     

Equalization of acoustic source using multi-pole sources and source strength estimation using inverse method

Sound measured at various points around the environment can be evaluated by a series of multi-pole sources and their acoustic strength can be acquired. In this numerical study, a method, called the inverse method, was examined to achieve this goal. A variety of arrangements of different sources were considered and the acoustic strength of these sources was acquired. Through the application of the mismatch criterion, good conformity was observed between these sound models and the original sound. Furthermore, with regard to results, sound was generated via different source arrangements which showed acceptable agreement with the original sound. Finally, an arrangement named ‘sources vertical arrangement’ was selected as the best approach.     

Estimation of the far-field directivity of broadband aeroengine fan noise using an in-duct axial microphone array

This paper presents a measurement technique for estimating the far-field directivity of the sound radiated from a duct using measurements of acoustic pressure made inside the duct. The technique is restricted to broadband, multi-mode sound fields whose directivity patterns are axi-symmetric, and whose modes are mutually uncorrelated. The technique uses a transfer function to relate the output from an in-duct axial beamformer to measurements of the far-field polar directivity. A transfer function for a hollow cylindrical duct with no flow is derived, and investigated in detail. Transfer functions for practical cases concerning aeroengine exhausts are also presented. The transfer function is shown to be insensitive to the mode-amplitude distribution inside the duct, and hence can be used to predict the directivity in practice where the noise source distribution is unknown. The technique is then validated using a no-flow facility, and is shown to be able to predict variations in the far-field directivity pattern and also estimate the far-field sound pressure levels to within 2 dB. It is suggested that the proposed technique will be especially useful for fan rig experiments, where direct measurement of directivity, for example by use of an anechoic chamber, is impossible.     

A method for evaluating the relation between sound source segregation and masking

Sound source segregation refers to the ability to hear as separate entities two or more sound sources comprising a mixture. Masking refers to the ability of one sound to make another sound difficult to hear. Often in studies, masking is assumed to result from a failure of segregation, but this assumption may not always be correct. Here a method is offered to identify the relation between masking and sound source segregation in studies and an example is given of its application.     

Influence of source directivity on noise levels in industrial halls: Simulation and experiments

Noise exposure of workers in industrial halls is mainly induced by noisy machines whose acoustical features are often globally known by Sound Pressure Level. The evaluation of the directivity of these noise sources can help to anticipate specific solutions for noise reduction.This study shows how the directivities of three wood-working machines have been characterized. Some characterisations have been achieved with a simple and fast acoustical intensity mapping which meets the constraints of industrial areas.When source directivity is evaluated, its influence on the noise field in industrial halls can be assessed. Some simulations and some experiments allowed the estimation of the noise field induced in workshops by both directional and omnidirectional sources. Comparison of the fields prove that the noise distribution is influenced by the source directivity if the halls are empty. As soon as the halls contain scattering objects, the directivity effect is reduced a lot and the noise field remains nearly the same far from the source whatever source used. Nevertheless, workers close to a machine are exposed to noise according to their position with respect to the machine. Exposure at the workplace can vary from 4 to 8 dB(A) according to the directivity of machines such as those measured in the trials.     

Influence of vehicle noise emission directivity on sound level distribution in a canyon street. Part I: Simulation program test

Simulation programs may be useful tools for controlling an environmental noise. The computer simulation program PROP11 that enables predictions of the time-average sound level within an urban system is used here. A roadway as a noise source is represented by a sum of the sound exposures due to individual vehicle pass-bys. Different representations of equivalent point sources for various classes of vehicles are allowed including directivity characteristics other than omnidirectional. Propagation throughout an urban system contains multi-reflections from the walls and single and double diffraction at their edges. In this paper, the PROP11 program is used to predict the sound level between opposing façades of buildings in a canyon street. A multi-lane road is assumed to contain two classes of vehicles (light and heavy) ones. The equivalent source representing vehicles is defined by the source power spectrum, its position above the ground and directivity characteristics. The consequences of introducing equivalent source directivity are analyzed.     

Prony's method applied to estimate noise source locations and their sound powers

An application of Prony's method for evaluating the acoustic power and location of sound sources from spatially sampled data is described. A sound source considered as a point source has an intensity proportional to the inverse square of the distance between source and observation point. The Fourier transform of this intensity function is an exponential function with a real exponent. The shift property of the Fourier transform results in a spectral change in the phase angle, which is expressed in the transform domain by a multiplicative exponential function of pure imaginary exponent. In this paper the usual time axis of the Fourier pair of time and frequency is treated as a variable denoting the location of the sound source. Accordingly, each spectral component of spatially sampled sound intensity generated by n point sources can be expressed as a linear combination of n complex exponentials. By applying Prony's method to the spectral data, these unknown exponents can be calculated numerically. This paper deals with an estimation procedure to find the location and power of a noise source. The estimation is done by minimizing the sum of the squares of the errors between the model and measured data. The proposed method has general applicability to problems where the so-called inverse square law for intensity can be assumed to be valid.     

