This paper describes a technique dedicated for the localization of acoustic sources in all directions and in the farfield. Classical beamforming techniques based on planar arrays provide an acoustic map restricted to a limited solid angle, but a spherical array does not have such a limitation since there is no preferential direction. In the processing called Spherical Harmonics Beamforming (SHB), the sound field on the sphere is decomposed with spherical harmonics functions, and then a corrected summation gives the acoustic contribution from a given direction. We have used a rigid spherical array, which has the advantage that cabling of microphones and integrated cameras can be hidden inside the sphere. A rigid surface also provides better numerical stability in connection with SHB. In this study, SHB is evaluated with respect to resolution and dynamic range. Simulated and experimental results are presented.
Acoustic impedance is typically measured using an impedance tube, which requires a material sample physically fitted to the tube. However, the impedance can vary greatly between the material mounted in the tube and the material located in a real environment, where the mounting conditions are likely to be different. Also, oblique incidence cannot be measured in an impedance tube. In this paper, we investigate the use of a double-layer microphone array for in-situ measurement of surface impedance and absorption coefficient. With the array positioned near the material surface, a source emits broad-band sound towards the array and the material. A measurement is taken, and the sound pressure and the surface-normal particle velocity at the material surface are calculated using Statistically Optimized Near-field Acoustical Holography (SONAH). From the surface pressure and velocity, the impedance across a selected area is calculated, and finally the absorption coefficient is calculated from the impedance. A set of tests has been performed on porous material samples in an anechoic chamber as well as in a fitted room. Different sample sizes and different sound incidence angles have been considered. The results show consistency between the measurements in the anechoic room and the ordinary room as well as good agreement with Miki's model up to large oblique incidence angles.
We propose a format and a set of tools in the rendering of computer musical notation. The proposed format is not to be considered as a universal standard. Nevertheless, it appears to be an efficient approach to musical notation. The proposed format might be thought as a contribution to future or existing implementations of musical editors. We will try to provide general guidelines and schemes that will be compatible with any type of programming language. This protocol has been implemented in the OpenMusic environment using the CLOS language environment. This paper is an a posteriori generalization of this implementation.
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