Acoustic Pressure Data Research Articles

Quantifying acoustic damping using flame chemiluminescence

Thermoacoustic instabilities in gas turbines and aeroengine combustors fall within the category of complex systems. They can be described phenomenologically using nonlinear stochastic differential equations, which constitute the grounds for output-only model-based system identification. It has been shown recently that one can extract the governing parameters of the instabilities, namely the linear growth rate and the nonlinear component of the thermoacoustic feedback, using dynamic pressure time series only. This is highly relevant for practical systems, which cannot be actively controlled due to a lack of cost-effective actuators. The thermoacoustic stability is given by the linear growth rate, which results from the combination of the acoustic damping and the coherent feedback from the flame. In this paper, it is shown that it is possible to quantify the acoustic damping of the system, and thus to separate its contribution to the linear growth rate from the one of the flame. This is achieved by postprocessing in a simple way simultaneously acquired chemiluminescence and acoustic pressure data. It provides an additional approach to further unravel from observed time series the key mechanisms governing the system dynamics. This straightforward method is illustrated here using experimental data from a combustion chamber operated at several linearly stable and unstable operating conditions.

Sequential source localization in coastal water under the circumstances of time-evolving sound speed profiles

The sound speed profile (SSP) in water column is highly variable in time and space and can not be accurately known among measurement periods in coastal water. These factors will lead to the modeling mismatch and influence the performance of source localization of matched filed processor (MFP). The possibility of sequential tracking of source range and depth under the circumstances of time-evolving sound speed profiles is discussed. In this paper, we attempt to describe the time evolution characteristics of SSPs by analyzing the oceanographic dynamics which is the physical reason of the SSP’s change. A sequential approach considering the oceanographic dynamics model into the state-space equation is developed and applied. Synthetic simulations are carried out with experimental SSP data and acoustic pressure data from a vertical line array. Result suggests the sequential approach based on oceanographic dynamics model can improve the tracking capabilities.

An airborne acoustic method to reconstruct a dynamically rough flow surface.

Currently, there is no airborne in situ method to reconstruct with high fidelity the instantaneous elevation of a dynamically rough surface of a turbulent flow. This work proposes a holographic method that reconstructs the elevation of a one-dimensional rough water surface from airborne acoustic pressure data. This method can be implemented practically using an array of microphones deployed over a dynamically rough surface or using a single microphone which is traversed above the surface at a speed that is much higher than the phase velocity of the roughness pattern. In this work, the theory is validated using synthetic data calculated with the Kirchhoff approximation and a finite difference time domain method over a number of measured surface roughness patterns. The proposed method is able to reconstruct the surface elevation with a sub-millimeter accuracy and over a representatively large area of the surface. Since it has been previously shown that the surface roughness pattern reflects accurately the underlying hydraulic processes in open channel flow [e.g., Horoshenkov, Nichols, Tait, and Maximov, J. Geophys. Res. 118(3), 1864-1876 (2013)], the proposed method paves the way for the development of non-invasive instrumentation for flow mapping and characterization that are based on the acoustic holography principle.

On Sparse Reconstructions in Near-Field Acoustic Holography Using the Method of Superposition

The method of superposition is proposed in combination with a sparse [Formula: see text] optimization algorithm with the aim of finding a sparse basis to accurately reconstruct the structural vibrations of a radiating object from a set of acoustic pressure values on a conformal surface in the near-field. The nature of the reconstructions generated by the method differs fundamentally from those generated via standard Tikhonov regularization in terms of the level of sparsity in the distribution of charge strengths specifying the basis. In many cases, the [Formula: see text] optimization leads to a solution basis whose size is only a small fraction of the total number of measured data points. The effects of changing the wavenumber, the internal source surface and the (noisy) acoustic pressure data in general will all be studied with reference to a numerical study on a cuboid of similar dimensions to a typical loudspeaker cabinet. The development of sparse and accurate reconstructions has a number of advantageous consequences including improved reconstructions from reduced data sets, the enhancement of numerical solution methods and wider applications in source identification problems.

