Immersive auditory display system 'sound cask': three-dimensional sound field reproduction system based on the boundary surface control principle

Sound cask was developed to realize the perfect 3D auditory display that creates 3D sound waves around the listener's head just the same as the primary sound field, based on the boundary surface control (BoSC) principle.

If we consider the sound pressure p(s) within a region V enclosed by a surface S, Kirchhoff-Helmholtz integral equation is given by

[EQUATION]

where n denote normal vectors on S. In the conventional theory of sound field reproduction, the Greenfs function G and its gradient ∂G/∂n are interpreted as monopole sound sources and dipole sound sources, respectively. Developing such sources is technically impossible. Perfect sound field reproduction is thus thought to be a hopeless idea [2]. However, the BoSC principle [4] presented another interpretation of the Kirchhoff-Helmholtz integral equation where p and ∂p/∂n are the acoustic pressure and particle velocity, and G and its gradient ∂G/∂n are those coefficients, respectively. Thus, designing the inverse system of the acoustic characteristics in the reproduction room reveals that perfect sound field reproduction can be realized without such acoustic problems. The basic concept of sound field reproduction based on the BoSC principle is shown in Fig. 1. To record sound in the primary field, the C80-shaped fullerene microphone array (BoSC microphone) was developed as shown in Fig. 2. The BoSC microphone is also used to measure the impulse responses between all possible combinations of loudspeakers and microphones in the secondary sound field (IRs in Fig. 1) to calculate the inverse filter matrix [5].

Compared with other sound reproduction method such as wave field synthesis (WFS) systems [1] or ambisonics systems [3], the sound cask has practical advantages in the following aspects:

(1) Sound image along the depth direction can be controlled even in the vicinity of the head of the listener.

(2) The whole system can be easily moved into any place.

(3) A theoretically assured combination with a recording system - an 80-channel fullerene-shaped microphone array in our case - can be constructed.

The main characteristic of the BoSC system is its ability to reproduce a sound field, not by points but in three dimensions. A listener can freely move his head, and the system can provide high performance in terms of spatial information reproduction such as sound localization and sound distance. Based on these system features, as an example of a more effective application of the BoSC system, we propose the design of a sound cask. In the design process of the sound field reproduction system based on the BoSC principle, space design, which is suitable for inverse filter calculation, will be important, since the quality of these filters directly affect the total performance of the system.

As a practical and reasonable compromise of the conditions, 96 loudspeakers are allocated inside the sound cask. Figure shows a picture of the sound cask designed in practice. In particular, a higher-grade loudspeaker unit (FOSTEX FX120) was adopted in the current version of the sound cask after several listening tests.