T
UNED MASS DAMPERS / TUNED VIBRATION ADSORBERS
OVERVIEW
Application of a Tuned Vibration Absorber (TVA) is sometimes the best option for control of unwanted noise/vibration. This countermeasure is particularly appropriate when the noise/vibration issue occurs for a single frequency, or across a very narrow frequency range. The TVA basically consists of a mass and spring system which is added to the structure at a location which exhibits high displacement for the objectionable event. The mass is often selected from considerations of the modal mass of the structure for the resonance of the structure which contributes to the objectionable vibration. (10%) of the modal mass is often the starting point in the design and analysis of a TVA. However, performance of the TVA is not strictly limited to resonance response, though it will have its most profound effect for resonance response of a structure. It is sometimes possible to induce a "valley" in the response of a structure for a narrow frequency range using the TVA approach, even when no resonance exists there. The TVA will be most effective approximately where its own (designed) actuation resonance frequency occurs. The tuning frequency is adjusted via the TVA mass and the stiffness of its spring element. This spring element is typically a complex spring having stiffness and damping. A viscoelastic material is typically utilized for the spring element of the TVA.

VISCOELASTIC MATERIAL SELECTION
The viscoelastic material requirements of the TVA are much different than the viscoelastic material requirements of a surface damping treatment. For example, the inherent damping of the material is usually required to be much lower. Also, in order to ensure that the TVA remains tuned across broad temperature ranges at the application site, it is very desirable that the viscoelastic material has very stable modulus relative to temperature and frequency.

HISTORY AND COMPLICATIONS OF TVA DESIGNS
TVA designs have been utilized for many years in noise and vibration applications. Conceptually, it is a very simple approach. However, with most of these old designs there were numerous problem associated with production of the TVA such that frequency tolerances were unacceptably high. This generally rendered the countermeasure as non-feasible for the production-environment. Often, a provision was built into the design to "tune" the TVA by applying static prestrain to the viscoelastic material, thereby varying its stiffness. Our experience with these designs has been that the long-term time domain variation in modulus as a function of static prestrain (relaxation effects) can cause the TVA to be mis-tuned after some time. In addition, this approach can result in variability of the inherent damping of the viscoelastic material, since loss factor will typically vary as a function of static prestrain. Finally, this approach incurred extreme expense via the necessity to tune each-and-every TVA by hand. Again, under these circumstances, the net results were that the TVA approach was rendered unacceptable for a cost and production standpoint.

TVA FABRICATION AND MATERIAL PROCESSING
Damping Technologies, Inc. has developed proprietary processing techniques to eliminate the concerns described above regarding TVA designs. Damping Technologies has demonstrated the capability to design and produce TVA assemblies to within (1%) to (2%) of the target tuning frequency. This capability has enabled TVA designs to become a powerful tool in the menu of noise/vibration countermeasures.

ENVIRONMENTAL EXPOSURE ISSUES
Correctly designing a TVA is only the first step in successful implementation of the design. The TVA must be produced repeatably, with quality control, and it must also survive the environment to which it is exposed. Damping Technologies can specify its TVA designs for applications periodically exposed to very harsh environments such as:
  • High Temperature Exposure
  • Engine Oil Exposure
  • Transmission Fluid Exposure
  • Methyl Ethyl Ketone Exposure
  • Skydrol (Aircraft Hydraulic Fluid) Exposure
  • High Thermal Conductivity Issues
TVA DESIGN ISSUES
Damping Technologies has experienced structural dynamics engineers on staff who are available to review your particular noise/vibration issue for TVA applicability. Damping Technologies has a full structural dynamics experimental laboratory which can be utilized for design and development support. The keys to a successful implementation of the TVA countermeasure include:
  1. Establish a Good Understanding of the Structural Dynamics of Interest.
  2. Optimize the TVA to the Application Regarding Added Mass, Damping, and Tuning Frequency.
  3. Control Raw Materials and Processing Parameters to Achieve TVA consistency (Within 1% to 2% of Target Tuning Frequency).