In a harmonic response step or modal dynamic step, the motion of nodes can be specified only as base motion. Restraints must be applied during the eigenfrequency extraction step to the degrees of freedom that will be prescribed in the harmonic response step. These degrees of freedom are grouped into a base. Base motion is a boundary condition used to specify the motion of nodes in a modal dynamic step, steady-state dynamic (mode-based or subspace-based) step, or random response step. Base motions can be defined by a displacement, a velocity, or an acceleration history. For an acoustic pressure, the displacement is used to describe an acoustic pressure history. If the prescribed excitation record is given in the form of a displacement or velocity history, the solver differentiates it to obtain the acceleration history. The default is to give an acceleration history for mechanical degrees of freedom and to give a displacement for an acoustic pressure. You prescribe the displacements and rotations that are associated with a base during the harmonic response step or modal dynamic step. | ||||||