Index of content:
Volume 125, Issue 3, March 2009
- ACOUSTIC SIGNAL PROCESSING 
Internal noise reduction from dependent signal mixtures using microphones and a piezoelectric device under blind condition125(2009); http://dx.doi.org/10.1121/1.3077218View Description Hide Description
The acoustical array composed of microphones and piezoelectric devices can reduce internally generated noise from the system such as a machine, a vehicle, or a robot. However, when the acoustical array combining microphones and piezoelectric devices is employed, it is necessary to estimate the gains of microphones and piezoelectric devices concerning the system that generates the noise in advance. The aim of this paper is to simplify this process, that is, to reduce internally generated noise without preknowledge concerning the gains of microphones and piezoelectric devices. Although the proposed method handles an anechoic mixing, it has some merits that complement the other typical blind source separation algorithms. In this paper, the problem is first formulated. The assumptions and the methodology of the proposed method are then given with some experimental results.
On the applicability of the spherical wave expansion with a single origin for near-field acoustical holography125(2009); http://dx.doi.org/10.1121/1.3068451View Description Hide Description
The spherical wave expansion with a single origin is sometimes used in connection with near-field acoustical holography to determine the sound field on the surface of a source. The radiated field is approximated by a truncated expansion, and the expansion coefficients are determined by matching the sound field model to the measuredpressure close to the source. This problem is ill posed, and therefore regularization is required. The present paper investigates the consequence of using only the expansion truncation as regularization approach and compares it with results obtained when additional regularization (the truncated singular value decomposition) is introduced. Important differences between applying the method when using a microphone array surrounding the source completely and an array covering only a part of the source are described. Another relevant issue is the scaling of the wave functions. It is shown that it is important for the additional regularization to work properly that the wave functions are scaled in such a way that their magnitude on the measurementsurface decreases with the order. Finally, the method is applied on nonspherical sources using a vibrating plate in both simulations and an experiment, and the performance is compared with the equivalent source method.
Reconstruction of vibroacoustic responses of a highly nonspherical structure using Helmholtz equation least-squares method125(2009); http://dx.doi.org/10.1121/1.3068449View Description Hide Description
The vibroacoustic responses of a highly nonspherical vibrating object are reconstructed using Helmholtz equation least-squares (HELS) method. The objectives of this study are to examine the accuracy of reconstruction and the impacts of various parameters involved in reconstruction using HELS. The test object is a simply supported and baffled thin plate. The reason for selecting this object is that it represents a class of structures that cannot be exactly described by the spherical Hankel functions and spherical harmonics, which are taken as the basis functions in the HELS formulation, yet the analytic solutions to vibroacoustic responses of a baffled plate are readily available so the accuracy of reconstruction can be checked accurately. The input field acoustic pressures for reconstruction are generated by the Rayleigh integral. The reconstructed normal surfacevelocities are validated against the benchmark values, and the out-of-plane vibration patterns at several natural frequencies are compared with the natural modes of a simply supported plate. The impacts of various parameters such as number of measurement points, measurement distance, location of the origin of the coordinate system, microphone spacing, and ratio of measurement aperture size to the area of source surface of reconstruction on the resultant accuracy of reconstruction are examined.
125(2009); http://dx.doi.org/10.1121/1.3075769View Description Hide Description
In the traditional approach to X-waves, the X-wave field is synthesized from a superposition of solutions to the homogenous wave equation (in three-dimensions) without regard to boundary conditions. As a consequence the synthesized solution is acausal. Here, it is shown that the solution to the inhomogenous scalar wave equation for the acoustic field from a supersonic source distribution consistent with the radiation condition, i.e., a Mach front, defines a causal X-wave. Using the connection between X-waves and a physical source, it is shown that an X-wave can be generated from a planar aperture using time-reversal. By appealing to the demonstrated self-adaptivity of time-reversal processes, the method should allow for the generation of X-waves in arbitrary (inhomogenous) media. Typically, the generation of approximate acoustic X-waves from a planar aperture is achieved using a complicated annular transducer arrangement. Here, the time-reversal method for the generation of approximate acoustic X-waves is experimentally proven using a line transducer array in two-dimensional geometry in free space.