Markus Noisternig

3D binaural sound reproduction using a virtual ambisonic approach

Convincing binaural sound reproduction via headphones requires filtering the virtual sound source signals with head related transfer functions (HRTFs). Furthermore, humans are able to improve their localization capabilities by small unconscious head movements. Therefore it is important to incorporate head-tracking. This yields the problem of high-quality, time-varying interpolation between different HRTFs. A further improvement of human localization accuracy can be done by considering room simulation yielding a huge amount of virtual sound sources. To increase the computational efficiency of the proposed system, a virtual ambisonic approach is used, that result in a bank of time-invariant HRTF filter independent of the number of sources to encode.

A comparative study of interaural time delay estimation methods

The Interaural Time Delay (ITD) is an important binaural cue for sound source localization. Calculations of ITD values are obtained either from measured time domain Head-Related Impulse Responses (HRIRs) or from their frequency transform Head-Related Transfer Functions (HRTFs). Numerous methods exist in current literature, based on a variety of definitions and assumptions of the nature of the ITD as an acoustic cue. This work presents a thorough comparative study of the degree of variability between some of the most common methods for calculating the ITD from measured data. Thirty-two different calculations or variations are compared for positions on the horizontal plane for the HRTF measured on both a KEMAR mannequin and a rigid sphere. Specifically, the spatial variations of the methods are investigated. Included is a discussion of the primary potential causes of these differences, such as the existence of multiple peaks in the HRIR of the contra-lateral ear for azimuths near the inter-aural axis due to multipath propagation and head/pinnae shadowing.

From ear to hand: the role of the auditory-motor loop in pointing to an auditory source.

Studies of the nature of the neural mechanisms involved in goal-directed movements tend to concentrate on the role of vision. We present here an attempt to address the mechanisms whereby an auditory input is transformed into a motor command. The spatial and temporal organization of hand movements were studied in normal human subjects as they pointed toward unseen auditory targets located in a horizontal plane in front of them. Positions and movements of the hand were measured by a six infrared camera tracking system. In one condition, we assessed the role of auditory information about target position in correcting the trajectory of the hand. To accomplish this, the duration of the target presentation was varied. In another condition, subjects received continuous auditory feedback of their hand movement while pointing to the auditory targets. Online auditory control of the direction of pointing movements was assessed by evaluating how subjects reacted to shifts in heard hand position. Localization errors were exacerbated by short duration of target presentation but not modified by auditory feedback of hand position. Long duration of target presentation gave rise to a higher level of accuracy and was accompanied by early automatic head orienting movements consistently related to target direction. These results highlight the efficiency of auditory feedback processing in online motor control and suggest that the auditory system takes advantages of dynamic changes of the acoustic cues due to changes in head orientation in order to process online motor control. How to design an informative acoustic feedback needs to be carefully studied to demonstrate that auditory feedback of the hand could assist the monitoring of movements directed at objects in auditory space.

Holophonic Sound in IRCAM's Concert Hall: Technological and Aesthetic Practices

This article presents a report on technological and aesthetic practices in the variable-acoustics performance hall, Espace de Projection, at the Institut de Recherche et Coordination Acoustique/Musique. The hall is surrounded by a 350-loudspeaker array for sound-field reproduction using holophonic approaches such as wave-field synthesis and higher-order Ambisonics. First we present the design and implementation of the audio system and discuss the challenges of both hardware and software architectures. This is followed by a discussion of spatial composition techniques, aesthetic approaches, and methodologies for composing computer music for high-density loudspeaker arrays, explored through the paradigmatic examples of pieces produced by two artist-in-research residencies.

Design of Spatial Microphone Arrays for Sound Field Interpolation

This paper presents a design method for microphone arrays with arbitrary geometries. Based on a theoretical analysis and on the magic points method, it allows for the interpolation of a sound field in a generic convex domain with a limited number of microphones on a given frequency band. It is shown that only a few microphones are needed in the interior of the considered domain to ensure a low interpolation error in the frequency band of interest, and that most of the microphones have to be located on the boundary of the domain, with a non-uniform density depending on the shape of the domain. Practical design constraints can be included in the optimization process. Comparisons for some particular array geometries with design methods known from the literature are given, showing that the proposed approach results in lower errors.

