Structural Acoustic Response Research Articles

Measurements and modeling of acoustic scattering from targets in littoral environments

Broadband laboratory and at-sea measurements systems have been built by NRL to quantify the acoustic target strength of objects sitting on or in the bottom of littoral environments. Over the past decade, these measurements and the subsequent modeling of the target strength have helped to develop an understanding of how the environment, especially near the bottom interface, impacts the structural acoustic response of a variety of objects. In this talk we will present a set of laboratory, at-sea rail and AUV based back scatter, forward scatter, and propagation measurements with subsequent analysis to understand the impact of the littoral environment. Simple targets such as spheres, along with UXO targets will be discussed. The analysis will be focused on quantifying the changes to target strength as a result of being near the bottom interface. In addition to the traditional backscatter or monosatic target strength, we focus upon efforts to investigate the multi-static scattering from targets. [Work supported by ONR.]

Structural–acoustic optimization of a pressurized, ribbed aircraft panel

A method to reduce the noise radiated by a ribbed, aircraft panel excited by turbulent boundary layer flow is presented. To compute the structural-acoustic response, a modal approach based on finite element/boundary element analysis was coupled to a turbulent boundary flow forcing function. A static pressure load was also applied to the panel to simulate cabin pressurization during flight. The radiated sound power was then minimized by optimizing the horizontal and vertical rib location and rib cross section using an evolutionary search algorithm. Nearly 10 dB of reduction was achieved by pushing the ribs to the edge of the panel, thus lowering the modal amplitudes excited by the forcing function. A static constraint was then included in the procedure using a low-frequency dynamic calculation to approximate the static response. The constraint limited the amount of reduction that was achieved by the optimizer.

Acoustic and Vibration Control for an Underwater Structure under Mechanical Excitation

Acoustic and vibration control for an underwater structure under mechanical excitation has been investigated by using negative feedback control algorithm. The underwater structure is modeled with cylindrical shells, conical shells, and circular bulkheads, of which the motion equations are built with the variational approach, respectively. Acoustic property is analyzed by the Helmholtz integration formulation with boundary element method. Based on negative feedback control algorithm, a control loop with a coupling use of piezoelectric sensor and actuator is built, and accordingly some numerical examples are carried out on active control of structural vibration and acoustic response. Effects of geometrical and material parameters on acoustic and vibration properties are investigated and discussed.

Coupled Structural-Acoustic Response of a Duct-Mounted Elastic Plate with Grazing Flow

An analytical solution for the fully coupled structural-acoustic response of a thin elastic plate mounted in a rectangular duct assuming linear dynamics is presented. The plate is subjected to harmonic planar acoustic waves at grazing incidence in the presence of a subsonic uniform mean flow in the duct. Both clamped and simply supported plates are considered. Regimes are identified in which the structural-acoustic coupling plays a significant role on the response of the plate. Four metrics characterizing the coupled response are considered: the deviation of the peak response frequency from the in vacuo natural frequency of plate, the amplitude of the peak response, the effective acoustic damping of the plate, and the plate intermodal coupling through the duct acoustic field. For a typical aerospace structure, the plate-fluid coupling does little to change the fundamental vibration frequencies from their in vacuo values but significantly alters the amplitudes of the plate vibration. This is a result of the acoustic radiation being a strong function of the in-duct flow Mach number . Further, the intermodal coupling through the acoustic field is also a strong function of . Therefore, reduced-order models relying on the panel’s in vacuo modes require Mach-number-dependent coupling coefficients for accurate predictions.

Acoustic imaging and structural acoustic analysis of laboratory measurements of scattering from buried targets above critical grazing angles

A broadband (3-40 kHz) compact range measurement technique has been developed to obtain the acoustic scattering from buried unexploded ordnance and objects simulating natural and man-made clutter. The targets—two 5 in. rockets with 0, 30, and 60 deg pitch angles, a large rock, and cinder blocks with 0 and 45 deg roll—are buried 10 cm beneath the surface of a water-saturated sandy bottom with a mean grain size of 240 μm. A 2D synthetic array is generated at a height of 20 cm above the sediment–water interface with an element spacing of 3 cm (25 kHz Nyquist). Waveforms collected on the synthetic array are processed to extract the structural acoustic response of the buried targets. A Relevance Vector Machine algorithm applied to the scattered data for target identification, which shows that the target features separate even as the receiver array size is considerably decreased. Similar results are presented for numerical simulations of the bistatic returns for the buried 5 in. rocket and a rock of comparable size. [Work supported by ONR and SERDP.]

