Near-field Acoustic Holography Method Research Articles

High-efficiency prediction method for helicopter global/ground noise based on near-field acoustic holography

Noise reduction program design is an effective approach that relies on efficient noise prediction for reducing ground noise during flight. The existing noise prediction methods have the limitations of being computationally expensive or only applicable to far-fields. In this paper, a High-Efficiency Prediction Method (HEPM) for helicopter global/ground noise based on near-field acoustic holography is proposed. The HEPM can predict the global noise based on acoustic modal analysis and has the advantages of high prediction accuracy and low time cost. The process is given as follows: firstly, the rotor noise on the holographic surface in the specified flight is obtained by simulations or experiments. Secondly, the global noise model, which maps time-domain noise to acoustic modes, is established based on near-field acoustic holography and Fourier acoustic analysis methods. Finally, combined with acoustic modal amplitude, the model established enables efficiently predicting the global/ground noise in the corresponding flight state. To verify the accuracy of the prediction method, a simulation study is conducted in hovering and forward flight states using a model helicopter with a 2-meter rotor and Rotor Body Interaction (ROBIN) fuselage. The comparison of HEPM with numerical results shows that the average prediction errors of the global and ground noise are less than 0.3 dB and 0.2 dB, respectively. For a region containing 100000 observers, the computation time of the HEPM is only one-fifth of that of the acoustic hemisphere method, demonstrating the rapidity of the proposed method.

Numerical Simulation Results of Double Nearfield Acoustic Holography method with extrapolation

In my laboratory, the new sound localization method, Double Nearfield Acoustic Holography (DNAH) method is being developed. This method has high resolution in low frequency sound localization. It is presented at former inter-noise. On the other hand, the extrapolation of measurement points is effective for Nearfield Acoustic Holography method in low frequency. Therefore, the extrapolation technique is also applied for DNAH method. In this presentation, the numerical simulation results of DNAH method with extrapolation of measurement points is reported. With extrapolation, the DNAH shows higher resolution in low frequency sound localization than DNAH method without extrapolation, and higher resolution in low frequency sound localization than NAH method with extrapolation.

Numerical Analysis and Measurement Techniques for Aircraft Cabin Noise Using JAXA's Jet-FTB "HISHO"

The reduction of aircraft cabin noise is one of the important issues not only to increase passenger comfort but also to strengthen international competitiveness in aircraft development. JAXA has been conducting research activities on numerical and experimental prediction techniques of aircraft cabin noise. This paper focuses on the development of our cabin noise prediction tool using the Finite-Difference Time-Domain method, and in-flight noise measurement in the JAXA's Jet-Flying Test Bed (Jet-FTB) "HISHO" with the aim of grasping the cabin noise characteristics. The measurement data include the noise level near interior walls using the near-field acoustic holography method, the wall impedance, and structural vibrations, acquired through some flight test / ground test campaigns. The authors also explain validation results of our numerical prediction tool using microphone data measured in the Jet-FTB.

Method of acoustic separation on the surface of cylindrical baffle

This paper suggests a statistically optimal near-field acoustic holography method based on the combination of plane waves and cylindrical waves, which is used for underwater cylindrical baffles. The method addresses the issue that the extraction of scattered acoustic waves from the surface of underwater cylindrical baffles requires too many measurement points in practical applications: separation of incident and dispersed sound fields and near-field measurements of sound fields at baffle surfaces.The underwater detection sound source is situated in the scatterer's far-field region, so in the actual application scenario for the surface acoustic scattering separation of the cylindrical baffle, the plane wave expansion is used to represent the surface incident acoustic wave and the cylindrical wave expansion is used to describe the scattered acoustic wave on the baffle surface. As a result, fewer expansion orders are required to describe the entire sound field on the baffle's surface. It is verified by simulation that the method is valid and efficient for near-field total sound field measurement and dispersed sound field separation of roughly cylindrical targets under underwater far-field incidence situations because the measurement holographic surface of the surface permits unlimited geometries.

