Volume 117, Issue 3, March 2005
Index of content:
- UNDERWATER SOUND 
Analysis of multipath scintillations from long range acoustic transmissions on the New England continental slope and shelf117(2005); http://dx.doi.org/10.1121/1.1835510View Description Hide Description
Observations of intensity fluctuations for broadband, 400-Hz multipath arrivals observed during the 1996 New England shelfbreak PRIMER study are described. Acoustic signals were generated by two bottom-mounted sources located on the continental slope in roughly 290-m water depth and were received on a 52-m-long vertical-line-array (VLA) located in 93-m water depth. Propagation ranges were 42.2 and 59.6 km. The bathymetry,oceanography, and bottom geology of the PRIMER site are described. Acoustic observables of point intensity, peak intensity, and integrated energy over the VLA are treated in terms of the scintillation index, log-intensity variance, and intensity probability density functions(PDFs). Variability of the observables are decomposed into high and low frequency components with time scales less than and greater than 2 h, to facilitate correlation to ocean processes at different timescales. Parabolic equation numerical simulations using a quasi-random undular tidal bore model are able to reproduce many of the observed intensity fluctuation to within a factor of 2, and they allow investigation of scintillation behavior as a function of range.
117(2005); http://dx.doi.org/10.1121/1.1855791View Description Hide Description
In this paper, the propagation of a broadband sound pulse in three-dimensional (3D) shallow water waveguides is investigated numerically. Two cases are examined: (i) the 3D ASA benchmark wedge, and (ii) the 3D Gaussian canyon. The numerical method used to solve the four-dimensional acoustic problem is based on a Fourier synthesis technique. The frequency-domain calculations are carried out using the fully 3D parabolic equation based model 3DWAPE, recently modified to include a wide-angle paraxial approximation for the azimuthal component. A broadband sound pulse with a central frequency of 25 Hz and a bandwith of 40 Hz is considered. For both test cases, 3D results corresponding to a 25 Hz cw point source are first presented and compared with predictions from a 3D adiabatic modal model. Then, the acoustic problem is solved considering the broadband source pulse. The modal structure of the received signals is analyzed and exhibits multiple mode arrivals of the propagating signal.
Effect of realistic grounds and atmospheric conditions on single-channel active control of outdoor sound propagation117(2005); http://dx.doi.org/10.1121/1.1859231View Description Hide Description
Engine run-up tests are a part of routine aircraft maintenance at the Vancouver International Airport. A source of noise complaints is the Dash-8 aircraft, which emits low-frequency, tonal noise.Active noise control is a potentially cost-effective alternative to passive noise-control methods, which are ineffective at controlling low-frequency noise. Since the run-up tests are performed outdoors, the effects of outdoor conditions on the performance of an active control system must be considered. In this paper, the results of a preliminary investigation of the effects of realistic meteorological conditions and ground impedance on the performance of a single-channel active-control system are presented. Computer simulations of single-channel active control of a monopole source were performed using the Green’s-function parabolic-equation method. Different realistic atmospheric conditions, and reflective or soft ground, were used in the simulations. The results show that atmospheric refraction causes fluctuations in the noise attenuation achieved by a single-channel control system, and has the overall effect of decreasing its performance, making the system ineffective in some cases.