Coal-Rock Character Recognition in Fully Mechanized Caving Faces Based on Acoustic Pressure Data Time Domain Analysis

For the technical problems of coal and rock character recognition in fully mechanized caving faces. A method on characterization and recognition of coal and rock traits were discussed based on the time domain indexes of acoustic pressure data according to the differences of physics and mechanical parameters of coal and rock, and the differences of acoustic pressure data when coal and rock falling impact the rear beam of the sublevel caving hydraulic support. Firstly, the top coal caving experiments were carried out with mining portable vibration recorder developed by China University of Mining and Technology (Beijing) in fully mechanized caving faces in the underground mines, and the acoustic pressure data in quantity were acquired; Then, signal preprocessing were carried on to remove trend items for the selected acoustic pressure data; Finally, the acoustic pressure dates were analyzed in time domain and the time domain features were acquired. Comparison found, peak to peak, variance and kurtosis index are sensitive to the working conditions and the variance with a higher recognition rate. Accordingly proposed an analytical method that based on time-domain features of acoustic pressure date which used variance as recognition indicator, providing technical support for improving the caving automation and intelligent in the fully mechanized caving face.

수중실험을 통한 원통구조물의 3차원 수중방사소음 패턴 산출기법 분석

In this study for the prediction of 3D underwater radiated noise pattern, a comparison between the proposed method(DHIE, Discrete Helmholtz Integral Equation) and the 3D underwater radiated noise calculation results using the measurement of near-field acoustic pressure data is performed. The near-field acoustic pressure in water is measured for the calculation of the far-field radiated noise pattern and the far-field acoustic power. Also the vibration field of the underwater structure is measured in simultaneously. Using the total far-field acoustic power and the vibration field on the surface of the structure, the proposed method(DHIE) can predict the underwater radiated noise pattern of the far-field The predicted results show the reasonable agreement within about 5dB comparing with the experiment result.

Transient, planar, nonlinear acoustical holography for reconstructing acoustic pressure and particle velocity fields

A steady-state, nonlinear nearfield acoustical holography procedure, based on the Westervelt Wave Equation (WWE), was developed by the authors of this article to accurately reconstruct nonlinear acoustic “pressure” fields. Here, a “transient,” nonlinear acoustic holography algorithm is introduced that can be used to reconstruct three-dimensional, nonlinear acoustic pressure as well as particle velocity fields from two-dimensional acoustic pressure data measured on a measurement plane. This procedure is based on the Kuznetsov Wave Equation (KWE) that is directly solved by applying temporal and spatial Fourier Transforms to the KWE. When compared to the WWE-based procedure, the proposed procedure can be used to reconstruct acoustic particle velocity fields in addition to acoustic pressure fields. It can also be applied to multi-frequency source cases where each frequency component can contain both linear and nonlinear components. The KWE-based procedure is validated by conducting four numerical simulations with: (1) an infinite-size panel vibrating at a single frequency, (2) a pulsating sphere with a bifrequency excitation, (3) a finite-size, vibrating panel generating bended wave rays, and (4) an ultrasound transducer with a transient excitation. The numerical results show that holographically projected acoustic fields match well with directly calculated ones.

Tracking of time-evolving sound speed profiles in shallow water using an ensemble Kalman-particle filter

This paper presents a tracking technique for performing sequential geoacoustic inversion monitoring range-independent environmental parameters in shallow water. The inverse problem is formulated in a state-space model with a state equation for the time-evolving sound speed profile (SSP) and a measurement equation that incorporates acoustic measurements via a hydrophone array. The particle filter (PF) is an ideal algorithm to perform tracking of environmental parameters for nonlinear systems with non-Gaussian probability densities. However, it has the problem of the mismatch between the proposal distribution and the a posterior probability distribution (PPD). The ensemble Kalman filter (EnKF) can obtain the PPD based on the Bayes theorem. A tracking algorithm improves the performance of the PF by employing the PPD of the EnKF as the proposal distribution of the PF. Tracking capabilities of this filter, the EnKF and the PF are compared with synthetic acoustic pressure data and experimental SSP data. Simulation results show the proposed method enables the continuous tracking of the range-independent SSP and outperforms the PF and the EnKF. Moreover, the complexity analysis is performed, and the computational complexity of the proposed method is greatly increased because of the combination of the PF and the EnKF.