Line source and site characterizations for defining the sound transmission loss of building facades

An analytical model is presented for defining the sound transmission loss of building facades exposed to noise from line sources. The model describes the non-diffuse sound field incident upon the facade in terms of both source and site parameters. The effects of facade orientation relative to the line source and the sound propagation with distance are introduced as a single term in the definition of the facade sound transmission loss. This term defines a mean angle of incidence for the exterior sound field that is equivalent to a point source location relative to a point on the facade. Numerical results are presented illustrating the magnitudes of these effects and it is shown that alternative methods for conducting field measurements of building facade sound transmission loss may be related by using this model.     

Objective and subjective evaluations of the multi-channel auralization technique as applied to solo instruments

Auralizations are commonly used today by architectural acousticians as a tool to model acoustically sensitive spaces. This paper presents investigations employing an auralization methodology known as multi-channel auralizations, to determine the benefits of using an increasing number of channels in such auralizations. First an objective evaluation was conducted to examine how acoustic parameters, such as reverberation time, vary when using “quadrant” (one fourth of a spherical source) or “thirteenth” sources to create the binaural room impulse responses. Large differences in the values were found between the different sections of the sphere, on the order of several just noticeable differences. Two subjective studies were then pursued, first to determine if auralizations made with an increasing number of channels sound more realistic and have an increased perceived source size, using solo musical instruments of varying directivity indices as the sources. Overall, subjects perceived the auralizations made with an increasing number of channels as more realistic, whereas results for perceived source size are less clear. The second subjective study assessed the ease with which subjects could identify the source orientation from the auralizations as a function of number of channels. Results indicate that more channels made it easier for subjects to differentiate between source orientations.     

调制气流声源和远程广播中语言可懂度的实验研究

调制气流声源已广泛地用来作为模拟强噪声的声源。本文的研究工作表明,适当展宽高频响应,它能作为重播语言的广播用声源。我们研制的声源;输出声功率2kw,1000Hz处输出功率不低于最大值4dB,用它来广播语言,在5.8km处,逐字记录短句的可懂度是56％。四个声源组合,在8.7km处是58％,10.2km处是32％;短文可懂度的主观评价,在8.7km处是基本听懂,10.2km处是比基本听懂差,比听懂一半又要强。由于逐字记录的条件较严,因此与测听人员的素质,主要是文化程度和责任心有关,实验表明具有高中文化程度的比初中的平均逐字记录短句的可懂度要高10％。实验值是在一般气象条件下,在室外自然环境中,多次测量的平均值,测听人员未经任何训练,所以,结论具有一定的实用价值。     

Mathematically trivial control of sound using a parametric beam focusing source

By exploiting a case regarded as trivial, this paper presents global active noise control using a parametric beam focusing source (PBFS). As with a dipole model, one is used for a primary sound source and the other for a control sound source, the control effect for minimizing a total acoustic power depends on the distance between the two. When the distance becomes zero, the total acoustic power becomes null, hence nothing less than a trivial case. Because of the constraints in practice, there exist difficulties in placing a control source close enough to a primary source. However, by projecting a sound beam of a parametric array loudspeaker onto the target sound source (primary source), a virtual sound source may be created on the target sound source, thereby enabling the collocation of the sources. In order to further ensure feasibility of the trivial case, a PBFS is then introduced in an effort to meet the size of the two sources. Reflected sound wave of the PBFS, which is tantamount to the virtual sound source output, aims to suppress the primary sound. Finally, a numerical analysis as well as an experiment is conducted, verifying the validity of the proposed methodology.     

A numerical method for computation of sound radiation from an unflanged duct

Some issues involved in establishing a numerical model for sound radiation from a straight duct are addressed in this paper. The main ingredient of the numerical method is solutions of linearized Euler equations using a high order compact scheme. Farfield directivity is estimated through an integral solution of Ffowcs-Williams Hawkings equations. A generic test case of planar wave radiation from an unflanged duct is studied. The sound pressure level and wave propagation in the nearfield are analyzed, together with the farfield directivity. Comparison with analytical solutions shows good agreement. The effect of grid resolution on the sound radiation pattern and the construction of integration surface on the farfield directivity are discussed.