Starting in LS-DYNA v971, nodal accelerations can be filtered before being output to the ASCII file “NODOUT”. The type of filter can either be a built-in filter based on low-pass Butterworth frequency filter or can be user-defined. The filtering process is turned on by setting the IACCOP=2 in *CONTROL_OUTPUT keyword. The filtering process works by storing the nodal instantaneous accelerations at every compute cycle until a output state is reached. When the output state is reached, then the previously stored acceleration profile starting from the previous output state (or from time zero if the current output state is the first) is filtered and the last ordinate value of the filtered data is written to the NODOUT ascii file. To allocate sufficient memory for the storage, DT2MS parameter in *CONTROL_TIMESTEP must be defined so LS-DYNA can estimate the amount of required storage. For more information, please see *CONTROL_OUTPUT keyword in the LS-DYNA Keyword Users Manual.