Takuma Okamoto

Experimental validation of spatial Fourier transform-based multiple sound zone generation with a linear loudspeaker array

Generating acoustically bright and dark zones using loudspeakers is gaining attention as one of the most important acoustic communication techniques for such uses as personal sound systems and multilingual guide services. Although most conventional methods are based on numerical solutions, an analytical approach based on the spatial Fourier transform with a linear loudspeaker array has been proposed, and its effectiveness has been compared with conventional acoustic energy difference maximization and presented by computer simulations. To describe the effectiveness of the proposal in actual environments, this paper investigates the experimental validation of the proposed approach with rectangular and Hann windows and compared it with three conventional methods: simple delay-and-sum beamforming, contrast maximization, and least squares-based pressure matching using an actually implemented linear array of 64 loudspeakers in an anechoic chamber. The results of both the computer simulations and the actual experiments show that the proposed approach with a Hann window more accurately controlled the bright and dark zones than the conventional methods.

Generation of multiple sound zones by spatial filtering in wavenumber domain using a linear array of loudspeakers

Novel signal processing is proposed for generating acoustically bright and dark zones at arbitrary horizontal positions using a linear array of loudspeakers. Most conventional methods are based on the numerical calculation of the inverse of the spatial correlation matrix between control points and the positions of the loudspeakers. However, such methods are quite unstable because the acoustic inverse problem is very ill-conditioned. On the other hand, in the proposed method, spatial filters in the wavenumber domain are analytically derived by introducing the spectral division method and modeling sound pressures at the control line as a rectangular window corresponding to bright and dark zones. Computer simulation results show that the proposed method can generate acoustically bright and dark zones more accurately than the conventional acoustic energy difference maximization method.

Implementation of a high-definition 3D audio-visual display based on higher-order ambisonics using a 157-loudspeaker array combined with a 3D projection display

We have implemented a high-definition 3D audiovisual reproduction system based on higher-order ambisonics (HOA) using a surrounding-157-loudspeaker array combined with a 3D projection display to reproduce information with high sense-of-presence. In this report, we introduce the system overview as well as an investigation to realize good system synchronization, which we are keenly devoted to. Results of the investigation show that all 157 channels of loudspeakers were completely synchronized within the one-sample level (48 kHz) controlled using a single PC with three Multichannel Audio Digital Interface (MADI) systems. Moreover, the latency among 157 audio signals and the video signal was only about 1.1 ms.

Analytical methods of generating multiple sound zones for open and baffled circular loudspeaker arrays

This paper proposes analytical methods for generating acoustically bright and dark zones on the horizontal plane using circular loudspeaker arrays without and with a cylindrical rigid baffle. Both sound fields on the horizontal plane produced by continuous circular monopole source distributions without and with a rigid baffle are analytically calculated using 2.5D sound field representation derived from the 3D cylindrical harmonics expansion. Efficient spatial filters in the spatial Fourier series domain are analytically derived by modeling sound pressures at the control circle as a rectangular window that corresponds to bright and dark zones. The computer simulation results show that the proposed methods can generate bright and dark zones near the loudspeakers more accurately than conventional least squares and beamforming methods.

Effects of microphone arrangements on the accuracy of a spherical microphone array (SENZI) in acquiring high-definition 3D sound space information

We propose a three-dimensional sound space sensing system using a microphone array on a solid, human-head-sized sphere with numerous microphones, which is called SENZI (Symmetrical object with ENchased ZIllion microphones). It can acquire 3D sound space information accurately for recording and/or transmission to a distant place. Moreover, once recorded, the accurate information might be reproduced accurately for any listener at any time. This study investigated the effects of microphone arrangement and the number of controlled directions on the accuracy of the sound space information acquired by SENZI. Results of a computer simulation indicated that the microphones should be arranged at an interval that is equal to or narrower than 5.7 to avoid the effect of spatial aliasing and that the number of controlled directions should be set densely at intervals of less than 5 when the microphone array radius is 85 mm.

