Toward better understanding the diffuse sound field

The diffuse sound field is an important idealized field in acoustics for standardized acoustic measurements, simple predictions, and a wide range of theoretical derivations. Surprisingly, there is no general agreement on the definition of the diffuse sound field, its quantification and characterization. Moreover, there is also limited understanding of the required characteristics of the room boundaries that would result in a diffuse sound field in a room. In contrast, the requirements for the presence of a diffuse sound field are imposed on a variety of spaces and facilities, such as music performance venues, critical listening rooms, and spaces with speech intelligibility requirements. Elements of acoustic treatment, diffusers, have historically been used to scatter sound, although their effectiveness in achieving a diffuse sound field is not predictable or measurable. The primary knowledge deficit is the lack of a predictable and measurable quantifier of the sound field diffuseness, i.e., the degree to which the sound field is diffuse. Such a quantifier would allow the evaluation of currently assumed diffuse sound field environments. This is particularly important for reverberation chambers, which are standardized test facilities in acoustics. Absorption coefficients of materials are measured in reverberation chambers and results vary unacceptably when measured in different laboratories. Unreliable absorption coefficients are particularly problematic because they are input parameters for acoustic calculations and simulations. As a result, many acoustic design decisions are based on incorrect predictions, leading to potentially flawed acoustic design. The identified knowledge gap regarding the partial understanding of the diffuse sound field needs to be addressed. This project aims to investigate the topic from several challenging perspectives: 1) Sound wave reflection from the diffuser is modeled using state-of-the-art wave-based and geometric acoustic modeling techniques. Incorporating the complex boundary conditions of a diffuser is challenging because acoustic modeling methods make several questionable assumptions, operate in a limited frequency range, or are simply computationally overdemanding. 2) By augmenting acoustic modeling with optimization routines, prototype diffusers are designed. The diffusers are also built and their influence on the sound field in a room is evaluated experimentally. Such a comprehensive approach provides insight into all relevant steps and validates the practical use of diffusers as elements of the acoustic treatment of rooms. 3) The project develops and builds an innovative automated acoustic measurement device (cable robot). The device, with 60 microphones, will allow the acquisition of acoustic response at a large number of room positions. Such an extensive acoustic scanning of the sound field has not been performed before. The acoustic responses thus acquired will enable the development of advanced diffuseness quantifiers. Overall, the proposed project provides a very holistic approach to the subject and includes theoretical investigations of sound field characterization parameters, numerical as well as experimental tests. In addition, the design, construction, and testing of diffuser prototypes provides important information on the efficiency of different types of diffusers. The goal of the project is to provide at least partial answers to the identified knowledge gaps. A better understanding of the diffuse sound field will lead to the development of the scientific field of acoustics, reduce critical measurement uncertainties, and enable more reliable design in architecture and building acoustics. The InnoRenew CoE has recently acquired a state-of-the-art set of acoustic measurement equipment. Combined with the applied and scientific credentials of Dr. Rok Prislan, the project's lead researcher, all conditions are in place for a successful outcome of the proposed project.