### 14.2.2. Dipole and Rotating Dipole Sources

Dipoles and rotating dipole sources arise from the fluctuation of forces on the surface. Sometimes these sources are also referred to as “loading noise.” Strictly speaking, these forces cannot be evaluated without knowledge of the acoustic field. However, the Lighthill analogy neglects any two-way coupling, and the forces due to acoustic fluctuations are much smaller than the dynamic forces. Hence, it is perfectly valid to only consider the dynamic forces created by the bulk fluid flow.

In addition, it is common to neglect the viscous component of the force because that is usually a smaller fraction of the normal (static pressure based) component of the force.

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