FREQUENCY RESPONSE
In general, larger models may be solved more efficiently with modal method, because of the smaller size of the system of equations. Modal method is particularly advantageous if natural frequencies and mode shapes were computed in a previous stage of the analysis, using RESTART.
If there is a large number of excitation frequencies, then the modal method is more efficient.
The main effort in modal method is the computation of modes. If a large number of modes is required, this operation can be as costly as a direct solution, specially for high frequencies, where a direct method is more efficient.
The modal method is an approximation, as not all modes are considered, so the direct method is more accurate. The modal method requires computation of modes until a frequency 2 or 3 times larger than the higher excitation frequency.