2.5D higher order ambisonics for a sound field described by angular spectrum coefficients

This paper derives an analytical solution to convert sound field representation from the angular spectrum to the circular harmonics expansion. A sound field is decomposed to plane waves by the spatial Fourier transform and represented by the angular spectrum. A plane wave is also described by the circular harmonics expansion. In the proposed formulation, these two representations are integrated and a sound field can be represented by the circular harmonics expansion with the angular spectrum coefficients. For actual implementations, the driving function of a circular sound source for 2.5D higher order Ambisonics is analytically derived from a sound field described by the angular spectrum. The results of the computer simulations show that the proposed method with a circular loudspeaker array can reproduce a sound field recorded by a linear microphone array with appropriate accuracy around the center of the circular array.

Analytical approach to 2.5D sound field control using a circular double-layer array of fixed-directivity loudspeakers

This paper provides a regularization-free analytical approach to 2.5D interior and exterior sound field control using a circular double-layer array of fixed-directivity loudspeakers not only to provide a desired sound field inside the array but also to reduce the sound energy outside of it in the horizontal plane. The proposed method analytically derives the driving functions of the inner and outer circular arrays based on 2.5D spherical harmonic expansion to simultaneously control both sound fields inside and outside the array by a modematching framework. The results of computer simulations show that the proposed method is certainly effective compared with the conventional least squares approach that requires regularization schemes in terms of synthesis accuracy inside the listening zone over a wideband frequency range and the acoustic contrast between the quiet zone and the synthesis center at low frequencies.

Near-field sound propagation based on a circular and linear array combination

This paper proposes a new method for realizing 3D near-field sound propagation. Its main concept is that the sound pressure radiated from a circular sound source outside the circle is completely canceled out using another linear sound source. The appropriate driving functions and the reproduced sound pressures of both sound sources are analytically derived based on the two-dimensional spatial Fourier transform. The radiation property reproduced by a circular source outside the circle is different from that inside the circle. As a result, three-dimensional near-field sound propagation can be realized within the radius of a circular source because the total radiated sound pressure outside the circle can only be completely canceled out. Compared with previous methods, the propagation distance can be controlled by changing the circles radius. The results of computer simulations suggest that the proposed method can realize effective three-dimensional near-field sound propagation.

Evaluation of a high-order Ambisonics decoder for irregular loudspeaker arrays through reproduced field measurements

High-order Ambisonics (HOA) is a sound field reproduction technique that defines a scalable and system-independent encoding of spatial sound information. Decoding of HOA signals for reproduction using loudspeaker arrays can be a difficult task if the angular spacing between adjacent loudspeakers, as observed from the listening position, is not uniform. In this research, one of such systems is considered: a 157-channel irregular loudspeaker array. The array is used to reproduce simple HOA-encoded sound fields. Three HOA decoding methods are evaluated: two conventional ones and a recently proposed decoder designed for irregular loudspeaker arrays. Reproduction accuracy is compared by directly measuring the sound pressure around the listening position, the so-called sweet spot. Coarse-resolution sound field measurements give an approximate size for the listening region generated by the different methods. In addition, dummy head recordings are used to evaluate interaural level and phase differences. The resul...

Extrapolation of Horizontal Ambisonics Data from Mainstream Stereo Sources

Ambisonics, a sound field reproduction technique, can present spatial audio with high accuracy. However, it has not been widely adopted due to the hardware requirements it imposes. In consequence, very few Ambisonics encoded contents are available. End-users will find it more attractive to invest in Ambisonics systems if they can be made backwards compatible with existing recordings. In this paper, a technique to spatially extrapolate stereo sources for listening using Ambisonics reproduction systems is introduced. The proposed approach makes use of the spatial information already encoded in most modern stereo recordings. This spatial information is usually used to decode multichannel audio signals, such as 5.1 channel surround. Unlike these technologies, the proposal attempts to extract a continuous description of the 360-degrees horizontal sound field which would result in the original stereo recording. The output is encoded using the circular harmonic functions, making it adequate for reproduction using horizontally-distributed loudspeaker arrays.