Shipping noise is an important part
of the ocean 'environment', and presents interesting modeling
problems. The Philippines Sea is chosen as an example of a
moderate sized ocean basin for the calculation of noise
directionality. The upper left shows the elevation with the
color bar giving negative elevations in meters. Green is also
used for land. Shipping noise source level densities, at 50 Hz,
are plotted in the upper right and are indicative of shipping
lanes. The colors represent dB re: 1 microPa2 /Hz @
1m/m2. A virtual receiving array is located in the
center of the basin. The calculated noise directivity is plotted
versus bearing and elevation angle for a hard (reflective) ocean
bottom, and for a soft (absorptive) bottom. Energy from near
surface sources, over slopes, is seen in a range of elevation
angles, around zero degrees. Differences between the two cases
clearly show the importance of the ocean bottom reflectivity.
ONR's BASSEX program is studying the acoustic shadows cast by seamounts. The example above shows the bathymetry, along with measured TL for a recent test near the Kermit-Roosevelt seamount in the North Pacific. The strong shadowing effects are clearly visible. Three-dimensional acoustic models (lower panels) are being validated using a benchmark problem with a conical seamount.
Click on the above figure to complete
the Ocean Acoustics crossword puzzle or click here to download a
PDF of same. Happy Holidays!.
The wireless internet extends to the
sea .... ONR's research in high-frequency acoustic propagation
and acoustic communications is providing the underlying science
that will enable flexible undersea networking. A prototype
system shown above has been developed in the navy's SeaWEB
program. The network is being used for real-time oceanographic
observations in the FRONT
Possible applications of time-reversal acoustics are being studied by several groups. A transmitted signal is received on the vertical array on the right as a complicated waveform. However, when that received waveform is back-propagated, a focused signal is received back at the source.