Browsing by Author "Lokki, Tapio, Prof., Aalto University, Department of Signal Processing and Acoustics, Finland"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Acoustic Scattering for Spatial Audio Applications
    (Aalto University, 2022) Gonzalez, Raimundo; Politis, Archontis, Dr., Tampere University of Technology, Finland; Tietotekniikan laitos; Department of Computer Science; Perustieteiden korkeakoulu; School of Science; Lokki, Tapio, Prof., Aalto University, Department of Signal Processing and Acoustics, Finland
    Modeling 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.
  • Thumbnail Image
    Item
    Numerical and perceptual evaluations of finite-difference time-domain simulations for room acoustics applications
    (Aalto University, 2022) Meyer, Julie; Ahrens, Jens, Prof., Chalmers University, Sweden; Tietotekniikan laitos; Department of Computer Science; Perustieteiden korkeakoulu; School of Science; Lokki, Tapio, Prof., Aalto University, Department of Signal Processing and Acoustics, Finland
    The finite-difference time-domain (FDTD) method has been and continues to be widely used as a computational room acoustic modeling technique. In that context, the method aims to numerically solve the wave equation in enclosed spaces. One impediment to the efficiency of the method is its large computational cost due to the volumetric discretization of the simulation domain. Although its performance can significantly be improved by using graphics processing units (GPUs), the accuracy of the FDTD-computed solutions remains limited by the numerical errors that the method entails. This dissertation focuses on the analysis of the discretization error which not only limits the numerical accuracy of the simulation results, but also the perceptual accuracy by giving raise to audible artefacts in the auralizations. Also considering some of the key elements which constitute an immersive audio application, the analysis is carried out in the context of binaural synthesis and head-related transfer function (HRTF) prediction. Publications I and II explore to what extent the numerical accuracy of the FDTD solutions is limited by the discretization error in the prediction of HRTFs for the simple case of a single sphere model and for the more complex case of a two-sphere model. The results, similar for both models, show that more accurate HRTF predictions can be obtained by using a series of grids instead of running a single simulation over a small grid. The perceptual accuracy of the FDTD computations is explored in Publications IV and V in the context of binaural auralizations. More specifically, Publication IV investigates spherical receiver arrays minimizing the audible artefacts induced by the discretization error. The results show that increasing the receiver density for a fixed array size, which increases the robustness of the array, does not necessarily render the error inaudible in the auralizations. In Publication V, perceptual detection thresholds for the numerical error are measured in binaural auralizations of two acoustically different rooms. The results show that the perceptual detection threshold is generally lower for the most reverberant room and greatly depends on the source signal. It is the most noticeable with an impulse as source signal, and almost unnoticeable with speech. The capability of an FDTD solver in predicting complex room acoustic scenarios is also evaluated in Publication III, by comparing FDTD-simulated results with both measurement data and the results from another well-established wave-based method. While the two numerical methods are in agreement, large deviations between the measurement data and the simulated results indicate that typical material data-sets poorly represent the behaviour of real materials in a room.