Characteristics Of Noise Sources Research Articles

Systematic analysis of acoustic source localization maps for performance assessment

Noise pollution is a raising problematic mainly associated to the development of industry, cities, and transportation. In that context, the characterization of noise sources is key point to help the understanding of noise generation mechanisms and to be able to develop effective noise reduction strategies. Microphones phased array measurements can be used as input to solve the inverse problem of source localization allowing to obtain source maps for each frequency of interest. Conventional beamforming (CBF) is the most famous source localization algorithm which uses the interpretation of the propagation delays measured between each array microphones. However, the analysis of output source maps can be made difficult by the low spatial resolution in the low-frequency range and the presence of side-lobes associated with microphones distribution that can lead to additional wrong sources. This paper presents a systematic analysis of source maps to identify potential noise sources by using different methods (local maxima, Metropolis-Hastings). The set of potential noise sources can then be compared to the set of real sources to assess the CBF performances. This methodology is successfully applied to measurements performed in anechoic room for various source configurations highlighting CBF limitations (spatial resolution, correlated sources, reflective environment).

An innovative approach for enclosure design of power generators to meet noise regulation targets

The use of diesel-powered engine generators as a power source is very common for various industrial and residential applications. There are strict regulatory requirements and legislations to limit the noise levels from these generators below statutory limits. The purpose of the paper is to present an approach using statistical energy analysis (SEA) augmented by experimental inputs for the prediction of noise levels for the generators and propose strategies for optimized enclosure design that effectively mitigate noise to achieve regulatory requirements. The study begins by developing an SEA model that represents the acoustic behavior of the generator and enclosure. The airborne sources such as engine, alternator, radiator fan and exhaust are modelled explicitly using experimental noise source characterization. Based on the established SEA model and excitation sources, noise levels are predicted through the calculation of energy flow between subsystems. The predicted sound levels for an existing Diesel Generator with enclosure were validated with its test data to ensure correctness of SEA model. Parametric studies were carried out using simulations on key variables, including critical noise paths, panel thickness, and noise control treatments. These investigations helped identify optimal process parameters that reduce noise levels emitted by genset in adherence to noise regulations.

Numerical and experimental study on the aerodynamic noise characteristics of 600-km/h high-speed maglev trains

Abstract High-speed maglev trains represent a new mode of transportation; however, their 600-km/h traveling speed causes serious aerodynamic noise problems, with very little research on the aerodynamic noise of such trains. To clarify the aerodynamic noise source and radiation characteristics of high-speed maglev trains, both large eddy simulations and wind tunnel experiments with a 1:8 scale three-train model were conducted to obtain the aerodynamic noise source spectrum and spatial distribution characteristics of the maglev train at different speeds; the contribution of the dipole noise source was also analyzed. The results revealed that the train shape has a significant impact on the frequency distribution of the far-field aerodynamic noise below 1 kHz, with the highest sound power concentrated on the train head, especially in the areas around the track joint and streamlined sections. The surface dipole sound source energy increases linearly with speed and is mainly distributed at the bottom of the train head and tail train body. A short-streamlined train exhibited the highest ratio, especially in the trailing edge area, up to 1.42×1011 dB. The far-field sound pressure level of the short-streamlined maglev train is significantly higher, with a maximum difference of about 10 dB(A). The distribution pattern of far-field sound pressure level is influenced by the train type, reflected in the tail train and wake area. Different frequency intervals have different relationships between the acoustic pressure level and frequency; long-streamlined models may enhance broadband features in the sound field distribution of high-speed maglev trains. These findings could be used to guide the aero-acoustic design of maglev trains with a designed speed of 600 km/h.