Elastic response of an acoustic coating on a rib-stiffened plate

This paper develops a three-dimensional analytical model of a fluid-loaded acoustic coating affixed to a rib-stiffened plate. The system is loaded by a plane wave that is harmonic both spatially and temporally. The model begins with Navier–Cauchy equations of motion for an elastic solid, which produces displacement fields that have unknown wave propagation coefficients. These are inserted into stress equations at the boundaries of the plate and the acoustic coating. These stress fields are coupled to the fluid field and the rib stiffeners with force balances. Manipulation of these equations develops an infinite number of indexed equations that are truncated and incorporated into a global matrix equation. This global matrix equation can be solved to determine the wave propagation coefficients. This produces analytical solutions to the systems’ displacements, stresses, and scattered pressure field. This model, unlike previously developed analytical models, has elastic behavior and thus incorporates higher order wave motion that makes it accurate at higher wavenumbers and frequencies. An example problem is investigated for three specific model results: (1) the dynamic response, (2) a sonar array embedded in the acoustic coating, and (3) the scattered pressure field. An expression for the high frequency limitation of the model is derived. It is shown that the ribs can have a significant impact on the structural acoustic response of the system.

PEM Based Sensitivity Analysis for Acoustic Radiation Problems of Random Responses

A new effective method for computing the acoustic radiation and its sensitivity analysis of a structure subjected to stochastic excitation is presented. Previous work in the area of structural and acoustic sensitivity analysis systems was mostly focused on the deterministic excitation. New methods are developed to account for stochastic excitation. The structural-acoustic response is calculated using finite element method and boundary element method combined with stochastic analysis techniques. An accurate and highly efficient algorithm series for structural stationary random response analysis, pseudo-excitation method (PEM), is extended to acoustic random analysis in this paper, which was used to calculate structural random analysis in the past. So the acoustic radiation problems of random responses are transformed to the structural-acoustic harmonic analyses. This is a time-saving progress in comparison with traditional method. Based on the PEM, the acoustic radiation sensitivities of the structure are developed in emphasis that are transformed to harmonic sensitivity analyses. They are validated by comparing with the results of finite difference sensitivity method. Numerical examples are given to demonstrate the effectiveness of the methods and the program.

Forward scatter and backscatter low-frequency synthetic array measurements of the structural acoustic response from proud targets using a 48-m-long rail in a littoral environment.

A series of short range (25-m) forward scatter and backscatter measurements in a littoral environment was conducted to quantify the structural acoustic response from proud targets in the 2–23 kHz frequency band. The water channel was 14 m deep in the Gulf of Mexico near Shell Island, Panama City FL. The bottom was a medium grained sand. The acoustic forward scatter response of the proud targets was measured in a bistatic configuration with the source 25 m from the target and a receiver mounted on a 48-m-long rail. The rail is used to position the receiver and synthetically quantify the structural acoustic forward scatter response. A second source was co-located with the receiver on the rail, and monostatic backscatter measurements were also taken for each target. The structural acoustic response was analyzed and will be reported. The synthetic array experimental results are compared and contrasted with laboratory measurements. The laboratory measurements are convolved with water channel propagation predictions using a parabolic equation model of the littoral environment. The convolved laboratory target strengths compare well with the synthetic array measurements, and a detailed discussion of the comparison will be presented. [Work supported by ONR.]

Sensitivity analysis of coupled structural–acoustic systems subjected to stochastic excitation

A development for computing the acoustic pressure spectral density and its sensitivity of coupled structural–acoustic systems subjected to stochastic excitation is presented. Previous work in the area of structural–acoustic response considered systems subject to deterministic excitations. The response computation depends on the excitation; therefore, new methods are developed to account for stochastic excitation. The structural–acoustic response is calculated using finite element methods and stochastic analysis techniques. An accurate and highly efficient algorithm series for structural stationary random response analysis, pseudo excitation method (PEM) is used. Numerical examples are given to demonstrate the effectiveness of the methods and the program.

The Broadband In‐Water Structural Acoustics of Unexploded Ordinance

The Laboratory for Structural Acoustics (LSA) at the Naval Research Laboratory consists of a 1 million gallon, deionized water, indoor cylindrical tank (17m diameter by 15m deep). The key features include: 1) vibration and temperature isolation, 2) feedback controlled heating and adiabatic materials (temperature variability <0.01°C), and 3) reverberation reducing anechoic materials. This laboratory has computer controlled robotic scanners and manipulators used for precision freefield measurements including nearfield acoustic holography and compact range scattering. The precision robotics, environmental control, and painstaking measures to insure homogeneity and stability result in a high fidelity, versatile, and unique underwater acoustic measurement laboratory. The LSA also contains an indoor rectangular tank (10m by 8m) laboratory, with a 3m deep sand bottom and 4m of water column. In a similar fashion to the freefield laboratory, this laboratory is used to study target scattering in a marine bottom environment. We discuss such databases focused on the challenging problem of unexploded ordinance (UXO) in water where we use the structural acoustic response measured in a series of laboratory experiments to detect and identify several common UXOs. The 360 degree broadband (1‐140 kHz) compact range monostatic and bistatic measurements taken in both laboratories will be discussed.

