Experimental modal analysis technique for three-dimensional acoustic cavities

Nowadays Acoustic Modal Analysis often plays a significant role in the optimisation process of the acoustic parameters of all kinds of cavities, e.g. the interior of a car, a train, an airplane etc. A problem with the existing experimental Acoustic Modal Analysis techniques is that one has to use volume acceleration sources which interact with the acoustic system. The Operational Acoustic Modal Analysis (OAMA) technique doesn't suffer from this problem. An important disadvantage of the OAMA approach is that the estimated mode shapes are not correctly scaled. This scaling problem will be tackled in this paper. We will introduce a sensitivity based rescaling technique for the acoustic domain. Local volume changes of the cavity will make a correct estimate of the mode shapes possible. The technique will be validated by some simulations and experiments.

Two research projects have been conducted to study the possibility of using acoustic emission and modal analysis techniques for in situ surveillance of offshore platforms. For acoustic emission, experiments onshore, in sheltered area and on platforms have been conducted. The experimental part of the program terminated with a test on a pre-cracked T-joint on a platform in the southern North Sea. A prototype industrial system and the corresponding interpretation software have been developped. The various tests show that acoustic emission allows detection of cracks and monitoring of crack growth in a limited area of a platform. Modal analysis has been studied through theoretical analysis, small scale tests and field tests on an operational platform. Two slightly different techniques have evolved from the study: – modal analysis per se which gives global information on the overall structure. – vibrodetection which provides more accurate measurements on individual bracings.

In shallow water, spatial and temporal variability of the water column often restricts accurate estimations of bottom properties from long-range acoustic propagation data. Two recent shallow-water experiments, conducted at a sandy site on the New Jersey Shelf and a muddy site on the New England Shelf, showed 5-10 dB acoustic transmission-loss differences between two sites. The New Jersey Shelf experiment was conducted during May 2015 and The New England Shelf experiment was conducted during November 2015, representing summer and winter propagation conditions, respectively. Acoustic modal analysis and pulse-decay tomography methods were used to map the geoacoustic variability of each site covering a 40 km by 40 km area. Both numerical simulations and the New England Shelf experimental results showed that geoacustic variability can be measured by geoacoustic tomography. Tomographic results from the New Jersey Shelf sites showed less geoacoustic variability that may be due to the presence of a harder bottom....

A steady state analysis is applied to study the voltage collapse problem. The modal analysis method is used to investigate the stability of the power system. Q-V curves are used to confirm the obtained results by modal analysis method and to predict the stability margin or distance to voltage collapse based on reactive power load demand. The load characteristics are considered in this research. Different voltage dependent loads are proposed in order to be used instead of the constant load model. The effect of induction machine load is considered in this study. The load is connected to several selected buses.;The analysis is performed for three well-known system; Western System Coordinating Council (WSCC) 3-Machines 9-Bus system, IEEE 14 Bus system and IEEE 30 Bus system. The modal analysis technique is performed for all systems using the constant load model, the voltage dependent load models and induction machine load model. Then, the most critical mode is identified for each system. After that, the weakest buses, which contribute the most to the critical mode, are identified using the participation factor. The Q-V curves are generated at specific buses in order to check the results obtained by the modal analysis technique and to estimate the stability margin or distance to voltage collapse at those buses.

Successful osseointegration of orthopedic implants relies on achieving mechanical stability of screw fixation. Various techniques have been utilizing to assess screw stability; however, some of these methods are destructive or time-consuming. Non-destructive techniques, such as modal analysis, have gained interest in evaluating screw stability. This study aimed to assess the primary and secondary stability of screws in an in-vivo animal experiment by comparing conventional methods with two modal methods, namely Periotest® and acoustic modal analysis (AMA). To achieve this goal, eight rabbits were implanted with cylindrical threaded screws. The screw fixation stability was evaluated using four different test methods. Initially, the peak insertion torque (PIT) was measured for each bone screw during the implantation process. Additionally, AMA and Periotest® were employed to assess the natural frequency (NF) and Periotest® value (PTV) of the bone-screw structure, respectively. Finally, a destructive pull-out test was conducted to determine the peak pull-out force (PPF). The average primary stability values were measured 5.15 N.cm for PIT, 2507±59 Hz for NF, 21.85±4.1 for PTV, and 94.45±12 for PPF. In contrast, the secondary stability showed varying results with an average increase of 39% for NF, 101% for PTV, and 194% for PPF. The research also compared the non-destructive AMA with the destructive pullout test, considered the gold standard for stability evaluation of bone screws. The study found that AMA exhibited a linear relationship with the pullout test, making it an accurate method for assessing orthopedic and spinal screw applications.