On the aeroacoustic simulation and far-field noise modeling of a cylinder turbulent wake at the Reynolds number of 3900

The turbulent wake flows of circular cylinders have been under extensive study due to their strong fluctuating forces and noise levels, yet the relationship between them has rarely been examined. In this study, a high-fidelity numerical simulation is performed for a cylinder wake flow at the Reynolds number of 3900, aimed at far-field noise modeling based on the investigation of noise source characteristics. Despite irregular multi-scale turbulent wake eddies, the primary vortex shedding that resembles the Karman vortex shedding is observed, and the modes extracted by the dynamic mode decomposition technique at the primary shedding frequency and its higher harmonics appear as nearly two-dimensional spanwise structures. A non-permeable Ffowcs Williams and Hawkings (FW-H) approach is used to formulate the far-field noise spectra as the surface integral of unsteady flow samples on the cylinder surface and the volume integral of unsteady stress in the turbulent wake. The former can be modeled as the dipole sound of unsteady lift and drag, and the latter can be modeled as the tonal noise of discrete two-dimensional (i.e., spanwise-averaged) vortex modes. The results show that the vortex sound is extremely weak. Thus, the far-field noise can be modeled almost entirely by the aerodynamic forces, and the quantitative relationship between them is established by the convective FW-H solutions.

Numerical investigation on the characteristics of underbody aerodynamic noise sources in electric vehicles

As the wind noise caused by the upper body is gradually controlled, the influence of high-speed underbody airflow on the interior noise of electric vehicles becomes increasingly prominent. The method involves the combined application of the Finite Volume Method (FVM) and Finite Element Analysis (FEA) to compute the aerodynamic noise inside a real vehicle model induced by high-speed airflow. A numerical simulation of the underbody wind noise in electric vehicles has been conducted to reveal the noise sources at the bottom of electric vehicles and cabin noise. Finally, the impact of side skirts and wind deflectors on chassis wind noise was evaluated.

Acoustical Holography and Coherence-Based Noise Source Characterization of an Installed F404 Engine

Understanding the acoustic source characteristics of supersonic jets is vital to accurate noise field modeling and jet noise reduction strategies. This paper uses advanced, coherence-based partial field decomposition methods to characterize the acoustic sources in an installed, supersonic GE F404 engine. Partial field decomposition is accomplished using an equivalent source reconstruction via acoustical holography. Bandwidth is extended through the application of an array phase-unwrapping and interpolation scheme. The optimized-location virtual reference method is used. Apparent source distributions and source-related partial fields are shown as a function of frequency. Local maxima are observed in holography reconstructions at the nozzle lipline, distinct in frequency and space. The lowest-frequency local maximum may relate to noise generated by large-scale turbulence structures in the convectively subsonic region of the flow. Other local maxima are correlated primarily with Mach wave radiation originating from throughout the shear layer and into the fully mixed region downstream of the potential core tip. Source-elucidating decompositions show that the order and behavior of the decomposition lend to the local maxima being related to distinct subsources. Between the local maxima, however, there may be a combination of sources active, which is likely the cause of the spatiospectral lobes observed in other full-scale, supersonic jets.

Mechanistic study of noise source and propagation characteristics of flow noise of a submarine

Flow noise, part of hydrodynamic noise, significantly impacts the stealth capabilities of submarines. Investigating the noise source location and formation mechanism of flow noise is crucial for enhancing submarine stealth performance. This study uses large eddy simulation to examine the evolution of vortex structure in the SUBOFF submarine. The propagation of far-field noise from dipole and quadrupole noise sources of the SUBOFF is calculated, by rationally constructing a permeable integral surface in conjunction with the Ffowcs-Williams and Hawkings and “Collapsing-Sphere” formulations. The numerical results for both flow and acoustic fields are consistent with experimental data and previous numerical findings. Sail and rudder are the predominant dipole noise sources, due to turbulence effects from vortex shedding. The permeable integral surface effectively captures the quadrupole noise source. The quadrupole noise in the 2 kHz frequency range contributes 17 % to 26 % of the total noise energy. The location of the quadrupole noise source and its generation mechanism is explored, through the evolution of the vortex structure and the Lamb vector amplitude. Approximate scaling laws for far-field dipole and quadrupole noise spectra are given, thus illustrating the general laws in the frequency domain of the far-field noise.