Broadband acoustic scattering measurements of underwater unexploded ordnance (UXO)

In order to evaluate the potential for detection and identification of underwater unexploded ordnance (UXO) by exploiting their structural acoustic response, we carried out broadband monostatic scattering measurements over a full 360 degrees on UXO's (two mortar rounds, an artillery shell, and a rocket warhead) and false targets (a cinder block and a large rock). The measurement band, 1-140 kHz, includes a low frequency structural acoustics region in which the wavelengths are comparable to or larger than the target characteristic dimensions. In general, there are aspects that provide relatively high target strength levels ( approximately -10 to -15 dB), and from our experience the targets should be detectable in this structural acoustics band in most acoustic environments. The rigid body scattering was also calculated for one UXO in order to highlight the measured scattering features involving elastic responses. The broadband scattering data should be able to support feature-based separation of UXO versus false targets and identification of various classes of UXO as well.

Experimentally measured diffraction of sonic boom waves around a house

During June 2006, a series of sonic boom flight tests were conducted as a part of the NASA low-boom/no boom program. One objective was to evaluate structural acoustic building response on a house. This presentation gives some results of that test regarding the outdoor pressure response on and near the house. The passage of the sonic booms over a nearby wall and the house itself have been examined to determine the maximum pressure loading. Both conventional N-waves and low-amplitude N-waves have been considered. The results indicate that the walls of the house facing the direction of the incoming sonic boom receive the maximum loading in agreement with image theory. The findings also confirm the temporal and spatial dependence of the structural loading. [The first author is supported by NASA and the FAA/NASA/Transport-Canada PARTNER Center of Excellence.]

Widening the sound absorption bandwidths of flexible micro-perforated curved absorbers using structural and acoustic resonances

In this paper, the sound absorptions of flexible curved micro-perforated panels that are backed by an air cavity are studied in detail. A theoretical model that is based on the classical plate equation coupled with an acoustic wave equation is developed for the prediction of sound absorption. This model considers both symmetrical and antisymmetrical structural acoustic responses. Using the electro-acoustic analogy approach, another model is developed that only considers single structural and acoustic modes. It is proposed to make use of panel and cavity resonances to widen the absorption bandwidth of a single/double perforated absorber. The absorption of a flexible micro-perforated panel can be further enhanced by adjusting its curvature to bring the resonant frequencies closer together. The effects of various parameters such as boundary condition, incidence angle, and curvature on the absorption are studied. Predicted results for the single and double layer absorbers show good agreement with measurements.

Building structural acoustic response to aircraft sonic booms

As part of the NASA Low Boom/No Boom flight test project, a series of low‐amplitude sonic‐boom tests was completed over a 3‐week period in June of 2006. This series of flight tests was designed to evaluate indoor/outdoor human subjective response, structural acoustic building response, and the effects of atmospheric turbulence for low‐amplitude sonic booms characterized by overpressures in the nominal range of 0.1 to 0.6 pounds per square foot (psf). Low‐amplitude sonic booms were generated by F‐18 aircraft using dive trajectories to produce a range of overpressures at the Edwards Air Force Base housing area. In addition, straight and level supersonic flights were used to generate normal level (nominally 1.4 psf) sonic‐boom overpressures at the housing area. This report will describe the structural acoustic building response measurements obtained during this flight test project. A single‐family ranch‐style home was instrumented with nearly 300 microphone and accelerometer sensors to determine the incident pressure and the building’s structural acoustic response. Data were acquired for over 100 sonic‐boom events. The test setup will be described and analysis of the measurements will be presented.

Structural-Acoustic Optimization of Sandwich Panels

Sandwich panels comprising face sheets enclosing a core are increasingly common structural elements in a variety of applications, including aircraft fuselages, flight surfaces, vehicle panels, lightweight enclosures, and bulkheads. This paper presents the optimization of various innovative sandwich configurations for minimization of their structural-acoustic response. Laminated face sheets and core geometries comprising honeycomb and trusslike structures are considered. The design flexibility associated with the class of considered composite structures and with truss-core configurations provides the opportunity of tailoring the structure to the load and dynamic response requirements of a particular application. The results demonstrate how the proper selection of selected key parameters can achieve effective reduction of the radiated sound power and how the identified optimal configurations can achieve noise reduction over different frequency ranges and for various source configurations.

