WFS and Ambisonic Systems in the Espace de Projection
The Wave Field Synthesis (WFS) technique is a type of holophonic reproduction process that enables, by analogy with visual holograms, to capture or synthesize a sound scene while preserving the direction and depth information of its components. This approach, initiated by the University of Delft, goes well beyond the limits of conventional systems in terms of reproduction fidelity in a wide listening zone. While traditional stereophonic techniques (e.g. stereo, 5.1) are a kind of trompe-l’oeil and can only be truly appreciated when one is positioned at the sweet-spot of the system, the goal of holophonic technology is to reproduce a sound field in which listeners can move freely while maintaining a coherent perception of the localization of the sound sources (parallax effect).
The Ambisonics technique consists of representing the sound field as an angular distribution of sound pressure expressed through its decomposition on the spherical harmonics (an ensemble of periodic functions of the space that form an orthonormal basis). Capture and reproduction are based on concentric transducer arrays composed of microphones (or networks of loudspeakers, respectively) mounted on the surface of a sphere and that sample the space. Encoding and decoding operations are used to switch from the transducers’ domain to the domain of the spherical harmonics. The higher the order of decomposition into spherical harmonics, the better the spatial
resolution. This is called Higher Order Ambisonics (HOA). The Ambisonics formalism is very powerful and allows a certain independence between the reproduction system (number and spatial distribution of the loudspeakers)
and the encoding/decoding format. It also enables efficient spatial morphing operations on 2D or 3D sound scenes, such as rotation, spatial warping and beam forming (directional virtual microphone). IRCAM was able to acquire and install a WFS and HOA system in the Espace de Projection with the support of the .le-de-France R.gion, the CNRS, and Sorbonne Universit.. The system inaugurated in 2012 is made up of a horizontal ring of 264 loudspeakers placed at regular intervals around the stage and the audience for WFS sound diffusion and of a dome of 75 loudspeakers for three-dimensional Ambisonic sound diffusion. This equipment is used to experiment with new methods of spatialization for musical creation and for scientific experiments with virtual reality and spatial cognition. Elements from this system are mobile and can be used outside the Espace de Projection. The system has been used in this way for numerous musical creations, theatrical performances, and sound installations (Operspective Hölderlin by P. Schoeller; Le Père by M. Jarrell; Mimesis by M. Garcia-Vitoria; La Tragédie du Roi Richard II based on W. Shakespeare’s work and directed J.-B. Sastre at the Cour d’honneur du Palais des papes in Avignon; Mon Coeur parle tout seul by D. Ghisi and D. Janneteau; Disenchanted Island by O. Neuwirth and T. Rosner; Pocket of Spaces by N. Barett and The OpenEnded Group)
Diagram of the installation of the sound diffusion system in the Espace de projection // The HOA system installed in the Grande salle at the Centre Pompidou for the performance of Pocket of Spaces by N. Barrett and The OpenEnded Group.
IRCAM's Team : Acoustic and Cognitive Spaces