Acoustic source characterization of shock-associated noise in an installed, full-scale F404 engine

Shock-associated noise in supersonic jets is an important factor in acoustic radiation, particularly in the upstream direction. A previous paper [L. T. Mathews et al., J. Acoust. Soc. Am. 154, A325 (2023)] discussed a preliminary source characterization of shock-associated noise in a full-scale, installed tactical jet engine. Acoustical holography was used to reconstruct the acoustic behavior near the jet and high-amplitude features were produced that were consistent with shock-associated noise sources. This paper presents a more detailed characterization of shock-related noise in the jet using higher-fidelity methods, such as subarray processing and bandwidth extension. The spatial and spectral characteristics of the shock cell noise sources are discussed. A virtual reference decomposition method is used to examine the source and radiative characteristics of each detected shock noise source. The results are compared with shock noise characteristics in the literature. [Work supported by ONR Grant No. N00014-21-1-2069.]

Source characterization of afterburning laboratory-scale jet noise with vector acoustic intensity

This paper presents an intensity-based characterization of noise sources in a highly heated laboratory-scale jet with a 3-inch exit diameter. Run conditions had nozzle pressure ratios between 2.7 and 3.5 and temperature ratios of ∼7. Measurements were performed using a microphone scanning rig containing 8 four-microphone intensity probes. Probes were separated by 1.5 nozzle diameters and the microphones were spaced 0.5 nozzle diameters apart. Each scan mapped out over 3,000 measurement locations to produce an intensity field map. Vector intensity was processed over a 200 Hz to 20 kHz bandwidth using the phase and amplitude gradient estimator (PAGE) method [D. C. Thomas, et al., J. Acoust. Soc. Am. 137, 3366–3376 (2015)]. Results show the dominant source region as a function of frequency and are compared to the acoustical holography source reconstruction of the T7-A afterburner engine condition [L. T. Mathews and K. L. Gee, AIAA J., in press (2024)]. Additionally, the intensity-derived source locations are connected to the jet’s supersonic and subsonic flow regions. [Work supported by ONR Grant No. N00014-21-1-2069].

System-Level Validation of Radiated Noise Source Characterization Using Only Near-Field Magnitude Information

System-Level Validation of Radiated Noise Source Characterization Using Only Near-Field Magnitude Information

Trailing-edge far-field noise and noise source characterization in high inflow turbulence conditions.

Airframe noise currently is a bottle neck in various applications, e.g., wind energy, maritime applications, and aircraft. Airframe noise is significantly increased by the presence of inflow turbulence. High inflow turbulence influences the boundary layer and wall-pressure fluctuations close to the trailing edge of airfoils. In this research, measurements of boundary layer and wall-pressure fluctuations near the trailing edge of an airfoil are conducted to investigate how the inflow turbulence affects the trailing-edge noise generation mechanism. Far-field noise measurements of additional three airfoils are shown to understand the role of the airfoil geometry in the dominant noise source for the cases of inflow turbulence and to generalize the observed increase in trailing-edge noise. Inflow turbulence leads to an increase in both the wall-pressure spectrum and spanwise correlation length. Trailing-edge noise increases due to the inflow turbulence in the entire frequency range at least 2 dB up to more than 15 dB for all the cases. The contribution of leading- and trailing-edge noise to the total noise varies with the airfoil geometry and inflow velocity, with the trailing-edge noise dominating in a larger frequency range for the thickest airfoil and for lower velocities.

