Foote Kenneth G.

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Kenneth G.

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Now showing 1 - 6 of 6
  • Article
    Detecting Atlantic herring by parametric sonar
    (Acoustical Society of America, 2010-03-17) Godø, Olav Rune ; Foote, Kenneth G. ; Dybedal, Johnny
    The difference-frequency band of the Kongsberg TOPAS PS18 parametric sub-bottom profiling sonar, nominally 1–6 kHz, is being used to observe Atlantic herring. Representative TOPAS echograms of herring layers and schools observed in situ in December 2008 and November 2009 are presented. These agree well with echograms of volume backscattering strength derived simultaneously with the narrowband Simrad EK60/18- and 38-kHz scientific echo sounder, also giving insight into herring avoidance behavior in relation to survey vessel passage. Progress in rendering the TOPAS echograms quantitative is described.
  • Article
    Acousto-optic effect compensation for optical determination of the normal velocity distribution associated with acoustic transducer radiation
    (Acoustical Society of America, 2015-09-28) Foote, Kenneth G. ; Theobald, Peter D.
    The acousto-optic effect, in which an acoustic wave causes variations in the optical index of refraction, imposes a fundamental limitation on the determination of the normal velocity, or normal displacement, distribution on the surface of an acoustic transducer or optically reflecting pellicle by a scanning heterodyne, or homodyne, laser interferometer. A general method of compensation is developed for a pulsed harmonic pressure field, transmitted by an acoustic transducer, in which the laser beam can transit the transducer nearfield. By representing the pressure field by the Rayleigh integral, the basic equation for the unknown normal velocity on the surface of the transducer or pellicle is transformed into a Fredholm equation of the second kind. A numerical solution is immediate when the scanned points on the surface correspond to those of the surface area discretization. Compensation is also made for oblique angles of incidence by the scanning laser beam. The present compensation method neglects edge waves, or those due to boundary diffraction, as well as effects due to baffles, if present. By allowing measurement in the nearfield of the radiating transducer, the method can enable quantification of edge-wave and baffle effects on transducer radiation. A verification experiment has been designed.
  • Article
    Range compensation for backscattering measurements in the difference-frequency nearfield of a parametric sonar
    (Acoustical Society of America, 2012-05) Foote, Kenneth G.
    Measurement of acoustic backscattering properties of targets requires removal of the range dependence of echoes. This process is called range compensation. For conventional sonars making measurements in the transducer farfield, the compensation removes effects of geometrical spreading and absorption. For parametric sonars consisting of a parametric acoustic transmitter and a conventional-sonar receiver, two additional range dependences require compensation when making measurements in the nonlinearly generated difference-frequency nearfield: an apparently increasing source level and a changing beamwidth. General expressions are derived for range compensation functions in the difference-frequency nearfield of parametric sonars. These are evaluated numerically for a parametric sonar whose difference-frequency band, effectively 1–6 kHz, is being used to observe Atlantic herring (Clupea harengus) in situ. Range compensation functions for this sonar are compared with corresponding functions for conventional sonars for the cases of single and multiple scatterers. Dependences of these range compensation functions on the parametric sonar transducer shape, size, acoustic power density, and hydrography are investigated. Parametric range compensation functions, when applied with calibration data, will enable difference-frequency echoes to be expressed in physical units of volume backscattering, and backscattering spectra, including fish-swimbladder-resonances, to be analyzed.
  • Article
    Discriminating between the nearfield and the farfield of acoustic transducers
    (Acoustical Society of America, 2014-10) Foote, Kenneth G.
    Measurements of the performance of acoustic transducers, as well as ordinary measurements made with the same, may require discriminating between the farfield, where the field is spherically divergent, and the complementary nearfield, where the field structure is more complicated. The problem is addressed for a planar circular piston projector, with uniform normal velocity distribution, mounted in an infinite planar rigid baffle. The inward-extrapolated farfield pressure amplitude pf is compared with the exact nearfield pressure amplitude pn , modeled by the Rayleigh integral, through the error 20 log |pf  /pn |. Three sets of computations are performed for a piston with wavenumber-radius product ka = 10: normalized pressure amplitudes and error versus range at angles corresponding to beam pattern losses of 0, 10, 20, and 30 dB; error versus angle at three ranges, a 2/λ, πa 2/λ, and 10a 2/λ, where λ is the wavelength; and range versus angle for each of two inward-bounded errors, 1 and 0.3 dB. By reciprocity, the results apply equally to the case of a baffled circular piston receiver with uniform sensitivity over the active surface. It is proposed that proximity criteria for measurements of fields associated with circular pistons be established by like modeling, and that a quality factor be assigned to measurements on the basis of computed errors.
  • Article
    Sonar-induced pressure fields in a post-mortem common dolphin
    (Acoustical Society of America, 2012-02) Foote, Kenneth G. ; Hastings, Mardi C. ; Ketten, Darlene R. ; Lin, Ying-Tsong ; Reidenberg, Joy S. ; Rye, Kent
    Potential physical effects of sonar transmissions on marine mammals were investigated by measuring pressure fields induced in a 119-kg, 211-cm-long, young adult male common dolphin (Delphinus delphis) cadaver. The specimen was instrumented with tourmaline acoustic pressure gauges used as receiving sensors. Gauge implantation near critical tissues was guided by intraoperative, high-resolution, computerized tomography (CT) scanning. Instrumented structures included the melon, nares, ear, thoracic wall, lungs, epaxial muscle, and lower abdomen. The specimen was suspended from a frame equipped with a standard 50.8-mm-diameter spherical transducer used as the acoustic source and additional receiving sensors to monitor the transmitted and external, scattered field. Following immersion, the transducer transmitted pulsed sinusoidal signals at 5, 7, and 10 kHz. Quantitative internal pressure fields are reported for all cases except those in which the gauge failed or no received signal was detected. A full necropsy was performed immediately after the experiment to examine instrumented areas and all major organs. No lesions attributable to acoustic transmissions were found, consistent with the low source level and source-receiver distances.
  • Article
    Quantitative imaging of calibrated acoustic backscatter data from the seafloor
    (Institute of Acoustics, 2015-09-07) Foote, Kenneth G. ; Robinson, Stephen P. ; Theobald, Peter D.
    Two factors have often led to the neglect of instrument calibration and the loss of information in the imaging process: the power of the image and the convenient signal processing expedient of disregarding the physical nature of data. Seafloor imaging by sonar is a case in point. Notwithstanding ambitions and needs to remotely detect and identify bottom objects, determine seafloor properties, and quantify benthos, among other things, images of acoustic backscattering data are often used, misleadingly, as proxies for physical data. Since image processing is inherently nonlinear, the loss of physical data is immediate. Three processes that are essential to the attainment and maintenance of the physical nature of backscattering data are elaborated: sonar calibration to determine the transfer characteristics of the sonar, range compensation that addresses both geometric and radiometric factors, and beam pattern measurement or estimation.