Acoustic Scattering for Spatial Audio Applications

Defence is held on 24.5.2022 12:00 – 16:00 Zoom, https://aalto.zoom.us/j/62282474989Modeling of sound propagation on the context of acoustic design and interactive applications have mainly focused on room acoustics as well as source and receiver modeling. In order to enrich the description and perceptual immersion of virtual sound-fields, modeling frameworks can also include the effects of scattering of bodies within the physical space. One of the main challenges in modeling the effects of scattering, is that its behaviour not only depends on the geometry of the scatterer but also the direction of arrival of the incident field. This thesis is a collection of five publications, the first two studies focus on the effects of near-field sources, and the last three studies involve the effects of scattering within spatial audio applications. The first publication explores the effects of near-field sources on High-order Ambisonics recording, processing and binaural reproduction. Results indicate that while near-field sources introduce low-frequency proximity gains in high-order microphones arrays, the regularization stages in Ambisonics recording prevents excessive gains. The second publication explores the directivity of near-field speech of 24 subjects and evaluates various repeatable speech reproduction alternatives. The third publication presents a scheme for encoding the acoustic scattering of arbitrary geometries into the spherical harmonic domain. After encoding, the scattering is represented as a multiple-input multiple-output matrix which describes the relation between the incoming and outgoing scattering modes of a geometry. This method allows for the standard transformations in the spherical harmonic domain (rotation, translation, scaling) and it is compatible with existing spatial audio frameworks such as Ambisonics and image-source methods. This method is validated using boundary element method simulations and indicates minimal synthesis error. The fourth publication presents a method to encode the space domain signals from a microphone array with arbitrary geometry and irregularly distributed sensors into Ambisonics. The algorithm relies on the array response and its enclosure's scattering properties to solve the direction of various active sources as well as the diffuse properties of the sound-field. Objective and subjective evaluations indicate that the proposed method outperforms traditional linear encoding. The fifth publication extends the method presented in the third publication by allowing sector-based encoding of acoustic scattering, optimal for geometries and surfaces which do not require entire spherical radiation. This last publication also presents a method to compress the data of the scattering matrix, allowing for more efficient memory storage. Methods proposed in the third and fifth publications can be used to introduce scattering geometries into interactive sound environments to produce more descriptive sound-fields while the fourth publication can be used to develop Ambisonic recording arrays on practical devices such as wearables and head-mounted displays