Parallel-swath-array analysis of passive source surface waves based on beamforming

The detection of mined-out coal area remains a pivotal subject in geophysical prospecting and presents an urgent issue requiring resolution. A refined and effective approach to tackle this challenge involves conducting dispersion analysis on passive surface waves, which are derived from ambient noises. While classical passive surface wave methods, such as the Spatial Autocorrelation Method (SPAC), rely on spatial and time domain stationary assumptions, these can be difficult to fulfill. This is especially true when using ambient noises with frequencies >1 Hz, leading to a growing disparity between basic assumptions and actual scenario. To reduce this discrepancy, it's crucial to account for the non-stationary features of noise sources. In this study, we introduce a pioneering approach for parallel-swath-array analysis of passive source surface waves based on beamforming, making full use of non-stationary ambient noises. The parallel-swath array allows for efficient data collection and high-density spatial observation. The method's core concept involves: 1) The collection of noise data by a 2-D array arranged in three parallel lines; 2) The use of a plane-wave basis function to directly scan the original noise records in a time-segment mode within the time domain. These are conducive to unveil the azimuths and time-variant characteristics of the noise sources, resulting in accurate surface wave dispersion information. To ascertain the feasibility and effectiveness of this innovative method, we simulated passive surface wave records for a geological model, accommodating both time-invariant and time-variant distribution of noise sources. The results demonstrate that the method successfully reveals the time-invariant or time-variant characteristics of noise sources and the associated surface wave dispersion information. Simultaneously, we employ the preprocessed steepest-descent (PSD) algorithm to invert the dispersion curves, and the low-velocity layer in the numerical tests can be clearly revealed. In conclusion, we deployed this method in a coal mine situated in Northern China, which encompasses abandoned mined-out areas. This procedure effectively disclosed the intricate azimuthal properties of the noise sources, yielding high-resolution dispersion images. The inversion outcomes depicted a 2-D S-wave velocity section plunging to a depth of 60 m. By interpreting the low-velocity features of the mined-out area, we can accurately delineate its boundaries. The results of the interpretation align coherently with the findings from the drilling data, to verify the feasibility and effectiveness of parallel-swath-array analysis of passive source surface waves based on beamforming in detecting coal mined-out area under non-stationary ambient noises.

Performance improvement and noise reduction analysis of multi-blade centrifugal fan imitating long-eared owl wing surface

This research was inspired by the long-eared owl's ability to fly silently. For the first time in this study, a wind turbine blade is designed to mimic the wing surface and the leading edge of the long-eared owl. The commonly used two-dimensional blade profile in previous studies is replaced by a more effective three-dimensional profile. This change leads to improved aerodynamic performance of the multi-blade centrifugal fan and reduced noise levels. The airfoil and leading edge profile parameters of the long-eared owl were extracted and utilized. These parameters were used to develop a fitting formula based on their correlation. This formula facilitated the design and optimization of a bionic blade (B-Blade). The results indicate a 4.1% enhancement in the maximum flow rate compared to the original blade fan, alongside a noise reduction of 1.3 dB(A) under identical static pressure conditions. An examination of the internal flow, noise, and sound source characteristics of both fan types was conducted, elucidating the aerodynamic noise mechanism. Fan noise propagation showed pronounced dipole sound source traits. The sound source area at the B-Blade fan's inlet and the volute tongue was more compact, leading to a decrease in mid-low frequency discrete noise. The sound source intensity was also diminished. The B-Blade fan also ameliorated the flow distribution at the impeller outlet, reducing the unstable interaction between the impeller and volute tongue, thereby effectively diminishing noise.

