Archontis Politis

Direction of arrival estimation of reflections from room impulse responses using a spherical microphone array

This paper studies the direction of arrival estimation of reflections in short time windows of room impulse responses measured with a spherical microphone array. Spectral-based methods, such as multiple signal classification (MUSIC) and beamforming, are commonly used in the analysis of spatial room impulse responses. However, the room acoustic reflections are highly correlated or even coherent in a single analysis window and this imposes limitations on the use of spectral-based methods. Here, we apply maximum likelihood (ML) methods, which are suitable for direction of arrival estimation of coherent reflections. These methods have been earlier developed in the linear space domain and here we present the ML methods in the context of spherical microphone array processing and room impulse responses. Experiments are conducted with simulated and real data using the em32 Eigenmike. The results show that direction estimation with ML methods is more robust against noise and less biased than MUSIC or beamforming.

Sector-Based Parametric Sound Field Reproduction in the Spherical Harmonic Domain

This papers presents a parametric method for perceptual sound field recording and reproduction from a small-sized microphone array to arbitrary loudspeaker layouts. The applied parametric model has been found to be effective and well-correlated with perceptual attributes in the context of directional audio coding, and here it is generalized and extended to higher orders of spherical harmonic signals. Higher order recordings are used for estimation of the model parameters inside angular sectors that provide increased separation between simultaneous sources and reverberation. The perceptual synthesis according to the combined properties of these sector parameters is achieved with an adaptive least-squares mixing technique. Furthermore, considerations regarding practical microphone arrays are presented and a frequency-dependent scheme is proposed. A realization of the system is described for an existing spherical microphone array and for a target loudspeaker setup similar to NHK 22.2. It is demonstrated through listening tests that, compared to a reference scene, the perceived difference is greatly reduced with the proposed higher order analysis model. The results further indicate that, on the same task, the method outperforms linear reproduction with the same recordings available.

A directional diffuse reverberation model for excavated tunnels in rock

Acoustic impulse responses of an excavated tunnel were measured. Analysis of the impulse responses shows that they are very diffuse from the start. A reverberator suitable for reproducing this type of response is proposed. The input signal is first comb-filtered and then convolved with a sparse noise sequence of the same length as the filters delay line. An IIR loop filter inside the comb filter determines the decay rate of the response and is derived from the Yule-Walker approximation of the measured frequency-dependent reverberation time. The particular sparse noise sequence proposed in this work combines three velvet noise sequences, two of which have time-varying weights. To simulate the directional soundfield in a tunnel, the use of multiple such reverberators, each associated with a virtual source distributed evenly around the listener, is suggested. The proposed tunnel acoustics simulation can be employed in gaming, in film sound, or in working machine simulators.

Controlling the perceived distance of an auditory object by manipulation of loudspeaker directivity

This work presents a method to control the perceived distance of an auditory object by changing the directivity pattern of a loudspeaker and consequently the direct-to-reverberant ratio at the listening spot. Control of the directivity pattern is achieved by beamforming using a compact multi-driver loudspeaker unit. A small-sized cubic array consisting of six drivers is assembled, and per driver beamforming filters are derived from directional measurements of the array. The proposed method is evaluated using formal listening tests. The results show that the perceived distance can be controlled effectively by directivity pattern modification.

Applications of 3D spherical transforms to personalization of head-related transfer functions

Head-related transfer functions (HRTFs) depend on the shape of the human head and ears, motivating HRTF personalization methods that detect and exploit morphological similarities between subjects in an HRTF database and a new user. Prior work determined similarity from sets of morphological parameters. Here we propose a non-parametric morphological similarity based on a harmonic expansion of head scans. Two 3D spherical transforms are explored for this task, and an appropriate shape similarity metric is defined. A case study focusing on personalisation of interaural time differences (ITDs) is conducted by applying this similarity metric on a database of 3D head scans.

Parametric Time-Frequency Domain Spatial Audio

This book provides readers with the principles and best practices in spatial audio signal processing. It describes how sound fields and their perceptual attributes are captured and analyzed within the time-frequency domain, how essential representation parameters are coded, and how such signals are efficiently reproduced for practical applications. The book is split into four parts starting with an overview of the fundamentals. It then goes on to explain the reproduction of spatial sound before offering an examination of signaldependent spatial filtering. The book finishes with coverage of both current and future applications and the direction that spatial audio research is heading in.

Direction-of-arrival and diffuseness estimation above spatial aliasing for symmetrical directional microphone arrays

A method for direction-of-arrival (DOA) and diffuseness estimation is presented, which proves to be effective above the spatial aliasing frequency of the microphone array in use. The method assumes symmetrical circular or spherical arrays of directional microphones or microphone mounted on a rigid baffle, and it exploits the inherent directionality of the array at high frequencies. The DOA and diffuseness estimators are shown to exhibit low estimation error above the spatial aliasing limit, compared to commonly used intensity-based estimators. A low-error broadband scheme that combines the intensity-based method and the new one is proposed for the ranges below and above aliasing respectively.

Spatial audio coding with spaced microphone arrays for music recording and reproduction.

Spaced microphone arrays are commonly used in multichannel recording of music, due to their inherent quality of natural incoherence between the surround channels at reproduction, at the expense of accurate localization. Recent methods in parametric spatial audio coding, such as Directional Audio Coding, exploit coincident microphone patterns to extract directional information and reproduce it in a perceptually optimal way. In this study, we present how Directional Audio Coding can be adapted for spaced arrays, offering improved localization cues at reproduction, without compromising the qualities of the spaced microphone recording techniques. Examples are presented for some well-established array configurations.