Resolution enhancement of nearfield acoustic holography by interpolation using band-limited signal restoration method

A new method based on interpolation using band-limited signal restoration method was proposed for enhancing the resolution of the nearfield acoustic holography. According to the band-limited property of the pressure on the hologram surface, a band-limited signal restoration method named Papoulis-Gerchberg algorithm was used to realize the interpolation of acoustic pressure. Therefore acoustic pressure data on the hologram surface were increased, the sampling interval was reduced, the information on evanescent waves which was lost because of the large sampling interval was partially recovered, and the resolution of nearfield acoustic holography image was improved. The experimental result shows that the method can enhance the resolution of the nearfield acoustic holography image efficiently.

Geoacoustic inversion based on both acoustic pressure and particle velocity

Conventional inversion schemes for environmental assessment depend on an objective function that exploits amplitude or phase information of acoustic pressure data alone. This work investigates the potential of vector sensors for geoacoustic inversion by defining an objective function that also takes into account acoustic particle motion. Calculations are performed on synthetic broadband data for a shallow water environment (South Elba) with an optimization scheme based on different metaheuristics. Differences in the inversion process, including sensitivity of the cost function to environmental parameters and convergence speed of the optimization algorithm, are presented by comparing inversion results for a sparse pressure-only array and a vector sensor array.

Acoustic spreading and boat noise within shallow Florida waterways

The West Indian manatee has become endangered partly because of watercraft collisions in Florida’s coastal waterways. Several boater warning systems, based upon manatee vocalizations, have been proposed to reduce the number of collisions. One aspect of the feasibility of an acoustically based system will rely upon the distance in which a manatee vocalization can be detected. The magnitude of environmental noise and manatee vocalizations, as well as the acoustic spreading properties of the habitat will help to estimate the detection range of a manatee. This study surveyed several shallow-water coastal areas in Florida (Crystal River, Cedar Key, and Indian River) which are likely to be inhabited by manatees. Using a chirp signal (1–10 kHz) broadcast by an underwater transducer, it was observed that the acoustic pressure data collected was best represented by the mixed and Lurton spreading models. The overall data obtained from passing a boat directly over the hydrophone was most closely represented by a spherical spreading model. However, for a boat that is approaching, a mixed spreading model is the most appropriate. The mean SPL for boat traffic was measured to be 140 dB and the background environment noise level ranged between 69 and 105 dB.

Comparison of Two Waveguide Methods for Educing Liner Impedance in Grazing Flow

Acoustic measurements taken with several liners in a flow impedance tube are used to assess two waveguide methods, the single mode method (SMM) and the finite element method (FEM), for impedance eduction in the presence of uniform grazing flow. Both methods use complex acoustic pressure data acquired over the liner length to educe the liner impedance. The SMM is based on the assumption that the sound pressure level and phase decay rates of a single progressive mode can be extracted from the measured complex acoustic pressures. No a priori assumptions are made in the FEM regarding the measured data. For no-flow conditions, the accuracy of each method is demonstrated by the excellent agreement between no-flow impedances educed in a grazing incidence tube and those acquired in a normal incidence tube. For grazing flow conditions (Mach numbers up to 0.5), the relative accuracy of the two waveguide methods is demonstrated by comparing the impedances educed with the FEM to the corresponding results for the SMM. Significant discrepancies occur for both methods for tests conducted at 0.5 kHz. Possible explanations for these discrepancies are explored with, as yet, no clear answer. Above 0.5 kHz, the results indicate the SMM can be used when the acoustic pressure profile is dominated by a single progressive mode, whereas the FEM can be used for all cases.

