ABSTRACT
In cases where an excitation frequency (for general periodic forcing this includes any of the integer multiples of the fundamental frequency) coincides with one of the shell’s natural frequencies, it may be desirable to shift the natural frequencies. This is a risky undertaking because avoidance of one resonance may create a resonance matching somewhere else in the frequency spectrum, except where all natural frequencies are shifted above a given excitation frequency. In any case, it is usually advantageous to stiffen rather than weaken a given shell to cause the desired shifts, because stiffening results in smaller vibration amplitudes in a general sense. Stiffening is generally accomplished by adding to a shell such supporting structures as beams (stringers), rings, shelves, and secondary shells.