Reconstruction of transient acoustic field using sparse real-time near-field acoustic holography

A sparse real-time near-field acoustic holography method is proposed to precisely and stably reconstruct a transient sound field. By a time-domain impulse response function, a time domain convolution equation between the pressure time-wavenumber spectra on the hologram and reconstruction planes is first established. Then, a smoothed ℓ0-norm optimization algorithm is applied to solve the serious ill-conditioned problem of the inverse process to get the pressure time-wavenumber spectrum of the reconstruction plane. The key part of solving this problem is to approximate replace the discontinuous ℓ0-norm by using a suitable continuous Gaussian function family, and a steepest ascent algorithm is introduced to minimize the continuous function for obtaining the optimal solution. Finally, the pressure time-wavenumber spectra of the reconstruction plane for all wavenumbers at all times are solved, and the corresponding time-dependent pressures are gained by the two-dimensional space inverse Fourier transform. A numerical simulation is conducted to observe the reconstruction capacity of the proposed method. The simulation effect prove that the proposed method can accurately reconstruct the transient sound field, and its reconstruction results are compared to those of the real-time near-field acoustic holography (RTNAH) with Tikhonov regularization and YALL1 modal for verifying the superiority of the proposed method. The YALL1 module utilizes an alternating direction algorithm for solving the ℓ1 minimization problems. Several parameters including the noise, position of the holograph plane and two initialization values are discussed in the simulation. The comparison and discussion results show that the reconstruction effect of the proposed method is much better than those of RTNAH with Tikhonov regularization and YALL1 modal under different parameter configurations. An experiment with a percussive steel plate is implemented for further validating the effectivity of the proposed method.

Conical near-field acoustic holography based on cylindrical wave function of variable radius and nonconformal plane measurement

In practical acoustical measurement for large cones, maintaining coaxial and conformal between the holographic surface and reconstructed surface is usually tedious. To overcome this flaw, a conical near-field acoustic holography (NAH) based on cylindrical wave function of variable radius and nonconformal plane measurement is proposed in this paper. First, cylindrical wave function of variable radius (VR)–based modified statistically optimal cylindrical NAH (SOCNAH) method, entitled VR-SOCNAH, is proposed, which is more applicable to conical surfaces. Second, with the measured sound pressure data on the holographic plane, sound pressure on the conical conformal surface is first reconstructed using statistically optimal planar NAH (SOPNAH), and sound pressure on the reconstructed surface is then reconstructed using VR-SOCNAH. Third, fixed and variable radius measurement methods are comparatively studied, and the former is adopted. Furthermore, a measurement parameter optimization method based on orthogonal experiment is proposed to determine appropriate value ranges. Finally, the proposed NAH is applied to a test bed, and the robustness and accuracy of sound field reconstruction for large cones are significantly enhanced, which provides reliable scientific basis and engineering guidance for noise monitoring and control of mechanical equipment.

A Cylindrical Near-Field Acoustical Holography Method Based on Cylindrical Translation Window Expansion and an Autoencoder Stacked with 3D-CNN Layers.

The performance of near-field acoustic holography (NAH) with a sparse sampling rate will be affected by spatial aliasing or inverse ill-posed equations. Through a 3D convolution neural network (CNN) and stacked autoencoder framework (CSA), the data-driven CSA-NAH method can solve this problem by utilizing the information from data in each dimension. In this paper, the cylindrical translation window (CTW) is introduced to truncate and roll out the cylindrical image to compensate for the loss of circumferential features at the truncation edge. Combined with the CSA-NAH method, a cylindrical NAH method based on stacked 3D-CNN layers (CS3C) for sparse sampling is proposed, and its feasibility is verified numerically. In addition, the planar NAH method based on the Paulis-Gerchberg extrapolation interpolation algorithm (PGa) is introduced into the cylindrical coordinate system, and compared with the proposed method. The results show that, under the same conditions, the reconstruction error rate of the CS3C-NAH method is reduced by nearly 50%, and the effect is significant.

Experimental Results of Double Near-filed Holography method for a commercial Product

In my laboratory, the new localization method, Double Near-field Acoustic Holography (DNAH) method is being Developed. This method is developed to overcome the limit of acoustic holography methods, that the acoustic holography methods cannot measure low frequency noise precisely. In former presentations, the numerical simulation results, and basic experimental results are reported. In these presentations, it is verified that the DNAH method has good resolution for low frequency sound sources. But they are carried out for simple sound source(s), simulated point source(s). In this presentation, the experimental results for a commercial product will be presented. From these results, it is verified that the DNAH method has good resolution for low frequency noise of a commercial product between 10Hz and 200Hz, and the DNAH method has enough resolution for low frequency sound localization.

