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12.2
Transmission Losses
The transmission of an acoustic pulse is generally called a ‘ping’. When the projector sends out the
acoustic pulse many factors operate on that pulse as it moves through the water column to the
bottom and also on its return upward. The major influence of the water column sound velocity
characteristics was detailed above; this affects the speed of transmission (and return). There are
other influences that will affect acoustic energy in water and these are transmission losses.
12.2.1
Spreading Loss
Spreading loss does not represent a loss of energy, but refers to fact that the propagation of the
acoustic pulse is such that the energy is simply spread over a progressively larger surface area, thus
reducing its density. Spreading loss is not frequency dependent.
12.2.1.1
Spherical Spreading
Spherical spreading loss is the decrease in the source level if there are no boundaries (such as the
water surface or sea floor) to influence the acoustic energy; all of the acoustic energy spreads out
evenly, in all directions, from the source. The loss in intensity is proportional to the surface area of
the sphere. The intensity decreases as the inverse square of the range for spherical spreading. With
Spherical spreading, the transmission loss is given as:
TL = 20log(R)
, where R is range
Figure 91: Concept of Spherical Spreading
12.2.1.2
Cylindrical Spreading
In reality the acoustic energy cannot propagate in all directions due to boundaries such as the sea
floor and the water surface; this give rise to Cylindrical Spreading. Cylindrical spreading is when the
acoustic energy encounters upper and lower boundaries and is ‘trapped’ within these boundaries;
the sound energy begins to radiate more horizontally away from the source. With Cylindrical
spreading the acoustic energy level decreases more slowly than with Spherical spreading. With
Cylindrical spreading, the transmission loss is given as:
TL = 10log(R)
, where R is range.
Point Source
of Acoustic
Energy
Summary of Contents for 2022
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