The theoretical prediction that significant regions of attenuation exist in the transmissibility curves of cantilever beams driven by dual forces of like magnitude and phase has been substantiated by the results of reciprocal experiments made on small aluminum beams and a laminated steel/viscoelastic beam. The attenuation provided by the laminated beam, which had high damping, was remarkably extensive and exceeded 15 dB at all frequencies above 100 Hz and 20 dB at all frequencies above 1.2 kHz. Agreement between theoretical prediction and reciprocal experiment has also been noted in two other cases:first, where the large region of attenuation predicted for a uniform cantilever beam was doubled in width and deepened by a factor of more than 10 when the beam was loaded by a suitably positioned lumped mass of similar magnitude to the beam mass (in fact, the resultant region of attenuation was bounded by frequencies that differed in value by a factor of 21 and was greater than 40 dB at its lowest point); andsecond, where a uniform cantilever beam was driven by forces that were not derived from an isolated source but, rather, were transmitted by two resilient mountings that supported a vibrating mass, which was used to represent a machine. Because the natural frequency of this simple mounting system fell well below the fundamental resonant frequency of the cantilever beam, the mounting system and the beam vibrated more or less independently. In consequence, the attenuation afforded by the simple mounting—essentially 12 dB/octave—superimposed on the region of attenuation noted previously in the beam transmissibility to provide an extremely large and continuous region of attenuation in the transmissibility across the combined simple mounting/beam system at all but low frequencies.