This paper presents an application of the Experimental Acoustic Modal Analysis (AMA) on a vintage 10 M Conn tenor saxophone. This technique originates from the traditional Modal Analysis which is commonly adopted to determine the dynamic behaviour of solid structures. The methodology is based on the measurement of Frequency Response Functions (FRFs) as a ratio of the acoustic pressure, measured by means of a set of microphones positioned along the saxophone body, and the volume velocity produced by an acoustic source and measured at the instrument input. The microphones are housed on the stripped saxophone keys by means of tailored three-dimensional printed adapters. The AMA makes use of the FRFs to extract modal parameters, such as resonance frequencies, loss factors, and mode shapes. These parameters pertain to the cavity modes of the saxophone passive resonator. The analysis has been performed for three different notes (B ♭, B, and C) and for two registers of the instrument in order to show the potential of the technique. Moreover, the influence of the mouthpiece volume on the cavity modes has been shown. The information obtained by AMA can find useful application in the validation of analytical or numerical models of this kind of musical instruments.

Recently, acoustic analysis techniques are used in the design of acoustic environments both inside and outside of rooms containing a sound source. However, when the analysis area is expanded or the frequency is increased, unknowns of a solving liner equation increase, and large-scale analyses becomes necessary. Therefore, in this study, a large-scale acoustic analysis method has been developed based on a parallel finite element method. In the proposed large-scale analysis method, an iterative method is applied to the interface problem as the iterative domain decomposition method, which is known to be an effective parallelization method. To improve the convergence of the iterative method, balancing domain decomposition has been applied as a preconditioner step, and its effectiveness in large-scale acoustic analysis is demonstrated. The acoustic analysis code is developed in this study that is shown to be capable of the large-scale analysis of finite element models on the order of tens of millions of elements with high accuracy.

The term sweet spot is used for tennis racket in describing that point or region where the ball should be hit for optimal results. From a vibration standpoint, the sweet spot is determined by the location of the nodal lines across the racket head. In this paper, several methodologies based on modal analysis techniques are explored in order to estimate such a sweet spot for a beach tennis racket. Particular attention was addressed in the detection of the optimal boundary conditions to which the beach tennis racket should be constrained. It is the first time this item is dynamically characterized by experimental techniques. Test results are presented and the sweet spot related to minimal vibration perception is identified for a beach tennis racket. A further experimental study is also carried out in order to identify the Maximum Coefficient of Restitution in beach tennis rackets; this coefficient is related to racket efficiency and characterized by a minimum absorption of the ball incident kinetic energy by the racket. This study could be useful for design, development and practical use of beach tennis rackets.

Despite its importance in structural dynamics and vibration, modal analysis is rarely performed in acoustics due to the high modal density of sound fields. A novel acoustic modal analysis (AMA) approach is proposed in this paper for enclosed acoustic fields, such as a room, from the perspective of state-space formulation of control systems. A single-input-multiple-output (SIMO) state-space model is established in light of the balanced realization (BR), given impulse response measurements. The BR model is then converted to a modal form such that the modal parameters, including natural frequencies, damping ratios, and mode shapes, can be estimated. To reconstruct mode shapes, plane wave decomposition (PWD) and compressive sensing (CS) techniques are exploited to solve the underdetermined problem for a spatially sparse representation of mode shapes under the Schroeder frequency. As a result, a model of a continuous system can be "interpolated" for any arbitrary source-receiver positions on the basis of the estimated mode shapes. With the identified modal parameters, the low-frequency and early reflection part of room impulse responses (RIRs) can be synthesized for arbitrary source-sensor pairs. The proposed AMA acoustic field interpolation is validated by extensive simulations and experiments.

Bulk viscosity is an important factor in the damping properties of fluid systems and exhibits frequency dependent behaviour. A comparison between modal analysis in ANSYS Acoustics, custom code and experimental data is presented in this paper. The measured system consists of closed ended water-filled steel pipes of different lengths. The influence of a pipe wall, flanges on both ends and longitudinal waves in the structural part were included in measurement evaluation. Therefore, the obtained values of sound speed and bulk viscosity are parameters of the fluid. A numerical simulation was carried out only using fluid volume in a range of bulk viscosity. Damping characteristics in this range were compared to measured values. The results show a significant influence of sound speed and subsequently, the use of sound speed value regressed from experimental data yields a better fit between the measurement and the computation.

