NMFS Biological Opinion on U.S. Navy training ... - Govsupport.us

FINAL PROGRAMMATIC BIOLOGICAL OPINION ON U.S. NAVY ACTIVITIES IN THE HAWAII RANGE COMPLEX 2008-2013
below 1000 Hz; however, is quite poor. On the other hand, the hearing sensitivity of most sea turtles appear to be
best at frequencies between about 200 Hz and 700 Hz. As a result, sea turtles might be expected to suffer more
harmful effects from loud, low frequency noise than would dolphins.
Because ears adapted to function underwater are physiologically different from human ears, comparisons using
decibels would still not be adequate to describe the effects of a sound on a whale. When sound travels away from its
source, its loudness decreases as the distance traveled by the sound increases. Thus, the loudness of a sound at its
source is higher than the loudness of that same sound a kilometer distant. Acousticians often refer to the loudness of
a sound at its source as the source level and the loudness of sound elsewhere as the received level. For example, a
humpback whale 3 kilometers from an airgun that has a source level of 230 dB may only be exposed to sound that is
160 dB loud. As a result, it is important not to confuse source levels and received levels when discussing the
loudness of sound in the ocean.
As sound moves away from a source, its propagation in water is influenced by various physical characteristics,
including water temperature, depth, salinity, and surface and bottom properties that cause refraction, reflection,
absorption, and scattering of sound waves. Oceans are not homogeneous and the contribution of each of these
individual factors is extremely complex and interrelated. The physical characteristics that determine the sound’s
speed through the water will change with depth, season, geographic location, and with time of day (as a result, in
actual sonar operations, crews will measure oceanic conditions, such as sea water temperature and depth, to calibrate
models that determine the path the sonar signal will take as it travels through the ocean and how strong the sound
signal will be at given range along a particular transmission path).
Sound tends to follow many paths through the ocean, so that a listener would hear multiple, delayed copies of
transmitted signals (Richardson et al. 1995). Echoes are a familiar example of this phenomenon in air. In order to
determine what the paths of sound transmission are, one rule is to seek paths that deliver the sound to the receiver
the fastest. These are called acoustic rays. If the speed of sound were constant throughout the ocean, acoustic rays
would consist of straight-line segments, with reflections off the surface and the bottom. However, because the speed
of sound varies in the ocean, most acoustic rays are curved.
Sound speed in seawater is general about 1,500 meters per second (5,000 feet per second) although this speed varies
with water density, which is affected by water temperature, salinity (the amount of salt in the water), and depth
(pressure). The speed of sound increases as temperature and depth (pressure), and to a lesser extent, salinity,
increase. The variation of sound speed with depth of the water is generally presented by a “sound speed profile,”
which varies with geographic latitude, season, and time of day.
In shallow waters of coastal regions and on continental shelves, sound speed profiles become influenced by surface
heating and cooling, salinity changes, and water currents. As a result, these profiles tend to be irregular and
unpredictable, and contain numerous gradients that last over short time and space scales. As sound travels through
the ocean, the intensity associated with the wavefront diminishes, or attenuates. This decrease in intensity is referred
to as propagation loss, also commonly called transmission loss. In general, in a homogeneous lossless medium,
sound intensity decreases as the square of the range due to somple spherical spreading. In other words, a source level
of 235 dB will have decreased in intensity to a received level of 175 dB after about 914 meters (1,000 yards).
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