### ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

## 3pSA3. Influence of dynamic absorbers on a three-dimensional truss.

**Denis Branthonne
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Ira Dyer
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**
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*Dept. of Ocean Eng., MIT, Cambridge, MA 02139
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The objective of this research is to explore experimentally the effects of
adding dynamic absorbers to a three-dimensional truss and to design analytical
models which predict the phenomena of attenuation. The truss is built with
cells made of aluminum rods and connected in series. 110 dynamic absorbers are
mounted on the rods in the center cell to achieve a local mass ratio of 3. The
truss is excited at one end with white noise to measure the spatial attenuation
averaged on octave bands as a function of axial distance. The data are compared
to the spatial attenuation for the undamped structure. The differences
determine the effect of the dynamic absorbers. The overall shape of the
differential curves of attenuation versus axial distance is a step function,
with the step located in the dynamic absorbers attachment area. Assuming the
equipartition of energy between the different wave types and using the
classical theory of attenuation of waves propagating on a semi-infinite rod
loaded with a continuous layer or dynamic absorbers, one predicts stronger
attenuation than experimental data at low frequencies. The assumption of
equipartition of energy proves to be incorrect and the applicability of the
semi-infinite model is questioned. Subsequently, a new theory is derived and
validated by experiment to describe the attenuation of waves on finite rods
loaded with a layer of dynamic absorbers. At low frequencies, this model
achieves a better estimation of the axial attenuation along the truss. [Work
supported by ONR.]