Machine learning aided near-field acoustic holography based on equivalent source method.

In recent times, equivalent source method-based near-field acoustic holography methods have been extensively applied in sound source localization and characterization. The most commonly used equivalent sources are spherical harmonics. In a non-reverberant environment with no reflections, these equivalent sources could be the best choice since spherical harmonics are derived for the Sommerfeld boundary condition. However, these methods are not the best fit for reverberating environments. In such cases, a new relationship can be calculated between the field weights and the measured pressure with enough training examples. The proposed machine learning models include linear regression (LR) with adaptive moment estimation (Adam), LR with limited-memory Broyden-Fletcher-Goldfarb-Shanno (L-BFGS), and multi-layer perceptron with one and two hidden layers. These methods are tested for multiple monopoles and vibrating plate simulations in a room with different wall absorption coefficients. The data-driven methods are also studied on loudspeakers numerically and experimentally in a free field environment. The results from these methods are compared with the results of one norm convex optimization (L1CVX). LR with L-BFGS performed the best among all the methods studied and performed better than L1CVX for less absorption coefficient for geometrically separable sources. LR with L-BFGS also has much faster inference times.

Research on joint training strategy for 3D convolutional neural network based near-field acoustical holography with optimized hyperparameters

Near-field acoustical holography (NAH) method based on 3D convolutional neural network and stacked autoencoder (CSA-NAH) can eliminate aliasing error or inverse ill-posedness of NAH methods with sparse sampling rate of measurement, ensuring the accuracy of NAH while reducing the measuring cost. However, the CSA-NAH method’s hyperparameters are not optimized, also the error amplification caused by networks interception and integration together with parameters freezing is neglected. To improve the accuracy of CSA-NAH, this paper modifies the implementation framework of it, and proposed a joint training CSA-NAH (JTCSA-NAH). Subsequently, the feasibility of JTCSA-NAH is verified by numerical example and the results show that the average reconstruction error on the near-field acoustic pressure decreases from 3.4% to 2.41%. Finally, JTCSA-NAH method is applied to the examples compared with other similar or latest NAH methods, the better performance verifies the applicability of the proposed method.

Fast prediction method for multirotor global tonal noise based on acoustic modal analysis

With the popularity of multirotor aircraft, the aerodynamic noise it generates has an increasingly serious impact on the environment. A fast method for globally predicting the tonal noise is proposed in this study. The goal of this method is to predict the global aerodynamic noise at a small computational cost, then providing a fast model for online noise prediction and control. Firstly, the global noise model of a single rotor in acoustic modal domain is established by using the near-field acoustic holography method. Then, the global noise model of a rotor with different configurations is obtained by using the mapping relationship of acoustic modal coefficients. Subsequently, the global noise of multirotor is predicted based on the configuration information of rotors by using the wave superposition principle. To validate the performance of this method, the predicted global noises of single rotor and multi rotors were compared with other numerical methods. It is revealed that the acoustic modal analysis can accurately predict global noise at a small computational cost. Moreover, a parametric study was conducted to evaluate the effect of different configurations on the global noise. The mechanism of phase synchronization method was analyzed comprehensively at the same time. Lastly, experiments were carried out in the anechoic chamber to validate the method proposed. When the separation distance between rotor shafts is greater than 2.8 R, the aerodynamic interference between the rotors can be ignored and the method can be well applied globally. Otherwise, the aerodynamic interference result in predictions that do not match the experiments at higher elevation angles, e.g. 60°. Meanwhile, as the elevation angle decreases, the prediction error decreases rapidly.

Data extrapolation and sound field reconstruction based on the boundary element method

Near-field acoustic holography (NAH) based on the boundary element method (BEM) is a powerful tool for noise source identification which is the premise of noise control. For large-scale structures, the measurement aperture should be large enough to ensure that the reconstruction results are accurate. However, the layout of field points often cannot meet the conditions necessary for reliable measurements. The patch NAH methods proposed in previous literature do not explore the influence of the initial and extrapolated aperture on the reconstruction of the vibration velocity. In this paper, the field distribution modes are redefined by the BEM and applied to extrapolation technology. First, the sound pressure of the field points on an expanded hologram is obtained by a data extrapolation method. The expanded data can then be used to reconstruct the vibration velocity of the structure and the sound pressure in the sound field via BEM-based NAH. A modified Tikhonov regularization is utilized in the inverse process. Different sound sources with simple or complex radiated sound fields are used for simulations to explore the influence of the initial and extrapolated aperture on the reconstruction. The application scope of the proposed extrapolation technology is identified and the conclusions are verified by experiments.

