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.     

Fan broadband noise shielding for over-wing engines

Increasingly demanding community noise targets are promoting noise performance ever higher on the list of airliner design drivers. In response, significant noise reductions are being made, though at a declining rate—it appears that a whole airframe approach is now needed to achieve significant further gains. As a possible step in this direction, over-wing engine installations are considered here, which use the airframe itself as a noise shield. The paper is the account of an experimental investigation of the comparative shielding performances of a range of relative engine positions on such a layout. Using the statistical modelling technique Kriging, we build an approximation of the noise shielding metric as a function of the position of the engines above the wing—this can serve as the input to multi-disciplinary design trade-off studies. We then compare the results found with the results of applying simple half-barrier diffraction theory to the same problem. We conclude that the latter could be considered as a first order, conceptual design tool, though it misses certain features of the design merit landscape identified by the experiment presented here.     

CAA broadband noise prediction for aeroacoustic design

The current status of a computational aeroacoustics (CAA) approach to simulate broadband noise is reviewed. The method rests on the use of steady Reynolds averaged Navier-Stokes (RANS) simulation to describe the time-averaged motion of turbulent flow. By means of synthetic turbulence the steady one-point statistics (e.g. turbulence kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations having statistics that very accurately reproduce the initial RANS target-setting. The synthetic fluctuations are used to prescribe sound sources which drive linear perturbation equations. The whole approach represents a methodology to solve statistical noise theory with state-of-the-art CAA tools in the time-domain. A brief overview of the synthetic turbulence model and its numerical discretization in terms of the random particle-mesh (RPM) and fast random particle-mesh (FRPM) method is given. Results are presented for trailing-edge noise, slat noise, and jet noise. Some problems related to the formulation of vortex sound sources are discussed.     

A novel feedback active noise control for broadband chaotic noise and random noise

The feedback active noise control (ANC) can be seen as a predictor, the conventional method based on filtered-x least mean square (FXLMS) algorithm can only be useful for linear and tonal noise, but for nonlinear and broadband noise, it is useless. The feedback ANC using functional link artificial neural networks (FLANN) based on filtered-s least mean square (FSLMS) algorithm can reduce some nonlinear noise such as chaotic noise, but the noise cancellation performance is not very well, at the same time, it is not useful to random noise. To solve the problem above, a new feedback ANC using wavelet packet FXLMS (WPFXLMS) algorithm is proposed in this paper. By decomposing the broadband noise into several band-limited parts which are predictable and each part is controlled independently, the proposed algorithm can not only suppress the chaotic noise, but also mitigate the random noise. Compared with FXLMS and FSLMS algorithms, proposed WPFXLMS algorithm also holds the best performance on noise cancellation. Numerous simulations are conducted to demonstrate the effectiveness of the proposed WPFXLMS algorithm.     

Causal impedance matching for broadband hybrid noise absorption

The complementary strengths and weaknesses of passive and active noise control (ANC) methods have motivated many researchers to develop hybrid noise absorbers that integrate both control strategies. The impedance matching technique (IMT) is the most effective for such a purpose. An unsolved problem with available IMT schemes is the a priori reference signal that limits IMT applications. This study proposes the use of the forward wave, available by the two-microphone method, as the reference signal. Due to inevitable errors in wave separation and inlet reflection of the control signal, the absorber becomes a feedback system. A simple and stable ANC is developed for impedance matching without the a priori reference signal. The proposed absorber has an absorption coefficient of 0.9 or above in a frequency range of 60-850 Hz. It is stable in the presence of sensor mismatch and robust with respect to significant variation of inlet boundary conditions.     

基于宽光谱干涉系统的拼接主镜共相位检测技术

 针对拼接式大口径望远镜主镜,提出一种基于迈克尔逊干涉原理基础上的宽带光谱（白光）干涉检测方法,对拼接子镜间相位误差进行实时检测,进而对失调子镜进行相应校正,以实现拼接子镜的共面排布。子镜间相位误差通过干涉图形间的不匹配性进行提取。应用双中心波长组合白光源来提高白光中心条纹的可见度,通过理论仿真,对不同中心波长组合的白光中心条纹可见度进行比较,结果表明：该双中心波长组合白光源系统的应用,可以提高白光干涉中心条纹的信号分辨能力,借以提高检测精度,使得该白光干涉检测系统对拼接子镜间的相位失调误差进行高精度提取。     

Noninvasive system identification for multichannel broadband active noise control

Many real-world applications of active noise control are characterized by transfer functions that vary significantly and unpredictably. The controller's transfer-function models must adapt to these variations. Presented here is a class of adaptive filters that accomplish quasiperiodic system identification updates for feedforward control by using blocks of input-output histories. The algorithms form a one-dimensional family linking normalized least-mean squares (LMS) adaptive filters and block recursive least-squares, termed "block projection" algorithms, and generalize the noninvasive system identification studied by Sommerfeldt and Tichy. The system identification proceeds noninvasively, producing nonparametric impulse responses. Simulations show that the algorithm's convergence is faster than that of normalized LMS, even after the additional overhead of computing the update is taken into account. Both the multichannel generalization and application of these algorithms to system identification are novel. Simulations of the algorithms' performance using measured data are presented here, while experimental results of an implemented algorithm are contained in the companion paper.     