Sequencing Seismic Noise Correlations for Improving Surface Wave Retrieval and Characterizing Noise Sources

Abstract Cross-correlating continuous seismic data is a commonly employed technique to extract coherent signals to image and monitor the subsurface. However, due largely to site effects and poorly characterized noise sources in oceanic environments, its application to ocean-bottom seismometer (OBS) recordings often requires additional processing. In this contribution, we propose a method to improve the quality of the retrieved surface waves from OBS data and characterize the noise sources. We first cluster the pre-stack noise cross-correlation functions (NCFs) based on a sequencing algorithm, followed by selectively stacking those consisting of coherent and stable signals that are consistent with predicted surface-wave arrival times. Synthetic tests show that the sequenced NCFs can be used to recover the spatial and temporal distribution of noise sources. Applying the method to an OBS array offshore California increases the signal-to-noise ratios of the obtained Rayleigh waves. In addition, we find that the annual temporal distribution of selected NCFs with frequencies ranging from 0.04 to 0.1 Hz is nearly homogeneous during the recording period. In contrast, many NCFs excluded for stacking are temporally clustered. This method has the potential to be applied to other OBS recordings or possibly onland deployments, thus helping to obtain high-quality surface waves and to analyze temporal noise source characteristics.

High-precision Time Synchronization Algorithm for Unmanned Aerial Vehicle Ad Hoc Networks based on Bidirectional Pseudo-range Measurements

The increasing demand for missions involving unmanned aerial vehicles (UAVs) raises the need for reliable and accurate time synchronization in ad hoc networks. Time synchronization plays a crucial role in coordinating the actions of multiple UAVs, particularly when it comes to positioning, coordination, and data fusion. However, achieving high-precision time synchronization is challenging due to the difficult-to-estimate communication transmission delay, measurement noise, relative motion, and clock source characteristics in wireless ad hoc networks formed by UAVs. To address this issue, this paper proposes a fully distributed high-precision network time synchronization algorithm that is suitable for multi-hop dynamic ad hoc networks. The proposed algorithm is based on bidirectional pseudo-range measurements, graph theory, and random matrix correlation theory. It is composed of relative clock skew estimation, logic clock skew compensation and logic clock offset compensation. The algorithm is rigorously derived, requiring no central node, and it is capable of obtaining relative clock skew even when nodes broadcast clock information asynchronously. In addition, the proposed algorithm enables real-time elimination of transmission delays and has noise-resistant capabilities. Simulation results validate the effectiveness of the algorithm, demonstrating that the time synchronization accuracy in UAV ad hoc networks can reach the sub-nanosecond level.

An investigation of shock cell-related noise in an installed, full-scale tactical jet engine using acoustical holography

Shock cells in imperfectly expanded supersonic jet flows have been found to contribute to the noise radiated by these jets. Acoustic phenomena such as screech and broadband shock-associated noise have been linked to the shock cell structure in jets. Acoustical holography has been used to study various jet noise source and radiation characteristics. Holography-based reconstructions near the acoustic source in an installed, full-scale tactical jet engine have yielded features indicative of the known shock cell structure in the jet. This paper presents these results and investigates the characteristics of these features, including their distribution and relative amplitudes.

Characterization of microseism noise sources in Indian Ocean due to ocean atmospheric dynamics

Microseism noise, which occurs in the period range of 2–20 s, is the most energetic band in the earth's background spectra. In the present study, we examined the amplitude spectra and directional characteristics of microseism in the Indian Ocean. We use the data from ten openly accessible land stations located all around the Indian Ocean. The probability power spectral density was used to characterize the microseism. To characterize the microseism, we employ the frequency dependent polarization approach, which is governed by the Eigen value decomposition of the 3 × 3 spectral covariance matrix. The spatial and temporal variation of microseism was investigated in order to better understand its distribution in the Indian Ocean region, which is regarded as a global source of microseism. For some stations, we observe the splitting of double frequency microseism into short period (2–5 s) and long period (6–10 s) microseism. The polarization analysis reveals the dominant sources of the microseism are located in the Southern Ocean. We also correlated the spatio-temporal variation of significant wave heights (swh) with the power spectral densities at each station. We observe a remarkable correlation between power spectral density with the significant wave height (swh) in both spatially and temporally in secondary microseism band. We also characterize the dominant surface wave types in the microseism band. In long period band Rayleigh waves are dominant and Love waves are prominent in the short period band.