Analysis of the near field acoustic signature from subsonic high-speed projectiles in an acoustic test tank

A series of tests to obtain the near field acoustic signature from subsonic high-speed supercavitating projectiles in an underwater acoustic test tank at NUWC/NPT have been recently obtained. The projectiles are approximately 0.1 meters in length and 0.02 meters in diameter and travel at speeds up to 900 m/s in the test tank. Underwater acoustic near field transient pressure data has been obtained with several sensors and a preliminary analysis of the data is reported. Possible noise source mechanisms which have been investigated include the radiated noise of the launcher, the near field hydrodynamic pressure field associated with the motion and shape of the projectile, acoustic multipath contributions from the walls of the tank, oscillations of the trailing cavity convected with the projectile and the collapse of bubbles from the aft end of the cavity. The data has been analyzed using an empirical mode decomposition/Hilbert–Huang transform (EMD/HHT) method and hybrid methods of EMD/ordinary spectral analysis, EMD/wavelet analysis, and EMD/short time Fourier transform (STFT) methods to obtain frequency–time information. Preliminary results for the relative importance of the launcher noise, hydrodynamic component and the bubble collapse are presented and discussed. [Work supported by ONR.]

Imaging of supersonic surface fields on a cylindrical shell

Conventional imaging techniques represent a radiating body as a distribution of equivalent monopole sources. This paper presents a more rigorous approach for imaging the supersonic components of both surface pressure and normal velocity produced by scattering or radiation from a nominally cylindrical body. The technique employs the Helmholtz integral equation to relate monopole and dipole strengths. A modal expansion of the surface pressure on a baffled cylinder forms the basis for determining an array-weighting function used to estimate the surface fields. The technique makes use of acoustic pressure data measured in a horizontal plane at ranges where the evanescent field is insignificant. No far-field constraints are imposed in the derivation of the method, allowing the measurements to be conducted within the geometric near-field of the body. When the array is located in the far field, this method is shown to yield similar array-weighting functions to a method derived by Williams [E. G. Williams, J. Acoust. Soc. Am. 99, 2022 (1996)]. Mid-frequency, 2≤ka≤20, scattering and radiation data from measurements of a ribbed cylindrical shell are used to illustrate the technique. The intensity images reveal membrane wave and rib scattering processes in both the time and frequency domains. [Work supported by NSWC and ONR.]

A comparison of Hankel transform algorithms’ performance in shallow-water waveguides and their effect on the determination of seabottom plane-wave reflection coefficients

Complex acoustic pressure data as a function of range may be decomposed into its constituent wave-number components by taking the Hankel transform. This paper investigates the accuracy of a number of discrete Hankel transform algorithms applied to computer simulated pressure data in a shallow water channel for a variety of seabed bottom types. The results are compared against the exact solution and the most accurate algorithm identified for shallow-water applications. The differing behavior of these algorithms is illustrated by their use in the Hankel transform method proposed by Frisk et al. [J. Acoust. Soc. Am. 68, 602–612 (1980)] for the determination of seabottom plane-wave reflection coefficients. The results from computer simulations and scale model tank experiments are compared.

A parametric windtunnel test on rotorcraft aerodynamics and aeroacoustics (Helishape) — test procedures and representative results

Within the framework of a major European co-operative research project on rotorcraft aerodynamics and acoustics (Helishape), a parametric model rotor test was conducted in the open test section of the DNW using DLR's MWM test rig, and a highly instrumented model of a fully articulated ECF rotor equipped with blades of advanced design with two exchangeable blade tips. One set of blade tips (7A) was rectangular, while the other (7AD1) was a swept-back parabolic-anhedral shape. The objectives of this experimental research were to evaluate noise reduction techniques (conceptionally by variation of rotor speed, dedicated tip shapes and advanced aerofoils, as well as operationally, by identifying low noise, BVI-minimising, descent procedures) and to validate the partners’ aerodynamic and aeroacoustic codes. A comprehensive set of simultaneous acoustic and aerodynamic blade surface pressure data and blade dynamic and performance data was measured. In addition, valuable information on the tip-vortex geometry and blade-vortex miss distance was obtained by LLS flow visualisation. The experimental equipment, the test procedures, and the test matrix are briefly described. A survey of the main results is presented and the trends of the most important parameter variations for both rotors are discussed.

