Tuned Absorbers/Dampers

Tuned Absorbers/Dampers

Tuned dynamic absorbers and tuned mass dampers are reactive devices used in structural and acoustic systems to either absorb oscillation at a certain forcing frequency or damp oscillation at a particular resonant frequency. The make up of tuned absorber and tuned damper are for the most part the same, i.e., they are both made up of an inertia element (mass), a resilient element (spring), and an energy dissipating element ( damper, mainly of viscous type). What distinguishes one from the other is the extent of energy dissipation in their dissipative element. Tuned absorbers have negligible but tuned dampers have sizeable amount of damping (energy dissipation).


Vibration Control

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Dynamic absorbers and tuned mass dampers are the realizations of tuned absorbers and tuned dampers for structural vibration control applications. The inertial, resilient, and dissipative elements in such devices are: mass, spring and dashpot (or material damping) for linear applications and their rotary counterparts in rotational applications. Depending on the application, these devices are sized from a few ounces (grams) to many tons. Other configurations such as pendulum absorbers/dampers, and sloshing liquid absorbers/dampers have also been realized for vibration mitigation applications.

Figure 1 shows an underdamped structure (M1, K1, C1) with the resonant frequency of 9.5 Hz subject to a periodic excitation at the frequency of 35 Hz. A tuned mass damper (M2, K2, C2) tuned to the resonant frequency of the structure, i.e., 9.5 Hz and a tuned dynamic absorber (M3, K3, C3) tuned to the excitation frequency of 30 Hz are appended to the structure to mitigate its vibration. Figure 2 depicts the frequency response functions of the structure without (the black/dashed line trace) and with (the red/solid line trace) the vibration mitigation treatment. Clear from Figure 2, tuned damping induced by the tuned mass damper and dynamic absorption induced by the tuned dynamic absorber, are substantial.

The point worth noting again is that, as shown in Figure 1, the make up of tuned mass dampers and dynamic absorbers are not much different. What distinguished the two from each other is the amount of damping (energy dissipation) incorporated into their make up; a sizeable amount in tuned mass dampers and a negligible amount in tuned dynamic absorbers.


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Figure 1 A structure treated with a tuned mass damper and a tuned dynamic absorber

TMD_dyn_abs

Figure 2 Frequency response function of the structure without and with vibration treatment

Sound Control

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Helmholtz resonators, quarter-wave tubes, perforated liners, and passive acoustic radiators are the common realizations of tuned absorbers/dampers for acoustic applications. They are commonly used in narrow-band acoustic treatment of various architectural, industrial, and aerospace acoustic environments.

perforated_HR

Figure 3 A perforated acoustic liner

Tuning of Tuned Absorbers/Dampers

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The parameters of a tuned absorber/damper are selected such that their resonant frequencies match the disturbance frequency (for tuned absorption) or resonant frequency of a particular mode (for tuned damping) of an structural/acoustic system being treated. In case of tuned damping, as stated before, enough energy absorption (damping) should be built into the tuned damper so that the resonant mode targeted for damping does truly get damped rather than get split into two modes (one with a higher and one with a lower natural frequency of the tuned frequency); a phenomenon commonly known as mode splitting.

Another important factor directly affecting the effectiveness of tuned absorbers/dampers in structural applications is the size of the inertia element. The larger the inertia element, the more effective the device. Constrained by the packaging and other limitations on the system being treated, e.g. a structure, the inertia element of the tuned treatment is normally around 1 to 5% of the oscillating inertia.

Active Tuned Absorbers/Dampers

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As in any technology, passive tuned absorbers/dampers have their own shortcomings of:

  • They are large in size when tuned to low frequencies
  • They are single frequency (narrow band) treatments. When multiple frequencies need to be treated, a number of tuned absorbers/dampers should be used; see Figures 1 and 2. This takes away from the simplicity and small size/mass attributes of the device.
  • They are rather sensitive (absorbers more than dampers) to accurate tuning, and lose effectiveness when detuned.
  • This issue has been addressed by the use of multiple tuned absorbers/dampers with slightly different tuning frequencies. Again, such remedy takes away form the simplicity and small size appeal of these tuned devices.

The alternative to passive tuned absorbers/dampers is active tuned absorbers/dampers, which is either a stand-alone active element (actuator) made to behave like a tuned absorber/damper or an active element introduced into the make up of a traditional tuned absorber/damper, e.g., a linear actuator placed in parallel with the spring, between the structure and the tuned mass, in a dynamic absorber or tuned mass damper. The latter configuration is also known as proof mass absorber/damper (or in more general terms, proof mass actuator). An attractive attribute of proof mass dampers is that they can be configured, via the active controller, to act also as a broadband (not just tuned) damper, e.g. like an added dashpot.

Figure 4 shows the schematic of an active tuned absorber/damper with the active element U (the actuator).

Active tuned absorbers/dampers have higher effectiveness than their passive counterparts and can readily be readjusted (re-tuned) via their electronics/software (manually or automatically). Moreover a single active system can be tuned to multiple frequencies, simultaneously. They are also smaller in size than their passive counterparts.

Figure 4 An active dynamic absorber/tuned mass damper

Optimally sized passive tuned absorbers/dampers used in very low-frequency applications would be too large and too massive to be practical. Such devices are normally undersized and as such have less than desireable effectiveness.

Semi-active Tuned Absorbers/Dampers

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Although more effective and versatile, active absorbers/dampers are more elaborate and costly than passive tuned devices. In addition, the usual concerns associated with all active systems, i.e., stability robustness, actuator saturation, etc. hold true for active tuned absorbers/dampers too.

An alternative to both passive and active tuned absorbers/dampers is passive control solution with some degree of adjustability known as semi-active vibration control. For example a tuned mass damper with an adjustable stiffness is a semi-active vibration control device. By occasional adjustment of the adjustable the tuned absorber/damper can be re-tuned, manually or automatically, so that its effectiveness even when the structural parameters are changing can be always maintained.


With extensive experience in design and implementation of tuned absorbers/dampers in both structural and acoustic applications, DEICON can analyze your sound/vibration problem and customize the most optimal solution, including tuned absorption/damping, for it.