A combined sound field reconstruction method for non-cylindrical rotative surfaces based on statistically optimal cylindrical near-field acoustic holography and multipoint Helmholtz equation least square

For the sound field reconstruction of non-cylindrical rotative surfaces, current near-field acoustic holography (NAH) methods have relatively poor applicability and low accuracy. To overcome the problem, a combined sound field reconstruction method is proposed. First, an improved multipoint Helmholtz equation least square (HELS) method is proposed, which is more applicable to reconstruct the sound field with an aspect ratio larger than 1. Second, based on single criterion traversing method (SCTM), an improved double criterion traversing method (DCTM) is proposed to determine the optimal truncation order, and reduce the reconstruction error of multipoint HELS. Third, with the sound pressure measured on the holographic surface, the sound pressure on the transitional surface is reconstructed using statistically optimal cylindrical near-field acoustic holography (SOCNAH), and the sound pressure on the reconstruction surface is reconstructed using multipoint HELS. Finally, typical numerical case studies and experimental studies on a test bed are carried out, which validate that the combined method can obviously improve the accuracy and robustness of sound field reconstruction for non-cylindrical rotative surfaces, and thus provide reliable evidences for noise monitoring and control of mechanical systems.

The development of Double Nearfield Acoustic Holography method

In my laboratory, a new holographic sound localization method, ‘Double Nearfield Acoustic Holography method,’ is being developed. The purpose of new method is increasing preciseness of low frequency noise localization. On the other hand, in the conventional localization method, ‘Nearfield Acoustic Holography method,’ the extrapolation of measurement plane is applied, to increase preciseness of low frequency sound localization. In this paper, the extrapolation of measurement planes is applied for the new method in the numerical simulations. Many numerical simulations are calculated, and several numerical simulation results are listed in this paper. As a result, it is proved that the new method with extrapolation is best method for the low frequency sound localization.

Conical Statistical Optimal Near-Field Acoustic Holography with Combined Regularization.

For the sound field reconstruction of large conical surfaces, current statistical optimal near-field acoustic holography (SONAH) methods have relatively poor applicability and low accuracy. To overcome this problem, conical SONAH based on cylindrical SONAH is proposed in this paper. Firstly, elementary cylindrical waves are transformed into those suitable for the radiated sound field of the conical surface through cylinder-cone coordinates transformation, which forms the matrix of characteristic elementary waves in the conical spatial domain. Secondly, the sound pressure is expressed as the superposition of those characteristic elementary waves, and the superposition coefficients are solved according to the principle of superposition of wave field. Finally, the reconstructed conical pressure is expressed as a linear superposition of the holographic conical pressure. Furthermore, to overcome ill-posed problems, a regularization method combining truncated singular value decomposition (TSVD) and Tikhonov regularization is proposed. Large singular values before the truncation point of TSVD are not processed and remaining small singular values representing high-frequency noise are modified by Tikhonov regularization. Numerical and experimental case studies are carried out to validate the effectiveness of the proposed conical SONAH and the combined regularization method, which can provide reliable evidence for noise monitoring and control of mechanical systems.

Comparison of roadwheel and roadway noise generated by a mono-pitch tire tread

Tire-pavement interaction noise (TPIN, aka tire-road noise or tyre-road noise) is most efficiently measured in acoustically controlled laboratories with large diameter roadwheels (drums) that have surface treatments which replicate some pavement properties, especially when comparing the acoustic performance of different tires. However, it is not clear how closely the roadwheel replicates the road surface, including differences that include road curvature and mechanical impedance of pavements. On the other hand, measuring on a moving vehicle with a microphone array presents it own set of challenges. In this study, a Nearfield Acoustical Holography (NAH) method is used to measure tire/pavement interaction noise on roadways and roadwheels with similar smooth pavement and rough pavement properties. Sound intensity fields, overall sound power levels, and sound pressure levels are reconstructed very close to the tire surface. An experimental passenger car tire with a mono-pitch tread is used in this study. The experimental tire has three circumferential grooves and 64 equally spaced transverse grooves cut into the tread. Differences in sound fields and levels between roadway and roadwheel test conditions for this tire are shown.

