Measuring short impulse responses with inverse filtered maximum-length sequences

Most loudspeakers have a non-flat frequency response which produces a long oscillating impulse response. An inverse filtering approach may be used to calculate the driving waveform necessary to equalize the response of the loudspeaker in order to radiate shorter acoustic pulses. When combined with the MLS technique, inverse filtering may be used to pre-emphasize the driving signal so that a shorter impulse response, with a prescribed waveform, is measured. This technique is described and illustrated by applying it to a distributed mode loudspeaker. Originally, this loudspeaker has a rather irregular response in a wide band. When the MLS signal is pre-emphasized with the proper inverse filter, a shorter impulse response is measured with a zero-phase cosine-magnitude spectrum.     

一维声波方程声阻抗反演问题的稳定性分析

讨论了一维声波方程的声阻抗反演问题。对于脉冲激发的情况,得到反问题适定的充分、必要条件,适定条件的形式非常简单,在实际计算中易于判定。对于子波激发的情况,由子波响应反演声阻抗通常是不适定的,不适定程度与子波的奇性有关,子波越光滑,相应的反问题的不适定程度越高。同时给出了定量的说明,为一维声波方程声阻抗反演问题的数值计算提供了理论。     

Causal impulse response for circular sources in viscous media

The causal impulse response of the velocity potential for the Stokes wave equation is derived for calculations of transient velocity potential fields generated by circular pistons in viscous media. The causal Green's function is numerically verified using the material impulse response function approach. The causal, lossy impulse response for a baffled circular piston is then calculated within the near field and the far field regions using expressions previously derived for the fast near field method. Transient velocity potential fields in viscous media are computed with the causal, lossy impulse response and compared to results obtained with the lossless impulse response. The numerical error in the computed velocity potential field is quantitatively analyzed for a range of viscous relaxation times and piston radii. Results show that the largest errors are generated in locations near the piston face and for large relaxation times, and errors are relatively small otherwise. Unlike previous frequency-domain methods that require numerical inverse Fourier transforms for the evaluation of the lossy impulse response, the present approach calculates the lossy impulse response directly in the time domain. The results indicate that this causal impulse response is ideal for time-domain calculations that simultaneously account for diffraction and quadratic frequency-dependent attenuation in viscous media.     

存在球差和失焦下电子光学传递函数和脉冲响应函数

根据电子光学理论,球差和失焦引起附加相位移为γ,相干的振幅和相位衬度传递函数为cosγ和sinγ。它们的傅里叶逆变换给出脉冲响应函数的实部和虚部。本文讨论了cosγ和sinγ的零点和极值点,这些点将衍射平面划分为一系列环带。证明了当无量纲失焦量Δ=N^1/2(N为正整数)时各环带上cosγ,sinγ的积分相同,即对脉冲响应函数中心值的贡献相同。本文讨论了使cosγ或sinγ的积分取极大值或极小值,亦即使脉冲响应函数中心值或中心强度取极大值或极小值的条件。本文还讨论了脉冲响应函数最佳均方 关键词：     

一种频域合成房间频率响应的人工混响方法

给出了一种频域合成房间频率响应的方法用于卷积法人工混响,基于频域内房间频率响应的后期部分为高斯随机过程的假设,用自回归滑动平均模型为其自协方差函数和功率谱密度进行参数化描述,在对自回归滑动平均模型中的参数求解后,通过逆滤波得到了房间频率响应后期部分,与房间频率响应前期部分组合后经过傅里叶反变换得到完整的房间脉冲响应。仿真结果表明该方法的混响效果与镜像源法接近,明显优于反馈延迟网络法,但其计算复杂度比镜像源法低,便于实时应用。     

Closed form solutions for the acoustical impulse response over a masslike or an absorbing plane

The transient sound field caused by a Dirac delta impulse function above an infinite locally reacting plane can be calculated by applying the inverse Fourier transform of the corresponding half-space Green's function in frequency domain. As a starting point, the representation given by Ochmann [J. Acoust. Soc. Am. 116(6), 3304-3311 (2004)] is used, which consists of discrete and continuous superposition of point sources. For a locally reacting plane with masslike character and also with pure absorbing behavior, it is possible to express the resulting impulse response in closed form. Such a result is surprising, because corresponding formulations in the frequency domain are not available yet. Hence, the first main result is the closed form solution Eq. (22) for an impulse response over an infinite plane with a pure imaginary impedance. The second main result is the closed form solution Eq. (53) for an impulse response over an infinite plane with a pure real impedance. As a particular application of both main results, a convolution technique is used for deriving formulas for point sources with a general time dependency. For special signals like an exponentially decaying time signal or a triangular shaped impulse, the resulting sound field can be presented in terms of elementary functions.     