<div class="htmlview paragraph">Recent advances in automotive sound quality have reduced the general level of noise in the passenger compartment and focused more attention on irritating noises. Traditional methods of prevention and elimination of irritating noises such as squeaks and rattles are no longer sufficient to develop acceptable products in shortened product development cycles. Basic causes, design rules for prevention, and evaluation techniques using integrated digital data acquisition, laboratory simulation, acoustic and modal analysis techniques have been successfully developed to push the prevention process upstream in the development cycle.</div>

: Launch loads, both mechanical and acoustic, are the prime driver of spacecraft structural design. Passive approaches for acoustic attenuation are limited in their low frequency effectiveness by constraints on total fairing mass and payload volume constraints. Active control offers an attractive approach for low frequency acoustic noise attenuation inside the payload fairing. Smart materials such as piezoceramics can be exploited as actuators for structural-acoustic control. In one active approach, structural actuators are attached to the walls of the fairing and measurements from structural sensors and/or acoustic sensors are fed back to the actuators to reduce the transmission of acoustic energy into the inside of the payload fairing. In this paper, structural-acoustic modeling and test results for a full scale composite launch vehicle payload fairing are presented. These analytical and experimental results fall into three categories: structural modal analysis, acoustic modal analysis, and coupled structural-acoustic transmission analysis. The purpose of these analysis and experimental efforts is to provide data and validated models that will be used for active acoustic control of the payload fairing. In the second part of the paper, this closed-loop acoustic transmission reduction is implemented and measured on a full-scale composite payload fairing.

A method of modal acoustic emission monitoring and waveform analysis is developed as a means for tracking two of the primary damage mechanisms in unidirectional composite lamina, matrix-cracking and fiber–matrix debonding. Unidirectional 30, 45 and 90° coupons of a rubber-toughened carbon-epoxy are monitored in this way for various loading histories. A method of comparing waveforms from different samples is proposed in order to establish the similarity of microcracking regardless of fiber direction. Histograms of waveform energy are used as a measure of average arrested crack length versus stress level. Larger faster cracks will emit more energy. Finally, an interpretation of the AE data is given based on an initial population of existing flaws where a cumulative distribution function (CDF) of microcracking is defined and used to study effects of stress history.

Since the noise generated by mechanical structures affects the merchantability, it is important to predict the noise and vibratory characteristics at the first design stage. The frames and panels or trim parts in automobiles are coupled systems of the rigid and flexible substructure. In these systems, vibration of the flexible structures often becomes one of the major noise sources when the excitation force is applied to the rigid structure. FEM and modal analysis technique are not sufficient to carry out the effective low noise design in higher frequency because of high modal density and complexity of mode shape. Modal energy propagation method, proposed in this paper, is based on the mode synthesis method and the vibration power flow between the eigen modes of the rigid and flexible structures. Previous studies have verified that the vibration level can be reduced by extracting modes that contribute highly to the vibration power flow between subsystems and changing their characteristics. In this report, we present the results of applying this method to a coupled system of a body and panel structures in automobiles and verifying its validity. The structural modifications obtained from this analysis method were verified for their effectiveness in decreasing vibration.

<div class="htmlview paragraph">A hybrid-integrated approach is presented to analyze the structure-borne booming noise in a passenger car. We identify the critical noise transfer path from the engine to the target by the transfer path analysis. However, it does not give the answer for why the noise transfer function is so high at that path. Therefore, an integrated approach which applies the analysis tools systematically is presented. The running mode analysis gives us the operating motion of each component in the body structure. However, there is no evidence that the components that vibrate severely are the sources of this problem. The modal characteristics from the structural modal test enable us to describe the real motion of the body completely in terms of the structural modes. Similarly, the acoustic modal characteristics from the acoustic modal analysis describe the fundamental behavior of the cabin cavity. The introduction of the experimental running mode data of the structure to the acoustic finite element model makes the hybrid analysis possible. Through the structural and the acoustic modal analysis, we verify the mode that contributes to the booming noise. The panel contribution analysis points out the components that have the greatest influence on the booming noise. The modification of the body based on the results of the hybrid-integrated approach results in the great decrease of the noise level.</div>