Launch vehicle acoustics. II - Statistics of the time domain data

Skewness and kurtosis coefficients were investigated as a means of quantifying the shock content and non-Gaussian characteristics of rocket noise. Rocket noise data measured at 3-5 locations during the launches of four different vehicles with thrusts ranging from 440 to 10,700 kN were analyzed. The kurtosis coefficients of the acoustic pressure data showed no discernible pattern of variation about the Gaussian value of 3. The skewness coefficients ranged from 0.02 to 0.55, all of them greater than the value of 0 expected for Gaussian data. Both the skewness and kurtosis coefficients for the derivative of the acoustic pressure showed much greater deviations from the values expected for Gaussian noise. This study has confirmed that the coefficient of skewness is a useful metric for the characterization of rocket noise. The statistics of the pressure gradient have been shown to be more sensitive indicators of shock content, and these metrics are much less sensitive to low-frequency instrumentation limits.

Experimental Results of the European HELINOISE Aeroacoustic Rotor Test

In a cooperative research program between eight European partners, a 40% geometrically and dynamically scaled and highly instrumented model of the ECD (formerly MBB) BO 105 helicopter main rotor was tested in the open-jet anechoic test section of the German-Dutch Wind Tunnel (DNW) in the Netherlands. The primary objectives of this experimental study were 1) to improve the physical understanding of the impulsive rotor noise sources by correlating blade pressure and acoustic characteristics and 2) to provide an extensive airload and acoustic database for code validation purposes.Consequently, a comprehensive set of simultaneous acoustic and aerodynamic blade surface pressure data as well as blade dynamic and performance data were measured for the standard rotor with rectangular blade tips. In addition, initial quantitative information of the blade- vortecx miss distance during blade-vortex interaction (BVI) was obtained.

From gigabytes to understanding

A near-field holography data set typically consists of about 0.5 Gbytes of acoustic pressure data. After a bit of routine processing, data sets totaling 2.0 Gbytes are soon in our computer’s hands. An example is given with a recent experiment of how a typical data set is handled, processed, and reprocessed to eventually provide insight into the physics of vibration, radiation, and/or scattering. The nuts and bolts of the processing and how they have become optimized through years of experience will be discussed, for example, how such large data sets are dealt with, what kind of computers and computer programs, what signal processing algorithms and libraries, how data is visualized, what kind of display devices, computer networks, etc. In the quest for physical understanding, of critical importance is time, that is, it is not only what can be done that counts, but how fast it can be done. This is the purpose of this paper.

Wave vector measurements of empirical design transfer functions

Empirical design transfer functions can be used to predict the radiated noise generated from various components in a system. The Darby method is one example of an approach used to find these empirical design transfer functions. For an immersed and radiating structure, near‐field measurements of the pressure field can be wave vector filtered to find the empirical design transfer function from the structural input point to the acoustic far field. Assuming a cylindrical body excited by a force and radiating, a discrete line array of pressure sensing devices are used to measure the near‐field pressure. The acoustic pressure data measured at each point are sampled, digitized, time averaged, and frequency transformed. The position‐frequency domain pressure is then transformed into the wave vector domain, and the far‐field radiation is predicted. The empirical design transfer function is found by dividing the predicted far‐field radiation by the input force. The mathematical development of this approach is presented. Simulations are presented for various radiation sources, including a monopole and a discrete line source. The pressure predictions of the wave vector are compared with the pressure directly computed for the sources. [Work supported by ONR.]