The Glass Organ: Musical Instrument Augmentation for Enhanced Transparency

The Organ and Augmented Reality (ORA) project has been presented to public audiences at two immersive concerts, with both visual and audio augmentations of an historic church organ. On the visual side, the organ pipes displayed a spectral analysis of the music using visuals inspired by LED-bar VU-meters. On the audio side, the audience was immersed in a periphonic sound field, acoustically placing listeners inside the instrument. The architecture of the graphical side of the installation is made of acoustic analysis and calibration, mapping from sound levels to animation, visual calibration, real-time multi-layer graphical composition and animation. It opens new perspectives to musical instrument augmentation where the purpose is to make the instrument more legible while offering the audience enhanced artistic content.

Design of a robust open spherical microphone array

This paper presents a method for designing a robust open spherical microphone array that overcomes the typical problems of open sphere geometries at frequencies related to the zeros of the spherical Bessel functions. The proposed array structure uses only a few additional sampling points inside the spherical volume whose optimal positions are determined by the eigenmodes of the sphere for a given wave number interval. This novel approach minimizes the interpolation error inside the sphere. With illustrate this approach with the design of a 10-th order array using 130 microphones and discuss the simulation results with regard to commonly used error measures (white noise gain, condition number, and interpolation error), and show that the proposed array design compares favorably to previously suggested array designs.

Organ Augmented Reality: Audio-Graphical Augmentation of a Classical Instrument

This paper discusses the Organ Augmented Reality ORA project, which considers an audio and visual augmentation of an historical church organ to enhance the understanding and perception of the instrument through intuitive and familiar mappings and outputs. ORA has been presented to public audiences at two immersive concerts. The visual part of the installation was based on a spectral analysis of the music. The visuals were projections of LED-bar VU-meters on the organ pipes. The audio part was an immersive periphonic sound field, created from the live capture of the organ sounds, so that the listeners had the impression of being inside the augmented instrument. The graphical architecture of the installation is based on acoustic analysis, mapping from sound levels to synchronous graphics through visual calibration, real-time multi-layer graphical composition and animation. The ORA project is a new approach to musical instrument augmentation that combines enhanced instrument legibility and enhanced artistic content.

EVERTims: Open source framework for real-time auralization in VR

Our demonstration presents recent developments of the EVERTims project, an auralization framework for virtual acoustics and real-time room acoustic simulation. The developments presented here concern the complete re-design of the scene graph editor unit, and the C++ implementation of a new spatial renderer based on the JUCE framework. EVERTims now functions as a Blender add-on to support real-time auralization of any 3D room model, both for its creation in Blender and its exploration in the Blender Game Engine. The EVERTims framework is published as open source software.

Design framework for spherical microphone and loudspeaker arrays in a multiple-input multiple-output system

Spherical microphone arrays (SMAs) and spherical loudspeaker arrays (SLAs) facilitate the study of room acoustics due to the three-dimensional analysis they provide. More recently, systems that combine both arrays, referred to as multiple-input multiple-output (MIMO) systems, have been proposed due to the added spatial diversity they facilitate. The literature provides frameworks for designing SMAs and SLAs separately, including error analysis from which the operating frequency range (OFR) of an array is defined. However, such a framework does not exist for the joint design of a SMA and a SLA that comprise a MIMO system. This paper develops a design framework for MIMO systems based on a model that addresses errors and highlights the importance of a matched design. Expanding on a free-field assumption, errors are incorporated separately for each array and error bounds are defined, facilitating error analysis for the system. The dependency of the error bounds on the SLA and SMA parameters is studied and it is recommended that parameters should be chosen to assure matched OFRs of the arrays in MIMO system design. A design example is provided, demonstrating the superiority of a matched system over an unmatched system in the synthesis of directional room impulse responses.