I've been running a random vibration model where I have Random Vibration/Properties/Frequency Options set to Upper bound frequency. But now when I run the model, the random vibration status window indicates that there is a predefined(?) number of natural frequencies = 120. Initially I had run the model with the option of Number of Frequencies = 120, but that box is now grayed out, since I've chosen the Upper bound frequency option.
Under global damping, options is set to Modal damping with the first mode = 0 and the last mode = 120. I've had this setting all along, so I don't understand how the model is now indicating a predefined number of natural frequencies? I don't think I've changed anything that would cause the software not to calculate the number of natural frequencies instead of setting the number equal to 120.