Intermediate scale measurement system for underwater acoustic measurements

The Intermediate Scale Measurement System (ISMS) provides high quality acoustic measurements on large-scale submarine models and other underwater structures in a controlled environment. Development and implementation of advanced marine technology designs require a thorough understanding of structural acoustic response mechanisms. The only practical and cost effective approach to developing this understanding is through the use of a combination of numerical models and large-scale physical models. The ISMS provides an affordable, unique capability of making precision acoustic measurements on models large enough to contain the structural detail and the instrumentation required to understand structural acoustic mechanisms and to confirm and guide the use of numerical models. This is a key element for prediction of full-scale performance and integration of advanced submarine technologies into operational systems.

Wave propagation and damping in linear viscoelastic laminates

This analysis is concerned with wave propagation and damping in linear viscoelastic laminates. A spectral finite-element method is developed and used to calculate the dispersion properties of the first few wave types of a given laminate; the proposed approach provides a robust and numerically efficient alternative to the transfer matrix method in certain applications. The proposed approach is also well suited to the calculation of the wave types of sections whose material properties vary continuously throughout the thickness of the section. A one-dimensional finite-element mesh is used to describe the through-thickness deformation of the laminate, and the dispersion equation for plane-wave propagation is formulated as a linear algebraic eigenvalue problem in wave number at each frequency of interest. The resulting eigenvectors and eigenvalues can be computed using standard numerical routines and used to investigate the dispersion characteristics of the propagating wave types of the section. The damping loss factor of each wave type is estimated from the cross-sectional strain energy distribution of the laminate. The proposed approach is well suited to modeling the structural-acoustic response of sandwich panels, constrained layer damping treatments, and general viscoelastic laminate sections in statistical energy analysis (SEA) codes.

Parametric design sensitivity analysis of high-frequency structural-acoustic problems using energy finite element method

A design sensitivity analysis of high-frequency structural-acoustic problems is formulated and presented. The energy finite element method (EFEM) is used to predict structural-acoustic responses in the high frequency range, where the coupling between structural junctions and the structural-acoustic interface are modelled using power transfer coefficients. The continuum design sensitivity formulation is derived from the governing equation of EFEM and the discrete method is applied in the variation of the structural-structural and structural-acoustic coupling matrices. The direct differentiation and adjoint variable method are both developed for the sensitivity analysis, where the difficulty of the adjoint variable method is overcome by solving a transposed system equation. Parametric design variables such as panel thickness and material damping are considered for sensitivity analysis, and numerical sensitivity results show excellent agreement as compared to analytical finite difference results.

Investigation of Curved Polymeric Piezoelectric Active Diaphragms

Piezoelectric active diaphragms hold promise as an alternative to using passive diaphragms driven by voice coils for sound generation and noise cancellation applications. This paper presents an in-depth investigation of the acoustic response for curved polymeric piezoelectric active diaphragms. Simple analytical models were derived and experimentally validated to predict the structural dynamic and acoustic responses for generic polyvinylidene fluoride (PVdF) constant curvature active diaphragms with variable geometric parameters (width, radius, subtended angle, thickness). These models are useful for design purposes and for capturing the overall behavioral trends. To analyze the acoustic response mechanisms further, three-dimensional numerical models were also developed and experimentally validated. Parametric studies based upon these models reveal the potential of high acoustic outputs (over 100 dB in the far field) from optimized geometric configurations with subtended angles differing from the conventionally utilized flat, semicircular or circular configurations. These studies, corroborated by experiments on a variety of active diaphragm prototypes, conclude that the acoustic output is governed by the structural ring resonance, which can be designed such that the most efficient acoustic radiation is within the particular frequency range of operation.

Feature Variation and its Impact on Structural Acoustic Response Predictions

This paper presents a screening technique to assess the impact on model fidelity introduced by variations in the properties or positions of features in harmonically forced fluid-loaded structural acoustic models. The perspective taken is one of knowledge of a reference state, with a desire to determine the impact on the total radiated acoustic power due to perturbations in the reference state. Such perturbations change the predicted resonance frequencies of a structure under consideration, and hence, change the predicted response amplitudes. The method uses a single degree of freedom response model in the local region of each fluid-loaded resonance, coupled with eigenvalue sensitivities or variations, to estimate the perturbation impact. The perturbation is scaled by the degree to which each given mode participates in the response quantity of interest. The SDOF model yields results that indicate that proportional bandwidth analysis will be less sensitive to perturbation than constant bandwidth analysis. This is demonstrated through comparison of a constant bandwidth analysis and a 1/3 octave analysis applied to the same system. Elements of the analysis method are not necessarily restricted to model perturbations nor acoustic power, rather they may be used to assess the perturbation of any quadratic response quantity of interest due to changes in resonance frequency.