mike944 points out one of the limitations behind using non-resonance vibratory stress relief:  limited effectiveness.  A more effective means of performing vibratory stress relief is to use resonance, but the equipment must have better vibrator speed regulation, plus be of a more rugged design to survive the "rough-ride" it will be subjected to. 

Also, independent research has shown that resonance is superior to non-resonance.  This research can be seen at http://vsrtechnology.net/library.htm    The works of Hahn while at Alfred University, Shankar at the Oregon Graduate Center, and Yang el al from the Edison Welding Institute, compare resonance vs. non-resonance vibratory stress relief effectiveness.

Our firm has been able to stress relieve a wide range of high-rigidity workpieces using resonance vibration, ranging from satelitte components to massive press frames and machine tool bases.

BK