Index of content:
Volume 104, Issue 2, August 1998
- ACOUSTIC SIGNAL PROCESSING 
104(1998); http://dx.doi.org/10.1121/1.423310View Description Hide Description
Matching pursuits is a nonlinear algorithm which iteratively projects a given signal onto a complete dictionary of vectors. The dictionary is constructed such that it is well matched to the signals of interest and poorly matched to the noise, thereby affording the potential for denoising, by adaptively extracting an underlying signature from a noisy waveform. In the context of wavescattering and propagation, there are basic constituents that can be used to construct most measured waveforms. A dictionary of such constituents is used here, in the context of wave-based matching-pursuit processing of acoustic wavesscattered from submerged elastic targets. It is demonstrated how wave-based matching pursuits can be utilized for denoising as well as to effect a detector, the latter being parametrized via its receiver operating characteristic (ROC). Results are presented using measured aspect-dependent (orientation-dependent) scattered waveforms, for the case of a submerged elastic shell.
104(1998); http://dx.doi.org/10.1121/1.423311View Description Hide Description
An array of hydrophones is towed below the sea surface so as to sample the underwater acousticpressure field in both space and time, while a land-based array of microphones is used to sense the atmospheric acoustic environment which, at the time, was dominated by a single source of broadband energy. After transformation from the time domain to the frequency domain, the sensor outputs from each array are weighted and combined in the spatial domain (beamformed) so as to produce a frequency–wave number power spectrum, which displays the power spectral density distribution of the various signal and noise sources as a joint function of frequency and wave number. The frequency-domain beamforming (or spatial filtering) process enables both conventional and optimal estimation of the frequency–wave number power spectrum. The optimal spatial filtering technique used here is commonly referred to as the Minimum Variance Distortionless Response (MVDR) beamformer which requires inversion of the observed narrow-band cross-power spectral matrix at each frequency of interest. A comparison of the frequency–wave number power spectra estimated by the two spatial filtering techniques shows that the MVDR beamformer enables the various sources of acoustic energy to be more clearly delineated in frequency–wave number space. The MVDR beamformer is a data-adaptive spatial filter which is observed to suppress sidelobes, to enhance the spatial resolution of an array through narrower beamwidths, and to provide superdirective array gain at frequencies well below the design frequency of an array. By extending the processing to include the data from another type of towed array, it is shown that frequency–wave number analysis, when incorporated with MVDR beamforming, constitutes a powerful diagnostic tool for studying the self-noise characteristics of towed arrays.
104(1998); http://dx.doi.org/10.1121/1.423312View Description Hide Description
Detection and estimation of aeroacousticshock waves generated by supersonic projectiles are considered. The shock wave is an -shaped acoustic wave emanating in the form of an acoustic cone trailing the projectile. An optimal detection/estimation scheme is considered based on a parametric signal plus white Gaussian noise model. To gain robustness and reduce complexity, we then focus on gradient estimators for shock waveedge detection, exploiting the very fast shock rise and fall times. The approach is cast in terms of a wavelet transform where the level of smoothing corresponds to scale. A multiscale analysis is described, consisting of multiscale products, to enhance edge detection and estimation. This method is effective and robust with respect to unknown environmental interference that will generally not exhibit singularities as sharp as the -wave edges. Experimental results are presented for discriminating waves in the presence of vehicle noise. Results are also shown, as a function of miss distance, for gradient-based detection of simulated small projectile shocks inserted into recorded tank noise.