Pj Shorter

The wave transmission coefficients and coupling loss factors of point connected structures

This analysis is concerned with the calculation of the elastic wave transmission coefficients and coupling loss factors between an arbitrary number of structural components that are coupled at a point. A general approach to the problem is presented and it is demonstrated that the resulting coupling loss factors satisfy reciprocity. A key aspect of the method is the consideration of cylindrical waves in two-dimensional components, and this builds upon recent results regarding the energetics of diffuse wavefields when expressed in cylindrical coordinates. Specific details of the method are given for beam and thin plate components, and a number of examples are presented.

On the use of the Sturm sequence to evaluate modal density

The modal density of a structural‐acoustic subsystem is usually obtained analytically by considering the dispersion of various propagating wave types. Closed form expressions are available for the modal densities of simple beams, plates and shells (with curvature in one or two directions). However, one is often interested in subsystems with complex geometry which may possess inhomogeneous material and physical properties. The classical asymptotic formulations are not appropriate for such subsystems and numerical methods are often adopted. The most straightforward approach is to perform a finite‐element‐based modal analysis and count the number of eigenvalues that fall within various frequency bands. However, the computational expense associated with solving the full eigenproblem is often prohibitive. Significant computational savings can be made by employing the Sturm sequence property to evaluate the modal density. This paper describes the approach in more detail and provides a numerical example.

Radiation efficiency, impedance, and the acoustics of rattle

Rattle issues consistently rank as one of the top consumer complaints in initial quality surveys for many new products. Predicting the acoustics of rattle is complicated by the need to model the vibro-acoustic response of large complex structures across a broad frequency range. The complexity of the analysis can be reduced by making use of standard methods derived almost 50 years ago. In particular, this paper discusses a computationally efficient method for assessing the propensity for rattle in large complex structures. A finite element model is used to predict the probability that impacts will occur when a product is exposed to a particular low frequency random vibro-acoustic environment. The expected contact forces arising from each impact are then estimated by making use of expressions involving the drive point impedances of infinite structures. Finally, the vibration and acoustic radiation associated with the various impacts are predicted and ranked using a SEA model. A number of examples are presen...

Vibro-acoustic analysis of large space structures using the Hybrid FE-SEA method

A number of advances have been made recently in the development of a Hybrid method for rigorously coupling deterministic and statistical descriptions of the dynamics of a vibro-acoustic system. The method provides an efficient way to describe the response of a complex vibro-acoustic system across a broad frequency range. This paper provides an overview of various numerical and experimental validation studies that have been performed using the method.

Hybrid deterministic‐statistical modeling of stiffened plate structures

Statistical energy analysis (SEA) is based on equations of power balance between resonant subsystems, and, given the excitation power input to the system, the resulting average vibration level in each subsystem may be calculated. However, due to the low modal density of beams, SEA cannot fully account for energy flow along stiffening members of typical stringer‐skin constructions used in the aerospace and marine industries. Thus, in a conventional SEA model of a built‐up structure to which excitation is applied through a stiffening member, it is not clear how to compute the power input to each resonant subsystem and the response of the nonresonant stiffening framework. The problem is addressed by using a coupled deterministic‐statistical approach: the stiffeners are modeled deterministically, while the reverberant two‐dimensional structural components are modeled as power absorbing systems, in which the power is carried by statistical cylindrical waves propagating towards an energy sink. This power is the...

Experimental validation of RESOUND mid‐frequency acoustic radiation models

RESOUND is a full‐spectrum structural acoustic analysis method that uses finite element analysis in the low frequency region, statistical energy analysis in the high frequency region and a hybrid approach in the mid frequency region. In this paper, acoustic radiation from a frame‐stiffened panel into a large acoustic space will be investigated. Over the frequency range of interest, the frame has relatively few modes and exhibits long wavelength global behavior, while the panel has a large number of modes and exhibits short wavelength local behavior. Numerical and experimental results will be presented which illustrate how the frame and panel interact to give rise to the radiated sound field.A system for generating loudspeaker-ready binaural signals comprises a tracking system for detecting the position and, preferably, the angle of rotation of a listeners head; and means, responsive to the head-tracking means, for generating the binaural signal. The system may also include a crosstalk canceller responsive to the tracking system, and which adds to the binaural signal a crosstalk cancellation signal based on the position (and/or the rotation angle) of the listeners head. The invention may also address the high-frequency components not generally affected by the crosstalk canceller by considering these frequencies in terms of power (rather than phase). By implementing the compensation in terms of power levels rather than phase adjustments, the invention avoids the shortcomings heretofore encountered in attempting to cancel high-frequency crosstalk.

Combining statistical energy analysis and finite element analysis in RESOUND mid frequency vibroacoustic analysis

At low frequencies, vibroacoustic systems exhibit a dynamic response characterized by spatially correlated motion with low modal density. These systems are typically modeled with deterministic methods. While at high frequencies, the dynamic response is characterized by weak spatial correlation and a large number of modes with high modal overlap. These systems are typically modeled with statistical methods. However many vibroacoustic systems have some regions with high modal density and some regions with low modal density. Such systems require a midfrequency solution technique. One such method has been developed based on a hybrid approach combining finite element analysis (FE) in the low modal density regions and statistical energy analysis (SEA) in the high modal density regions. This method is called RESOUND [Langley and Bremner, J. Acoust. Soc. Am. 105, 1657–1671 (1999)]. Recent developments of RESOUND have focused on predicting the appropriate dynamic interactions and mechanisms for energy flow between...

Including acoustics in the RESOUND hybrid approach to the mid‐frequency problem

The prediction of the response of a complex structural acoustic system across a broad frequency range presents a number of challenges to an analyst. It is quite common to find that (i) the uncertainty associated with the local modal properties of a subsystem, (ii) the wave number content of the local modes, and (iii) the local modal density, can all vary significantly across the various subsystems of a system. This mismatch in the local statistical and dynamic properties of a system is often referred to as the mid‐frequency problem. One approach to the mid‐frequency problem is to construct a hybrid model that combines deterministic and statistical representations of the system dynamics. Previously, this approach has been applied to in‐vacuo structural problems. In this presentation we discuss the extension of the hybrid formulation to encompass enclosed acoustic cavities. The various mechanisms by which energy flows between the deterministic and statistical parts of the structural–acoustic model are discu...

On the spatial coupling between deterministic and fuzzy subsystems in the hybrid approach to the midfrequency problem

The analysis of the dynamic behavior of a structural–acoustic system across a broad frequency range presents a number of challenges to an analyst. For a typical structural–acoustic system it is quite common to find that the modal density varies significantly between the various subsystems, across the frequency range of interest. The vast number of modes in the system as a whole can render a detailed deterministic analysis impractical, while the low modal density of certain subsystems is problematic for statistical energy analysis. One approach to the problem is to construct a hybrid model that combines deterministic and statistical descriptions of the system dynamics. One of the key requirements of the hybrid approach is an accurate estimate of the spatial coupling that occurs between the deterministic and statistical (or fuzzy) parts of the model. This paper discusses recent work which has investigated both a spatial correlation approach to calculating this coupling and also an asymptotic modal approach. The relative merits of the different approaches are discussed and a number of numerical examples are presented.