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Todd Hefner

Chair - Acoustics Department & Senior Principal Physicist





Department Affiliation



B.A. Physics, Bard College, 1994

M.S. Physics, Washington State University, 1997

Ph.D. Physics, Washington State University, 2000


2000-present and while at APL-UW

The impact of the spatial variability of the seafloor on midfrequency sound propagation during the Target and Reverberation Experiment 2013

Hefner, B.T., D. Tang, and W.S. Hodgkiss, "The impact of the spatial variability of the seafloor on midfrequency sound propagation during the Target and Reverberation Experiment 2013," IEEE J. Ocean. Eng., EOR, doi:10.1109/JOE.2024.3361968, 2024.

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14 Mar 2024

To support the modeling of reverberation data collected during the Target and Reverberation Experiment in 2013 (TREX13), transmission loss was measured in the 1.5–4.0 kHz band using a towed source and two moored vertical line arrays. The experiment site was located off the coast of Panama City Beach, FL, and the transmission loss measurements took place along a 7-km-long isobath, which ran parallel to the shore with a water depth of approximately 19 m. The seafloor at the TREX13 site consists of sand ridges, which run perpendicular to the track of the experiment, with narrow bands of softer sediments on the western sides of the ridges and in the ridge swales. Using data from a multibeam echosounder survey and direct measurements of the seafloor properties, a geoacoustic description of the seafloor is developed and used to model the transmission loss at the site. Although the soft-sediment bands only occur in 27% of the seafloor, they are found to have a significant impact on the transmission loss, increasing it by roughly 5 dB at 4 km over what would be expected from an entirely sand sediment. This is consistent with the previous work by Holland who showed that lossiest sediments play the largest role in propagation over range-dependent seabeds. Simulations also show that the exact locations of the soft sediments are less important for controlling propagation in the TREX13 environment than the proportions of the sediments. This suggests that a range-independent, effective media description of the sediment could be used to model propagation at the site. The limits of the use of an effective medium in describing both propagation and reverberation measurements made during TREX13 are considered.

Subsurface acoustic ducts in the Northern California current system

Xu, G., R.R. Harcourt, D. Tang, B.T. Hefner, E.I. Thorsos, and J.B. Mickett, "Subsurface acoustic ducts in the Northern California current system," J. Acoust. Soc. Am., 155, 1881-1894, doi:10.1121/10.0024146, 2024.

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7 Mar 2024

This study investigates the subsurface sound channel or acoustic duct that appears seasonally along the U.S. Pacific Northwest coast below the surface mixed layer. The duct has a significant impact on sound propagation at mid-frequencies by trapping sound energy and reducing transmission loss within the channel. A survey of the sound-speed profiles obtained from archived mooring and glider observations reveals that the duct is more prevalent in summer to fall than in winter to spring and offshore of the shelf break than over the shelf. The occurrence of the subsurface duct is typically associated with the presence of a strong halocline and a reduced thermocline or temperature inversion. Furthermore, the duct observed over the shelf slope corresponds to a vertically sheared along-slope velocity profile, characterized by equatorward near-surface flow overlaying poleward subsurface flow. Two potential duct formation mechanisms are examined in this study, which are seasonal surface heat exchange and baroclinic advection of distinct water masses. The former mechanism regulates the formation of a downward-refracting sound-speed gradient that caps the duct near the sea surface, while the latter contributes to the formation of an upward-refracting sound-speed gradient that defines the duct's lower boundary.

Observed correlations between the sediment grain size and the high-frequency backscattering strength

Wendelboe, G., T. Hefner, and A. Ivakin, "Observed correlations between the sediment grain size and the high-frequency backscattering strength," JASA Express Lett., 3, doi:10.1121/10.0017107, 2023.

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1 Feb 2023

In March 2019, Teledyne RESON and the Applied Physics Laboratory at the University of Washington conducted surveys with a calibrated multibeam echosounder at ten sites in Sequim Bay, a shallow sheltered bay in Washington State, USA. For each site, the mean grain size was obtained from a diver core sample, and estimates of the backscattering strength at frequencies ranging between 200 and 350 kHz were calculated. The correlation between the backscattering strength and the normalized grain size have been investigated for the grazing angles 45° and 75°. For 45°°, a correlation consistent with previous results has been found. It demonstrates the potential for simple seabed classification.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center