A transient boundary element method model of Schroeder diffuser scattering using well mouth impedance

Room acoustic diffusers can be used to treat critical listening environments to improve sound quality. One popular class is Schroeder diffusers, which comprise wells of varying depth separated by thin fins. This paper concerns a new approach to enable the modelling of these complex surfaces in the time domain. Mostly, diffuser scattering is predicted using steady-state, single frequency methods. A popular approach is to use a frequency domain Boundary Element Method (BEM) model of a box containing the diffuser, where the mouth of each well is replaced by a compliant surface with appropriate surface impedance. The best way of representing compliant surfaces in time domain prediction models, such as the transient BEM is, however, currently unresolved. A representation based on surface impedance yields convolution kernels which involve future sound, so is not compatible with the current generation of time-marching transient BEM solvers. Consequently, this paper proposes the use of a surface reflection kernel for modelling well behaviour and this is tested in a time domain BEM implementation. The new algorithm is verified on two surfaces including a Schroeder diffuser model and accurate results are obtained. It is hoped that this representation may be extended to arbitrary compliant locally reacting materials

Surface diffusion coefficients for room acoustics : free-field measures

A surface diffusion coefficient is needed in room acoustics to enable the quality of diffusing surfaces to be evaluated. It may also facilitate more accurate geometric room acoustic models. This paper concentrates on diffusion coefficients derived from free-field polar responses. An extensive set of two- and three-dimensional measurements and predictions was used to test the worth of different diffusion coefficient definitions. The merits and problems associated with these types of coefficients are discussed, and past parameters reviewed. Two new coefficients are described. The new measure based on the autocorrelation function is forwarded as the best free-field coefficient. The strengths and weaknesses of the coefficient are defined. © 2000 Acoustical Society of America

A transient boundary element method for acoustic scattering from mixed regular and thin rigid bodies

Boundary Element Methods (BEMs) may be used to predict the scattering of sound by obstacles, which has accelerated the prototyping of new room acoustic treatments such as diffusers. Unlike the more popular frequency domain method, the time domain BEM is usually solved in an iterative manner which means it can exhibit instability, a crucial impediment to its widespread use. These instabilities are primarily associated with the resonance of cavities formed by closed surface sections, but may also be caused by discretisation or integration error corrupting physical damped resonances. Regular BEM implementations cannot model objects with thin sections due to a phenomenon known as Thin Shape Breakdown. This paper develops an algorithm which combines an accepted approach for modelling thin plates with the Combined Field Integral Equation which eradicates cavity resonances, thereby permitting models of mixed regular and thin bodies. Accuracy and stability are tested by comparison to verified frequency domain BEMs, examination of the transient response, and pole decomposition. This is done for a simple obstacle and a Schroeder diffuser, which comprises a series of wells separated by thin fins. The approach is successful but universal stability cannot be guaranteed for the diffuser. It is suggested that instability is caused by the lightly damped resonances of the wells being corrupted into divergent behaviour by numerical errors

Intermediate Outcomes, Strategies, and Challenges of Eight Healthy Start Projects

Site visits were conducted for the evaluation of the national Healthy Start program to gain an understanding of how projects design and implement five service components (outreach, case management, health education, depression screening and interconceptional care) and four system components (consortium, coordination/collaboration, local health system action plan and sustainability) as well as program staff’s perceptions of these components’ influence on intermediate outcomes. Interviews with project directors, case managers, local evaluators, clinicians, consortium members, outreach/lay workers and other stakeholders were conducted during 3-day in-depth site visits with eight Healthy Start grantees. Grantees reported that both services and systems components were related to self-reported service achievements (e.g. earlier entry into prenatal care) and systems achievements (e.g. consumer involvement). Outreach, case management, and health education were perceived as the service components that contributed most to their achievements while consortia was perceived as the most influential systems component in reaching their goals. Furthermore, cultural competence and community voice were overarching project components that addressed racial/ethnic disparities. Finally, there was great variability across sites regarding the challenges they faced, with poor service availability and limited funding the two most frequently reported. Service provision and systems development are both critical for successful Healthy Start projects to achieve intermediate program outcomes. Unique contextual and community issues influence Healthy Start project design, implementation and reported accomplishments. All eight projects implement the required program components yet outreach, case management, and health education are cited most frequently for contributing to their perceived achievements