Low noise broadband modulated preamplifiers for a variety of transducers

Often, increases in signal-to-noise ratio and basic sensitivity are gained by the use of carrier modulation. Further advantage can be taken of this process by the use of varactors. These are suitably biased solid-state diodes and transistors, usually used to provide capacitance reactances. Their capacitance is changed by applying a voltage. The change in capacitance available by applying to their terminals the electrical output of a transducer—such as many of the sensors for light, heat, sound, mechanical stress and deformation—can be used in suitable modulating circuits. The magnitudes of reactance available from varactors fall conveniently into the range of values suitable for the carrier frequencies now accessible by means of another stable, mass produced solid-state device, the crystal clock oscillator.In general, the complexity of the carrier circuit to be chosen depends upon the sensitivity and noise floor requirements. This paper describes circuits for applying the modulation process, and discusses the hierarchy of choices for the methods used. The authors have built circuits with an input noise level equivalent to 0·3 μV for signals over the entire audiofrequency range, having a gain in excess of 50 dB.     

Sharpness and amplitude envelopes of broadband noise

This paper discusses how amplitude envelope shapes, sound-pressure level, and duration of broadband noise affect sharpness and brightness. In the first experiment, sharpness, brightness, and similarity were judged for paired stimuli by 13 subjects. The stimuli consisted of broadband noise with different types of amplitude envelopes, sound-pressure levels, and duration. Experimental results were analyzed with a multidimensional scaling technique. In addition, the second experiment measuring the point of subjective equality (PSE) of sharpness was carried out for four subjects. The results of these two experiments showed large individual differences in sharpness judgment, difficulty in brightness judgment for the stimuli used in the experiments, and influence of the amplitude envelope shapes on sharpness. The individual differences observed in sharpness judgment were explained by the differences between weights given to two psychological dimensions: loudness and subjective duration. This study forms a basis for understanding sharpness of nonsteady sounds.     

Modulation and gap detection for broadband and filtered noise signals

Modulation detection thresholds (as a function of sinusoidal amplitude modulation frequency) and temporal gap detection thresholds were measured for three low-pass-filtered noise signals (fc = 1000, 2000, and 4000 Hz), a high-pass-filtered noise signal (fc = 4000 Hz), and a broadband signal. The two latter noise signals were effectively low-pass filtered (fc = 6500 Hz) by the earphone. Each of the filtered signals was presented with a complementary filtered noise masker. Modulation and gap detection thresholds were lowest for the broadband and high-pass signals. Thresholds were significantly higher for the low-pass signals than for the broadband and high-pass signals. For these tasks and conditions, the high-frequency content of the noise signal was more important than was the signal bandwidth. Sensitivity (s) and time constant (tau) indices were derived from functions fitted to the modulation detection data. These indices were compared with gap detection thresholds for corresponding signals. The gap detection thresholds were correlated inversely (rho = -1.0, p less than 0.05) with s (i.e., smaller gap detection thresholds were correlated with greater sensitivity to modulation), but were not correlated significantly with tau, which was relatively invariant across signal conditions.     

Virtual sensing for broadband noise control in a lightly damped enclosure

Available virtual sensing schemes either depend on assumptions that are valid for isolated frequencies, or require heavy online adaptations. A simple method is proposed here to predict the virtual signal exactly for broadband noise control in a lightly damped enclosure. The proposed method requires two physical sensors installed judiciously in a sound field to predict a virtual signal. The method is based on an exact mathematical relation between the virtual and physical sensors, which is valid for the entire frequency of interest. It is possible to use multiple sensor-pairs to reduce the sensitivity of the proposed method with respect to acoustic parameters, such as speed of sound or sensor mismatching. Experimental results are presented to verify the analytical results.     

高超噪比宽带毫米波噪声信号光子学产生研究

受电子器件工作频率及功率的限制,传统电子学方法产生的噪声源的超噪比通常小于20 dB,针对这一问题,本文提出了一种基于非相干光拍频产生高超噪比宽带毫米波噪声技术.首先,用两个光滤波器对宽带放大自发辐射光源进行滤波整形.将获得的两束频率不同的放大自发辐射光耦合进入光电探测器进行拍频,从而产生电噪声信号.理论分析发现,通过...     