A 3D convolutional neural network based near-field acoustical holography method with sparse sampling rate on measuring surface

Near-field acoustical holography (NAH) is an efficient noise diagnosis method with the deficiencies including wraparound error, which will increase when the spatial sampling rate is reduced below the minimum specified by Shannon-Nyquist theorem. Based on 3D convolutional neural network (3D-CNN) and stacked autoencoder (SAE), a method called CSA-NAH is proposed to reduce the wraparound error under sparse measuring. Subsequently, numerical calculations are carried out to illustrate the feasibility and performance of CSA-NAH. The results show that when holographic measurement point number is 64, average reconstruction error of CSA-NAH on an aluminum plate for sound pressure within 2000 Hz is 4.32%, while the latest existing methods is greater than 10%. For error in 1200 Hz~2000 Hz, the error is reduced from more than 15% of the existing methods to 5.5%. Therefore, the application of the proposed CSA-NAH can cut down the measuring cost by reducing the number of microphones without wraparound error.

Near-field acoustical holography incorporating compressive sampling: Effect of measurement distance and array density

To identify sound source locations and strength by using near-field acoustical holography (NAH), many microphones are generally required in order to span the source region and to ensure a sufficiently high spatial sampling rate. It is often the case that hundreds of microphones are needed, so such measurements are costly, which has limited the application of NAH in industrial settings. Recently, however, it has been shown that it is possible to accurately identify concentrated sound sources with a limited number of microphones based on compressive sampling theory. Here, the theory of the four NAH methods that were studied in the present work, that is, statistically optimized near-field acoustical holography (SONAH), wideband acoustical holography (WBH), l1-norm minimization, and a hybrid compressive sampling method, is briefly reviewed. Note that the latter three procedures incorporate elements of compressive sampling. Then, a simulation with one monopole as the sound source was conducted to illustrate some basic characteristics of these algorithms. In the experimental portion of the work, a multi-element loudspeaker was used as the sound source. A near-field intensity scan was conducted to measure both the true intensity spatial distribution and the sound power generated by the loudspeaker to provide a basis against which the values obtained from the holography reconstructions could be compared. The sound field was reconstructed by using both near- and far-field measurements, and the number of microphone measurements used to reconstruct the sound field was systematically decreased by increasing the spacing between microphones. Both in the simulation and experiment, the sound field was reconstructed by using the four NAH methods mentioned above. Then, the reconstruction results were comparedwith the measured intensity results in terms of spatial localization and sound power, and the benefits of the compressive sampling approach are illustrated.

An improvement of the generalized discrete Fourier series based patch near-field acoustical holography

Patch near-field acoustic holography (NAH) is an effective tool for reconstructing sound field with a small microphone array. The generalized discrete Fourier series based near-field acoustic holography (GDFS-NAH) is a natural patch NAH method. Although Fourier-based, the spectral leakage is greatly reduced in GDFS-NAH by using generalized discrete Fourier series rather than discrete Fourier transform, as more wavenumber components are used in the reconstruction. However, more wavenumber components also lead to the instability in a noisy case. In this study, the GDFS-NAH is improved by remodeling the reconstruction processing and introducing the sparse regularization. Different from the original GDFS-NAH, in which the pressure on the measurement plane is decomposed into discrete Fourier series, the reconstruction model is reformulated by decomposing the particle velocity on the reconstruction plane, which will avoid the inversion in the original model. Meanwhile, the problem is solved by using sparse regularization, which will suppress the influence of high wave number components and promote the stability. Numerical results show that the proposed method can effectively improve the reconstruction accuracy within 100–2000 Hz, and is capable of interpolating the sound filed simultaneously. The advantages of the proposed method are also verified by experiments.

Research on patch near-field acoustic holography based on two-level iteration

In order to solve the “window effect” and “aperture repetition effect” caused by limited measurement aperture, a patch near-field acoustic holography method based on two-level iteration is proposed. After analyzing the principle and process of patch near-field acoustical holography based on orthogonal spherical wave, a patch near-field acoustical holography method based on two-level iteration is proposed. The validity of the method is verified by simulation and experiment. Compared with the reconstruction results of patch near-field acoustical holography based on orthogonal spherical wave, it shows the superiority of patch near-field acoustical holography based on two-level iteration.