Stabilization of time domain acoustic boundary element method for the interior problem with impedance boundary conditions

The time domain boundary element method (BEM) is associated with numerical instability that typically stems from the time marching scheme. In this work, a formulation of time domain BEM is derived to deal with all types of boundary conditions adopting a multi-input, multi-output, infinite impulse response structure. The fitted frequency domain impedance data are converted into a time domain expression as a form of an infinite impulse response filter, which can also invoke a modeling error. In the calculation, the response at each time step is projected onto the wave vector space of natural radiation modes, which can be obtained from the eigensolutions of the single iterative matrix. To stabilize the computation, unstable oscillatory modes are nullified, and the same decay rate is used for two nonoscillatory modes. As a test example, a transient sound field within a partially lined, parallelepiped box is used, within which a point source is excited by an octave band impulse. In comparison with the results of the inverse Fourier transform of a frequency domain BEM, the average of relative difference norm in the stabilized time response is found to be 4.4%.     

INPUT FORCES ESTIMATION OF BEAM STRUCTURES BY AN INVERSE METHOD

An on-line recursive inverse method to estimate the input forces of beam structures is presented. The inverse method is based on the Kalman filter and a recursive least-squares algorithm. The filter models the system dynamics in a linear set of state equations. The state equations of the beam structures were constructed using the finite element method. The practicability and accuracy of the estimation method were examined with numerical simulations from which the input forces of a cantilever beam with a lumped mass on the free end were estimated from the output responses. In the numerical experiments, the cantilever beam was subjected to five types of input forces, i.e., sinusoidal, triangular impulse, rectangular impulse, a series of impulses and random. The simulation results show that the inverse method has an excellent performance to estimate the input forces of beam structural systems from the noisy measurements.     

Characterization of ultrasonic imaging systems using transfer functions

The transfer functions for focussed and defocussed, coherent and confocal optical imaging systems have been applied to the equivalent ultrasonic imaging systems. The transfer functions with varying degrees of defocus were calculated to show the defocus effects for ultrasonic imaging systems. Assuming that the acoustic waves are reflected perfectly on the surface of the step edge, the theoretical line-scans for small amplitude signals across a step edge, with various degrees of defocus, were generated. The first derivative of the line-scan for a step edge is shown theoretically to yield the same impulse response as that calculated using the inverse Fourier transform applied to the original transfer function. These results show how the real and imaginary parts of the transfer functions contribute to ultrasonic image formation. A method for the experimental determination of the impulse response, and the transfer functions for the characterization of an ultrasonic imaging system, such as an acoustic microscope, is provided.     

Three dimensional diffusion delay time tomography

Stable numerical convolution of the diffusion impulse time response with a Gaussian wave packet produces a wave packet whose delay time is governed by an eikonal equation where the diffusivity distribution plays the role of the square of the velocity distribution. Diffusion delay time tomography data can approximately image the diffusivity distribution by solving an inverse problem for the eikonal equation with multiple rays that traverse low diffusivity regions not traversed by earlier related methods, and consequently the resolution in such regions is improved. This is important for medical imaging.     

Barker码激励超声导波在长骨检测中的应用

单脉冲激励的超声导波在长骨中传播时,信号的衰减大,导致接收信号的幅度很小,且噪声严重。Barker码激励能有效增大接收信号幅度,提高信噪比(SNR)。将其应用到超声导波长骨检测中,进行仿真和长骨实验,得到的信号分别用加权匹配滤波器和有限冲激响应-最小均方误差(FIR-LS)逆滤波器进行压缩,并与单脉冲激励的结果进行了对比。结果表明,对于13位的Barker码,采用加权匹配滤波器进行解码时,压缩信号幅度是单脉冲激励接收信号的13倍;而FIR-LS逆滤波器则达到-63.59 dB的峰值旁瓣水平(PSL),更好地抑制噪声。说明可以将Barker码激励超声导波应用于长骨的检测中。      

窄脉冲声用于大样品的吸声测量

本文利用逆滤波器原理,在空间产生了波形可控、长度在毫秒量级的窄脉冲声信号,分别采用脉冲分离法和脉冲叠加法,对一种毛毡材料和三种不同厚度的海绵材料进行了吸声系数的测量。实验证明,基于窄脉冲声信号的吸声测量结果与ISO13472-1:2002中的MLS脉冲法及阻抗管的测量结果基本吻合。采用窄脉冲进行吸声测量,可以减少样品边缘和周围环境对测量信号的干扰,提高现场测量的准确性。     

Time impulse response and time frequency response of optical pupils

We define the time impulse response of optical pupils and relate this function to the optical characteristics of the pupils. Then we link the time impulse response to the spatial impulse response of the pupil.     

