Index of content:
Volume 126, Issue 2, August 2009
- ACOUSTIC SIGNAL PROCESSING 
126(2009); http://dx.doi.org/10.1121/1.3159587View Description Hide Description
Many noise guidelines currently use -weighted equivalent sound pressure level as the noise metric and the equal energy hypothesis to assess the risk of occupational noises. Because of the time-averaging effect involved with the procedure, the current guidelines may significantly underestimate the risk associated with complex noises. This study develops and evaluates several new noise metrics for more accurate assessment of exposure risks to complex and impulsive noises. The analytic wavelet transform was used to obtain time-frequency characteristics of the noise. 6 basic, unique metric forms that reflect the time-frequency characteristics were developed, from which 14 noise metrics were derived. The noise metrics were evaluated utilizing existing animal test data that were obtained by exposing 23 groups of chinchillas to, respectively, different types of noise. Correlations of the metrics with the hearing losses observed in chinchillas were compared and the most promising noise metric was identified.
Travel-time sensitivity kernels versus diffraction patterns obtained through double beam-forming in shallow water126(2009); http://dx.doi.org/10.1121/1.3158922View Description Hide Description
In recent years, the use of sensitivity kernels for tomographic purposes has been frequently discussed in the literature. Sensitivity kernels of different observables (e.g., amplitude, travel-time, and polarization for seismic waves) have been proposed, and relationships between adjoint formulation, time-reversal theory, and sensitivity kernels have been developed. In the present study, travel-time sensitivity kernels (TSKs) are derived for two source-receiver arrays in an acoustic waveguide. More precisely, the TSKs are combined with a double time-delay beam-forming algorithm performed on two source-receiver arrays to isolate and identify each eigenray of the multipath propagation between a source-receiver pair in the acoustic waveguide. A relationship is then obtained between TSKs and diffraction theory. It appears that the spatial shapes of TSKs are equivalent to the gradients of the combined direction patterns of the source and receiver arrays. In the finite-frequency regimes, the combination of TSKs and double beam-forming both simplifies the calculation of TSK and increases the domain of validity for ray theory in shallow-water ocean acoustictomography.
126(2009); http://dx.doi.org/10.1121/1.3158819View Description Hide Description
Patch near field acoustic holography (PNAH) based on sound pressuremeasurements makes it possible to reconstruct the source field near a source by measuring the sound pressure at positions on a surface that is comparable in size to the source region of concern. Particle velocity is an alternative input quantity for NAH, and the advantage of using the normal component of the particle velocity rather than the sound pressure as the input of conventional spatial Fourier transform based NAH and as the input of the statistically optimized variant of NAH has recently been demonstrated. This paper examines the use of particle velocity as the input of PNAH. Because the particle velocity decays faster toward the edges of the measurement aperture than the pressure does and because the wave number ratio that enters into the inverse propagator from pressure to velocity amplifies high spatial frequencies, PNAH based on particle velocitymeasurements can give better results than the pressure-based PNAH with a reduced number of iterations. A simulation study, as well as an experiment carried out with a pressure-velocity sound intensity probe, demonstrates these findings.