Technical Note:
Narrowband Damping with Tuned Vibration Absorbers

The primary reason why Newport RS Series Passive Damped Optical Tables are superior to all other products is their use of tuned vibration absorbers for narrowband damping. Commonly called "Tuned Dampers," these devices selectively eliminate a fundamental structural table mode and its entire harmonics. Multiple tuned dampers can be selected to eliminate multiple modes along with the resulting mode harmonics. Tuned damping is the most effective means of reducing relative motion across the table surface.

Narrowband "Tuned" Dampers vs. Broadband "Dry" Dampers

Tuned damping techniques use individual mode selected vibration absorbers to both eliminate a particular "narrow" mode and mode harmonics across the broader band. On the other hand, "dry" damping techniques use one design for all modes to indiscriminately absorb moderate amounts of vibration over the broadband. Using a single design "dry damper "is less expensive than specifically selected "tuned dampers" but not as effective in reducing the amplitude of resonance modes. This is especially true for the lower frequency resonances found on large tables where broadband techniques are not very effective. In contrast, Newport stocks tuned dampers for large tables down to 20 Hz. For smaller tables, dampers up to 480 Hz are also stocked. Figure 1 compares the typical effects of narrow-band vs. broadband damping on an individual mode.

Newport Tuned Damping - Broadband Damping

Damping charts-S
Figure 1: Newport tuned dampers (left) concentrate damping where it's needed most, at the frequencies of dominant resonance modes. Since broad-band dampers (right) are designed to provide moderate damping over a wide range of frequencies, they are not as effective at damping the dominant modes of table vibration.

Another problem with dry dampers is their weight. Since such dampers are not selective, they must be quite heavy to be effective. Unfortunately, this additional weight is added to the ends of the table and can significantly degrade the bending mode performance. The heavy ends and poor bending mode performance greatly increase unwanted relative motion in the center of the table where most experiments are built. Newport tuned dampers are much lighter and are distributed to produce minimal mass loading impact on the fundamental table modes.

Finally, "dry damping" techniques use adhesive tapes in a constrained layer configuration. Although industrial tapes have improved, they can still be subject to delamination - especially if constraining plates are not properly cleaned before assembly, or are not flat. The mechanical properties of the constrained layer can change over time resulting in a degradation of their damping effects. Newport "Tuned Dampers" use a rugged, fully welded construction. The low vapor pressure oil viscous damping fluid is hermetically sealed into the dampers and will not degrade over time.

3 Damper, Tunable
Figure 2: Schematic of Newport vibration absorber (left) used in tuned damping table systems compared to less effective constrained layer (right) damper.

Unlike competitive offerings that can only offer several table tops with obscure names such as "nominal" damping, Newport tables can be tuned damped into thousands of configurations depending on the table geometry and individual experimental need. Most of these configurations fall into 4 product offerings: RS4000™, RS3000™, RS2000™ and RS1000™. However, custom tuned tables are available to account for special geometries and heavily loaded surfaces.

How an RS Series table is "Tuned"

A vibration engineer trained in structural analysis tunes each RS Series table design. The process begins with an FEA model of the table estimating the structural modes, what dampers should be selected and where they should be located. The table is then built with provisions for adding the dampers. After construction, the table is tested using modal analysis techniques to experimentally determine the actual modes and to verify the accuracy of the FEA model. Different table sizes will exhibit different fundamental modes. Even tables that are close in size can be dramatically different. This is a primary reason why the "one size fits all" approach to damping is a poor choice.

For example: Newport's RPR Series Industrial and Educational Grade Optical Tables are the mechanical equivalent of the RS Series Passive Damped Optical Tables but without the tuned dampers. The RPR-38-8 (3 ft wide, 8 ft long, 8 in. thick) exhibits a fundamental "bending" mode at 223 Hz. The most effective location for the damper to absorb the bending mode is in each end of the table. In contrast, the RPR-48-8 (4 ft wide) exhibits a fundamental "torsion" mode at 217 Hz. The most effective location for the damper to absorb the torsion mode is in the corners of the table. The plots in Figure 3 show the FEAs for these tables.

VC-Torsion mode_large-S
Figure 3
Bending mode rev1-S
First bending mode

The example above shows the basic process used for determining the proper damper and location to eliminate the fundamental mode. Once the dampers are built into the table to attenuate this mode, the table is transformed from an RPR to an RS1000. When building an RS2000 the process is repeated and the next major mode eliminated. This process is reiterated for the RS3000 and RS4000 consecutively.

Specialized Tuning

Newport's tuned damping techniques allow the table construction to be tuned for specific applications.

Doubled Tables: Modular Doubled Tables are multiple tables that are doubled together into complex monolithic structures that can exhibit very low frequency fundamental modes. When building these large structures, Newport assembles the doubled tables into their final configuration and tunes them as a monolithic system. Broadband techniques are very ineffective for such complex low frequency structures.

Large Table Loads: Tuned damping techniques are not effected unless the equipment load exceeds 25% of the table weight. For example, an RS4000-48-12 weighs close to 1200 lbs - if the table load starts to exceed 300 lbs, the tuned damper will not be as effective. In these cases, Newport will simulate the positional load and specially damp the table to the application requirement. Fortunately most optical table experiments rarely exceed 25% of the table mass.

Special Shapes: Table structures with special shapes, cutouts, multiple levels and other custom features are modeled and tuned using the same process described above.

Advantages of Narrowband Tuned Dampers

  • Eliminate fundamental mode resonances
  • Offer much better vibration absorption at low frequencies
  • Lighter weight does not mass load table construction
  • Hermetically sealed damping fluid does not change over time

Problems with "Dry Dampers"

  • Less effective in reducing vibration amplitudes
  • Does not work well for large tables with low frequency modes
  • Heavy weight reduces table performance
  • Elastomer tape can degrade with time