Subjective and objective verifications of the inverse functions of binaural room impulse responses

The binaural room impulse responses (BRIRs) can be applied to 3-D sound field reconstruction, virtual reality, noise control, et al. Because the BRIRs are non-minimum phase functions, it is difficult to find the exact inverse functions of the BRIRs, especially when there are two or more sources in a reverberant space. In this work, a method was proposed to find the inverse functions of BRIRs with two sound sources in a reverberant space. The concept of time delays and the method of weighted least squares were used to find the causal, however, approximate inverse functions. The accuracy of the inverse functions was first evaluated objectively by a dummy head system. The result shows that the distortion due to crosstalk and room reverberation can be improved by 16∼18 dB. The inverse functions were also verified subjectively by 20 students. The result of subjective evaluation also shows that the inverse functions can be used successfully to reduce the crosstalk effect and the room reverberation.     

Reverberation time measurement by the product of two room impulse responses

The decay function for the evaluation of the reverberation time is often obtained by the method of the backward integration of a squared room impulse response as suggested by M.R. Schroeder more than four decades ago. Since then much work has been published about its implementation. However, soon after the initial exploitation of the method, it was realized that the effects of the background noise contaminating the room impulse response required a careful consideration for accomplishing better results.This paper describes an alternative method dealing with the problem of the backward integration of noisy room impulse responses. This method is based on the processing of two impulse responses sequentially recorded for a fixed source and receiver arrangement in a room. Statistical criteria are proposed to identify how the effect of the noise corrupts the level decay curve using a noise-free synthesized room impulse response as well as measurements performed in a real room.     

一种精确计算换能器空间脉冲响应的快速方法

空间脉冲响应被认为是最有效的瞬态声场计算方法,针对空间脉冲响应直接计算时需要很高的采样频率,导致数据量大、计算效率差的问题,推导了空间点与其在换能器平面的投影点脉冲响应之间的关系,提出了一种计算脉冲响应的快速算法,探讨了计算采样频率和插值采样频率对计算精度和计算效率的影响,研究发现,采用1000 MHz采样频率能够保证精度要求,采用500 MHz作为插值时的采样频率是较优的选择。该方法与直接求解相比,可提高计算效率18倍。      

Determination of the Parameters of Hydrogen Transport through Membranes by the Concentration Impulse Method

A nonlinear inverse problem of determining the parameters of hydrogen transmission from the experimental data is examined. The model considers not only diffusion but also adsorptive-desorptive processes. An algorithm of model identification based on the concentration impulse method is suggested.     

浅海水声信道冲激响应空变特性研究

浅海声传播对空间参数的变化十分敏感,信道冲激响应随空间的变化使水声信号处理的性能受到很大限制。文中采用射线理论模型估计信道冲激响应,用互相关方法仿真计算了信道响应对空间参数的敏感性,并对信道冲激响应空变特性的实际应用进行了讨论。     

Reconstruction of dynamic displacement and velocity from measured accelerations using the variational statement of an inverse problem

This paper presents two types of finite impulse response (FIR) filters to reconstruct dynamic displacement induced by structural vibration from measured acceleration. The governing equation for the reconstruction is derived by taking the variation of a minimization problem, which defines an inverse problem on displacement. A regularization function for overcoming the ill-posedness of the inverse problem is included in the minimization problem. The governing equation of the inverse problem becomes the same type of differential equation as that of a beam on an elastic foundation. The conventional FIR (CFIR) filter directly approximates the transfer function of the governing equation, while the FEM-based FIR (FFIR) filter is formulated by the discretization of the minimization problem with the finite element method. For the finite element discretization, the Hermitian shape function is utilized. The proposed FFIR filter is capable of reconstructing displacement and velocity simultaneously. The fundamental characteristics of the proposed filters are investigated in the frequency domain using the transfer and accuracy functions. It is shown that the proposed FIR filters suppress low frequency noise components in measured accelerations effectively, and reconstruct physically meaningful displacement accurately. The validity of the proposed filters is demonstrated through a numerical simulation study, a field experiment and an evaluation of flutter derivatives using measurements taken from a wind tunnel test.     

Time-domain sweeplets for acoustic measurements

This paper proposes new types of time-domain generated sine sweeps for impulse response measurements. A general time-domain analytical formulation method, combined with numeric phase alignment and frequency-domain inverse filtering is presented. It is applied to derive three new families of controllable spectrum sine sweeps called sweeplets, capable of matching 1/f^β background noises, producing finite band defocusing and single frequency focusing shapes. Mathematical properties concentrating on practical control of the signal shapes are examined. Effects of various perturbations, such as stationary and transient background noise, harmonic distortion and finite length are presented. Applicability of the proposed method is experimentally verified by a room acoustic measurement example.