Download LYBIN 6.0 - User Manual

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negligible at low frequencies, but significant close to the bubble resonance frequency
around 55 kHz.
Bottom loss - is estimated using empirical data for a set of predefined bottom types.
Refer to 7.1.5 for how to set the bottom type. The loss is a function of bottom type and
grazing angle. Predefined angles and corresponding losses are stored in LYBIN.
Thermal absorption - conversion of the elastic energy of a sound wave into heat. This
results in a heating up of the medium. Takes into account boric acid relaxation,
magnesium sulphate relaxation and viscosity.
Scattering – not by itself a loss mechanism, but the results of scattering can be
measured as loss of energy. Unlike several other models which treat scattering as
a loss, LYBIN attempts to simulate the scattering process itself. When a sound ray
hits the surface, it is reflected. Due to ocean waves, the reflection is not
necessarily specular. Scattering refers to the fact that the reflection angle is
somewhat random.
To read the plot, use the colour coding. To determine the transmission loss at an arbitrary position
in the plot, search for the colour of that location in the colour coding. The value of the intensity
loss (dB) is written above this colour.
5.3
Reverberation Curves
The reverberation curves plot graphically illustrates the calculated reverberation from the sea
surface, the water volume, and the sea bottom. The total noise level, calculated from ambient and
self noise, is also included in the plot. The reverberation and noise are plotted in dB/µPa as a
function of distance from the sonar. Noise is shown in green, the surface reverberation in blue, the
volume reverberation in red, the bottom reverberation in brown, and the total masking level is
shown in black. The total masking level is the sum of all reverberation and noise present.
Figure 5.3 gives an example of the reverberation curves plot.
FFI-rapport 11/00205
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