Volume 130, Issue 6, December 2011
Index of content:
- ACOUSTIC SIGNAL PROCESSING 
130(2011); http://dx.doi.org/10.1121/1.3652855View Description Hide Description
Orthogonal frequency division multiplexing (OFDM) communications in the presence of motion is investigated using data collected from the Kauai Acomms MURI 2008 (KAM08) experiment, conducted off the western side of Kauai, Hawaii, in June–July 2008. The experiment involved a vertical array moored in 106 m deep shallow water and a source towed at a speed of 3 knots at ranges between 600 m and 6 km. In order to attain reliable communications with only a single receive element, a synthetic aperture approach is applied. After combining multiple transmissions, an error-free reception is achieved with a low-density parity-check code, confirming the feasibility of coherent synthetic aperture communications using OFDM.
130(2011); http://dx.doi.org/10.1121/1.3658448View Description Hide Description
In the field of noise identification with microphone arrays, conventional delay-and-sum (DAS) beamforming is the most popular signal processing technique. However, acoustic imaging results that are generated by DAS beamforming are easily influenced by background noise, particularly for in situwind tunnel tests. Even when arithmetic averaging is used to statistically remove the interference from the background noise, the results are far from perfect because the interference from the coherent background noise is still present. In addition, DAS beamforming based on arithmetic averaging fails to deliver real-time computational capability. An observer-based approach is introduced in this paper. This so-called observer-based beamforming method has a recursive form similar to the state observer in classical control theory, thus holds a real-time computational capability. In addition, coherent background noise can be gradually rejected in iterations. Theoretical derivations of the observer-based beamforming algorithm are carefully developed in this paper. Two numerical simulations demonstrate the good coherent background noise rejection and real-time computational capability of the observer-based beamforming, which therefore can be regarded as an attractive algorithm for acoustic arraysignal processing.
130(2011); http://dx.doi.org/10.1121/1.3658473View Description Hide Description
The coherent processing of signals from multiple hydrophones in an array offers improvements in angular resolution and signal-to-noise ratio. When the array is steered in a particular direction, the signals arriving from that direction are added in phase, and any signals arriving from other directions are not. Array gain (AG) is a measure of how much the signal arriving from the steering direction is amplified relative to signals arriving from all other directions. The subject of this paper is the manner in which the AG of an acoustic array operating in water that contains air bubbles is affected by scattering from nearby bubbles. The effects of bubbles on acoustic attenuation and dispersion are considered separately from their effects on AG. Acoustic measurements made in bubbly water using the AB Wood tank at the Institute of Sound and Vibration Research, University of Southampton, in June 2008 show that as bubble density increases, relative phase shifts in individual hydrophone signals increase and signal correlation among the hydrophones is reduced. A theory and numerical simulation linking bubble density at the hydrophone to the AG is in good agreement with the measurements up to the point where multiple scattering becomes important.
130(2011); http://dx.doi.org/10.1121/1.3658443View Description Hide Description
The design and construction of a circular microphone array (CMA) that has a wide frequency range suitable for human hearing is presented. The design of the CMA was achieved using a technique based on simulated directivity index (DI) curves. The simulated DI curves encapsulate the critical microphone array performance limitations: spatial aliasing, measurement noise, and microphone placement errors. This paper demonstrates how the non-regularized DI curves for a given beamforming order clearly define the bandwidth of operation, in other words, the frequency band for which the beamformer has relatively constant and maximum directivity. Detailed and comprehensive experimental data that characterizes the CMA beamformer are also presented.
Acoustic quasi-holographic images of scattering by vertical cylinders from one-dimensional bistatic scans130(2011); http://dx.doi.org/10.1121/1.3652901View Description Hide Description
When synthetic aperture sonar (SAS) is used to image elastic targets in water, subtle features can be present in the images associated with the dynamical response of the target being viewed. In an effort to improve the understanding of such responses, as well as to explore alternative image processing methods, a laboratory-based system was developed in which targets were illuminated by a transient acoustic source, and bistatic responses were recorded by scanning a hydrophone along a rail system. Images were constructed using a relatively conventional bistatic SAS algorithm and were compared with images based on supersonic holography. The holographic method is a simplification of one previously used to view the time evolution of a target’s response [Hefner and Marston, ARLO 2, 55–60 (2001)]. In the holographic method, the space-time evolution of the scattering was used to construct a two-dimensional image with cross range and time as coordinates. Various features for vertically hung cylindrical targets were interpreted using high frequency ray theory. This includes contributions from guided surface elastic waves, as well as transmitted-wave features and specular reflection.