Ballistic diffusion induced by a thermal broadband noise

We present a thermal broadband noise from the difference between two Ornstein-Uhlenbeck noises, which can induce a ballistic diffusion, i.e., long-time mean square displacement of a free particle driven by this noise reads proportional to t(2). We apply this noise to a flashing ratchet and the mean velocity of the particle is calculated via Langevin simulation. The results show that a double peak of the mean velocity and flux reversal appears for the ratchet with large and small asymmetries, respectively; the inertia effect induces a large mean velocity and multireversal of flux. These rich and interesting phenomena are explained.     

Robust active control of broadband noise in finite ducts

This study focuses on robust active control of broadband noise in finite ducts. Our analytical and experimental studies suggest the existence of several technical flaws in the path models of conventional active noise control (ANC) systems. These are sensitivity of the path model with respect to boundary conditions, and nonminimum phase (NMP) secondary and reference paths. For finite ducts with small cross sections, the traveling wave model (TWM) may be adopted to find an effective solution to these problems and lead to a robust ANC system. Since many practical "noisy" ducts are finite with small cross sections, the proposed ANC has many practical applications. Its robustness and ability to suppress broadband noise will be explained theoretically and verified experimentally.     

Suppression of spin projection noise in broadband atomic magnetometry

We demonstrate that quantum nondemolition measurement, combined with a suitable parameter estimation procedure, can improve the sensitivity of a broadband atomic magnetometer by reducing uncertainty due to spin projection noise. Furthermore, we provide evidence that real-time quantum feedback control offers robustness to classical uncertainties, including shot-to-shot atom number fluctuations, that would otherwise prevent quantum-limited performance.     

A modal separation measurement technique for broadband noise propagating inside circular ducts

A measurement technique which separates broadband noise propagating inside circular ducts into the acoustic duct modes is developed. The technique is also applicable to discrete frequency noise. The acoustic modes are produced by weighted combinations of the instantaneous outputs of microphones spaced around the duct circumference. The technique is compared with the cross spectral density approach presently available and found to have certain advantages, and disadvantages. Considerable simplification of both the new technique and the cross spectral density approach occurs when no correlation exists between different circumferential mode orders. The properties leading to uncorrelated modes and experimental tests which verify this condition are discussed. The modal measurement technique-is applied to the case of broadband noise generated by flow through a coaxial obstruction (nozzle or orifice) in a pipe. Different circumferential mode orders are shown to be uncorrelated for this type of noise source.     

Variable-duration notched-noise experiments in a broadband noise context.

A variable-duration notched-noise experiment was conducted in a noise context. Broadband noise preceded and followed a tone and notched noise of similar duration. Thresholds were measured at four durations (10, 30, 100, and 300 ms), two center frequencies (0.6, 2.0 kHz), and five relative notch widths (0.0, 0.1, 0.2, 0.4, 0.8). At 0.6 kHz, 10-ms thresholds decrease 6 dB across notch widths, while 300-ms thresholds decrease over 35 dB. These trends are similar but less pronounced at 2 kHz. In a second experiment, the short-duration notched noise was replaced with a flat noise which provided an equivalent amount of simultaneous masking and thresholds dropped by as much as 20 dB. A simple combination of simultaneous and nonsimultaneous masking is unable to predict these results. Instead, it appears that the elevated thresholds at short durations are dependent on the spectral shape of the simultaneous masker.     

Random particle methods applied to broadband fan interaction noise

Predicting broadband fan noise is key to reduce noise emissions from aircraft and wind turbines. Complete CFD simulations of broadband fan noise generation remain too expensive to be used routinely for engineering design. A more efficient approach consists in synthesizing a turbulent velocity field that captures the main features of the exact solution. This synthetic turbulence is then used in a noise source model. This paper concentrates on predicting broadband fan noise interaction (also called leading edge noise) and demonstrates that a random particle mesh method (RPM) is well suited for simulating this source mechanism. The linearized Euler equations are used to describe sound generation and propagation. In this work, the definition of the filter kernel is generalized to include non-Gaussian filters that can directly follow more realistic energy spectra such as the ones developed by Liepmann and von Kármán. The velocity correlation and energy spectrum of the turbulence are found to be well captured by the RPM. The acoustic predictions are successfully validated against Amiet’s analytical solution for a flat plate in a turbulent stream. A standard Langevin equation is used to model temporal decorrelation, but the presence of numerical issues leads to the introduction and validation of a second-order Langevin model.     

Continuous production of glass-coated microwires and the applications for broadband noise suppression composite sheets

A system and a method were developed for continuous production of glass-coated microwires with controllable glass thickness and metallic core diameter. Fe-Si-Al flakes were fabricated by planetary ball milling using ethanol as a dispersant. A composite noise suppression sheet was prepared with glass-coated microwires combined with Fe-Si-Al flakes. The characteristics of the conduction noise suppression of the composite sheet were evaluated on a microstrip line in gigahertz frequencies. The conduction electromagnetic noise was effectively suppressed by the composite sheet in broad-band frequency region; the power loss is greater than 70% in the frequency range from 0.7 to 8.5 GHz.