Trevor J. Cox

From a profiled diffuser to an optimized absorber

The quadratic residue diffuser was originally designed for enhanced scattering. Subsequently, however, it has been found that these diffusers can also be designed to produce exceptional absorption. This paper looks into the absorption mechanism of the one-dimensional quadratic residue diffuser. A theory for enhanced absorption is presented. Corresponding experiments have also been done to verify the theory. The usefulness of a resistive layer at the well openings has been verified. A numerical optimization was performed to obtain a better depth sequence. The results clearly show that by arranging the depths of the wells properly in one period, the absorption is considerably better than that of a quadratic residue diffuser.

Prediction and evaluation of the scattering from quadratic residue diffusers

The scattering from quadratic residue diffusers has been predicted using methods based on the Helmholtz–Kirchhoff integral equation. The methods consisted of two boundary integral methods, and two more approximate solutions utilizing Kirchhoff’s boundary conditions. The predictions were compared to measurements made in both the near and far fields. An accurate boundary integral method which works below and above the well cutoff frequency has been found. The limitations of the more approximate methods have been defined. The approximations behind the quadratic residue diffuser’s design have been tested. The important assumption of a phase change local reacting admittance over each well face was found to be good over a wide range of frequencies. The frequency limit of this approximation has also been defined.

Event-related potential study to aversive auditory stimuli.

In an auditory oddball task emotionally negative (aversive) sounds (e.g. rubbing together of polystyrene) and everyday sounds (e.g. ringing of a bicycle bell) were presented as task-irrelevant (novel) sounds. Both the aversive and the everyday sounds elicited the orientation-related P3a component of the event-related potentials (ERPs). In the 154-250 ms range the ERPs for the aversive sounds were more negative than the ERP of the everyday sounds. For the aversive sounds, this negativity was followed by a frontal positive wave (372-456 ms). The aversive sounds elicited larger late positive shift than the everyday sounds. The early negativity is considered as an initial effect in a broad neural network including limbic structures, while the later is related to the cognitive assessment of the stimuli and to memory-related processes.

Diffusor application in rooms

Abstract This paper discusses the application of diffusors in music production rooms, such as performance facilities, and music reproduction rooms, such as recording studios. In the former, diffusors help create an acoustic which enhances the sound heard by both musicians and audience, whereas in the latter diffusors are used to provide a more neutral critical listening environment. Diffusors are primarily intended to control first order reflections, in contrast to the statistical application of absorption to control reverberation. By diffusing early reflections, specularity effects such as coloration, image shifting, harshness and acoustic glare can be mitigated. Diffusors can also be used to help maintain ambiance, promote even scattered energy coverage, improve spaciousness and remove echoes. Some case studies from different room types are presented.

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.

Acoustic band gaps in periodically and quasiperiodically modulated waveguides

We report experimental observation of the formation of phononic band structure in one-dimensional periodically and quasiperiodically (based on the Fibonacci and Thue-Morse number sequences) area modulated waveguide structures. The experimental results are compared to model calculations considering the interference of multiply reflected waves using a transfer matrix method formulation. It was found that both the scattering due to the changes in area (causing an impedance discontinuity) and also the radiation impedance acting at each area discontinuity must be considered to accurately model the experimental results. For the quasiperiodic structures, complicated transmission spectra are seen to result, characterized by wide acoustic pseudo-band-gaps interrupted by narrow defect modes around the center of the gap.

A New Variable Tap-Length LMS Algorithm to Model an Exponential Decay Impulse Response

This letter proposes a new variable tap-length least-mean-square (LMS) algorithm for applications in which the unknown filter impulse response sequence has an exponential decay envelope. The algorithm is designed to minimize the mean-square deviation (MSD) between the optimal and adaptive filter weight vectors at each iteration. Simulation results show the proposed algorithm has a faster convergence rate as compared with the fixed tap-length LMS algorithm and is robust to the initial tap-length choice

Evaluation of methods for predicting the scattering from simple rigid panels

Abstract The accuracies and computational times of theoretical methods used to predict the scattering from rigid plane and curved reflectors have been evaluated. These methods were based on different solutions of the Helmholtz-Kirchhoff integral equation. The prediction results were compared to measurements. It was found that a boundary integral method provided accurate predictions for both panel types. For the more approximate prediction methods their limitations were defined in terms of the accuracy achieved and the range over which the methods were applicable. These methods were also used to investigate the use of a cut-off frequency to describe the limit above which specular type reflections dominate the scattering. It was found to be applicable only close to the geometric scattering angle.

The optimization of profiled diffusers

Methods have been developed to produce profiled diffusers that create a large amount of diffusion. The methods are iterative and required the development of a new parameter to measure diffusion. Achieving scattering independent of angle has been attempted over a wide bandwidth. The methods are also applicable to other diffusion criteria. The diffusers consists of a series of wells of the same width but of different depths similar to Schroeder diffusers. Applications include concert halls, theatres, and studio monitor rooms. The new diffusers have been shown to create better, more uniform diffusion than the previous designs of Schroeder. This is due to the new designs being reliant on accurate boundary element prediction methods rather than more approximate techniques.

Acoustic Absorbers and Diffusers: Theory, Design and Application 2nd Edition

This definitive guide covers the design and application of absorbers and diffusers in acoustics. Surface diffusion is a relatively young subject area, and diffuser design, application and characterisation are often not well understood. Although there is greater knowledge of absorption, it is also informed by new research. As two of the main design tools for altering the acoustic conditions of rooms, the correct use of absorbers and diffusers is important to the creation of quality acoustics. This text details the evolution and the current state of the art in diffuser and absorber research and application. It covers a range of practical and theoretical aspects, with extensive examples of installations and case studies to cater to practitioners working in the measurement, modelling and design of rooms, semi-enclosed spaces as well as in noise control. It is also invaluable for students and researchers wanting a grounding in acoustic treatment, as well as understanding the latest developments. All chapters have been revised and brought up to date in this new edition, with new applications, absorbers and diffusers featured. Sustainability, portable vocal booths, and fast time domain models for diffusers are just a few of the new sections. Improved techniques for measurement and prediction are included, as well as bringing old methods up-to-date with the latest refinements from standards and research. Most of the prediction methods in the book are now linked to open source implementations and downloadable MATLAB scripts, enabling readers to exploit the knowledge in this book more readily in design and research.