Compressive matched-field processing

Source localization by matched-field processing (MFP) generally involves solving a number of computationally intensive partial differential equations. This paper introduces a technique that mitigates this computational workload by "compressing" these computations. Drawing on key concepts from the recently developed field of compressed sensing, it shows how a low-dimensional proxy for the Green's function can be constructed by backpropagating a small set of random receiver vectors. Then the source can be located by performing a number of "short" correlations between this proxy and the projection of the recorded acoustic data in the compressed space. Numerical experiments in a Pekeris ocean waveguide are presented that demonstrate that this compressed version of MFP is as effective as traditional MFP even when the compression is significant. The results are particularly promising in the broadband regime where using as few as two random backpropagations per frequency performs almost as well as the traditional broadband MFP but with the added benefit of generic applicability. That is, the computationally intensive backpropagations may be computed offline independently from the received signals, and may be reused to locate any source within the search grid area.     

Sensitivity kernel for surface scattering in a waveguide

Using the Born approximation, a linearized sensitivity kernel is derived to describe the relationship between a local change at the free surface and its effect on the acoustic propagation in the water column. The structure of the surface scattering kernel is investigated numerically and experimentally for the case of a waveguide at the ultrasonic scale. To better demonstrate the sensitivity of the multipath propagation to the introduction of a localized perturbation at the air-water interface, the kernel is formulated both in terms of point-to-point and beam-to-beam representations. Agreement between theory and experiment suggests applications to sensitivity analysis of the wavefield for sea surface perturbations.     

Transmission loss of periodically stiffened laminate composite panels: shear deformation and in-plane interaction effects

This paper investigates the transmission loss of symmetric and asymmetric laminate composite panels periodically reinforced by composite stiffeners. A comprehensive model based on periodic structure theory is developed. First order shear deformation theory is used and the coupling of the in-plane motion of the panel with its out-of-plane motion is taken into account. Stiffeners interact with the panel through three forces (two in-plane, one out-of-plane) and a torsion moment. Three types of cross sections are investigated for the composite stiffeners: I-shaped, C-shaped, and omega-shaped cross-sections. The model is validated numerically by comparison with the finite element/boundary element method. Experimental validations are also conducted. In both cases, excellent agreement is obtained. Numerical results show that the in-plane coupling effect is increased by increasing the panel thickness and the stiffener's eccentricity. The in-plane coupling effect is also found to increase with frequency.     

Comparing perceived auditory width to the visual image of a performing ensemble in contrasting bi-modal environments

Despite many studies investigating auditory spatial impressions in rooms, few have addressed the impact of simultaneous visual cues on localization and the perception of spaciousness. The current research presents an immersive audiovisual environment in which participants were instructed to make auditory width judgments in dynamic bi-modal settings. The results of these psychophysical tests suggest the importance of congruent audio visual presentation to the ecological interpretation of an auditory scene. Supporting data were accumulated in five rooms of ascending volumes and varying reverberation times. Participants were given an audiovisual matching test in which they were instructed to pan the auditory width of a performing ensemble to a varying set of audio and visual cues in rooms. Results show that both auditory and visual factors affect the collected responses and that the two sensory modalities coincide in distinct interactions. The greatest differences between the panned audio stimuli given a fixed visual width were found in the physical space with the largest volume and the greatest source distance. These results suggest, in this specific instance, a predominance of auditory cues in the spatial analysis of the bi-modal scene.     

Audience noise in concert halls during musical performances

Noise generated by the audience during musical performances is audible and sometimes disturbing. In this study, an attempt to estimate such audience noise was carried out. From the recordings of performances in five performance spaces (four concert halls and one opera house), probability density functions of the sound pressure levels were obtained in octave bands, which were fitted with three Gaussian distribution curves. The Gaussian distribution curve with the lowest mean value corresponds to a mixture of the technical background noise and audience generated noise, which is named the mixed background noise. Finally, the audience noise distribution is extracted by energy subtraction of the technical background noise levels measured in an empty condition from the mixed background noise levels. As a single index, L(90) of the audience noise distribution is named the audience noise level. Empirical prediction models were made using the four orchestra concert halls, revealing that the audience noise level is significantly correlated with the technical background noise level. It is therefore concluded that a relaxation of the current background noise recommendations for concert halls is not recommended.     

Scattering of time-harmonic elastic waves by an elastic inclusion with quadratic nonlinearity

This paper considers the scattering of a plane, time-harmonic wave by an inclusion with heterogeneous nonlinear elastic properties embedded in an otherwise homogeneous linear elastic solid. When the inclusion and the surrounding matrix are both isotropic, the scattered second harmonic fields are obtained in terms of the Green's function of the surrounding medium. It is found that the second harmonic fields depend on two independent acoustic nonlinearity parameters related to the third order elastic constants. Solutions are also obtained when these two acoustic nonlinearity parameters are given as spatially random functions. An inverse procedure is developed to obtain the statistics of these two random functions from the measured forward and backscattered second harmonic fields.     

How the cross-sectional discontinuity between ear canal and probe affects the ear canal length estimation

Many ear canal probes both deliver and measure sound via narrow tubes. This study investigates the effect of the cross-sectional discontinuity at the interface between ear canal models and the connecting tubes of a commercially available otoacoustic emission probe on the "acoustically" estimated cavity lengths. Rigid cavities having the same length but different diameters were produced, and modeled by the finite element method. Cavities with a diameter larger than 8 mm had acoustic lengths that considerably overestimated the real geometry. A length correction was derived, which, in most applications, compensates for the measurement errors emerging from the discontinuity effects.     

Resolving meso-scale seabed variability using reflection measurements from an autonomous underwater vehicle

Seabed geoacoustic variability is driven by geological processes that occur over a wide spectrum of space-time scales. While the acoustics community has some understanding of horizontal fine-scale geoacoustic variability, less than O(10(0)) m, and large-scale variability, greater than O(10(3)) m, there is a paucity of data resolving the geoacoustic meso-scale O(10(0)-10(3)) m. Measurements of the meso-scale along an ostensibly "benign" portion of the outer shelf reveal three classes of variability. The first class was expected and is due to horizontal variability of layer thicknesses: this was the only class that could be directly tied to seismic reflection data. The second class is due to rapid changes in layer properties and/or boundaries, occurring over scales of meters to hundreds of meters. The third class was observed as rapid variations of the angle/frequency dependent reflection coefficient within a single observation and is suggestive of variability at scales of meter or less. Though generally assumed to be negligible in acoustic modeling, the second and third classes are indicative of strong horizontal geoacoustic variability within a given layer. The observations give early insight into possible effects of horizontal geoacoustic variability on long-range acoustic propagation and reverberation.     

A detection-theoretic framework for modeling informational masking

There has been growing interest in recent years in masking that appears to have its origin at a central level of the auditory nervous system-so-called informational masking (IM). Masker uncertainty and target-masker similarity have been identified as the two major factors affecting IM; however, no theoretical framework currently exists that would give precise meaning to these terms necessary to evaluate their relative importance or model their effects. The present paper offers a first attempt at such a framework constructed within the doctrines of the theory of signal detection.     

Modification of the loop filter design for a plucked string instrument

A modified loop filter design method for plucked string instruments is introduced. Previous loop filter designs do not properly represent the frequency-dependent damping of a silk stringed instrument. To solve this problem, the modified method evaluates how many harmonics are required and which portion of the sound should be chosen to effectively replicate the instrument. Then, the most reasonable filter parameters are determined based on the frequency signal-to-noise ratio of the synthesized sound.