The bounda_unknown or bounda_force record with the same index is imposed with the sum of the sine functions; the first sine function has frequency freq_0 and amplitude amp_0, the second sine function has frequency freq_1 and amplitude amp_1, etc.. The sine functions starts at time 0. More general behavior in time can be imposed by using bounda_time records. For a specific index only one of bounda_time and bounda_sine can be specified.
As a typical application the response due to the excitation with a frequency spectrum can be analyzed. Just print the relevant response by control_print_history and extract the frequency spectrum of that response signal.
The sine loads will be only imposed after start_time.