Flow dynamics and noise generation mechanisms in supersonic underexpanded rectangular and planar jets

In this study, large eddy simulations of two rectangular jets with different aspect ratios and one planar jet with spanwise periodic boundaries are carried out in an attempt to investigate and clarify some key issues regarding the flow dynamics and noise generation mechanisms for the screech tone. The substantial effects of the nozzle configuration on shock structures and noise are first revealed in detail. It is found that when the lateral confinement is weakened, the spacing of the shock cells increases and the jet oscillates more intensively in the minor-axis plane, increasing the noise level and altering the screeching frequency. By analyzing the pressure fluctuations of the shear layer in the wavenumber-frequency space, different kinds of waves in these supersonic jets are examined. Importantly, it is revealed that the guided jet wave should play a dominant role in closing the resonance loops rather than the acoustic wave. Moreover, the energy-containing structures with pertinent frequencies are extracted by employing the reduced-order variational mode decomposition, and some underlying flow dynamics are presented. Especially, a novel mechanism has been identified: the low-frequency stretching motions of shock cells have a significant modulation effect on the screeching amplitude in the planar jet. Furthermore, with the aid of the multi-process acoustic theory, the characteristics of physical noise sources are diagnosed, particularly the source mechanisms related to the screech tone. The general structures and distribution of the kinematic noise source Sβ and entropy noise source Se are presented. Sβ mainly exhibits a vortex-like structure near the nozzle, while Se exhibits a lamellar bilayer structure. The spatiotemporal correlations between the physical sources and far-field noise show that the dominant mechanisms for the screech tone rely on the nozzle configuration and the screech tone tends to be produced by multiple sources in all cases.

A study on the shaft stability and prediction of electromagnetic and NVH characteristics of permanent magnet synchronous machine according to the different pole/slot combination

In this study, electromagnetic noise sources of permanent magnet synchronous motors with 8-pole 9-slot and 12-slot fractional slot concentrated windings were analyzed through finite element analysis, and the radiated noise/vibration levels were compared through structural-acoustic analysis. In addition, the stability of the rotational shaft generated vibration was analyzed. Electromagnetic noise sources are classified into cogging torque, torque ripple, and unbalanced magnetic force. The effect of electromagnetic noise sources on vibration/noise was analyzed in the frequency domain through the electromagnetic-mechanical coupling analysis. The noise level was compared to the electromagnetic noise source characteristics for each frequency using the waterfall diagram, and additional vibration due to mechanical resonance was analyzed using critical speed analysis while taking the rotor's bearing stiffness into account.

Speech comprehension in noise-considerations for ecologically valid assessment of communication skills ability with cochlear implants.

Nowadays, cochlear implant (CI) patients mostly show good to very good speech comprehension in quiet, but there are known problems with communication in everyday noisy situations. There is thus aneed for ecologically valid measurements of speech comprehension in real-life listening situations for hearing-impaired patients. The additional methodological effort must be balanced with clinical human and spatial resources. This study investigates possible simplifications of acomplex measurement setup. The study included 20adults from long-term follow-up after CI fitting with postlingual onset of hearing impairment. The complexity of the investigated listening situations was influenced by changing the spatiality of the noise sources and the temporal characteristics of the noise. To compare different measurement setups, speech reception thresholds (SRT) were measured unilaterally with different CI processors and settings. Ten normal-hearing subjects served as reference. In acomplex listening situation with four loudspeakers, differences in SRT from CI subjects to the control group of up to 8 dB were found. For CI subjects, this SRT correlated with the situation with frontal speech signal and fluctuating interference signal from the side with R2 = 0.69. For conditions with stationary interfering signals, R2 values <0.2 were found. There is no universal solution for all audiometric questions with respect to the spatiality and temporal characteristics of noise sources. In the investigated context, simplification of the complex spatial audiometric setting while using fluctuating competing signals was possible.