# Vibration absorber

Special types of vibration dampers are referred to as vibration absorbers (also known as absorbers or absorber pendulums for short ) . Vibration absorbers are not attached between two objects, but only on one. The natural frequency of the damper is matched to the resonance frequency of the object to be eliminated . At this frequency, the object then only performs small movements, below and above this frequency the amplitude of the object is greater than without a damper.

## Working principle

The world's largest pendulum absorber in the Taipei 101 building

Together with its own damper spring, the damper mass forms a mass-spring system , specifically a pendulum whose natural frequency is set to the vibration frequency to be eliminated (for example of the building). At this frequency, the damper can perform large deflections - the forces at the spring attachment point (= attachment point with the structure to be calmed) are therefore also large. The vibration absorber escapes at this frequency the structural vibrational energy for its own swinging motions, which is then finally converted into heat through friction.

However, the coupling of the two vibratory structures creates new natural frequencies below and above the damper natural frequency, which arise from the combination of the structure with the damper. At these two frequencies, the structure worsens as a matter of principle (stronger vibrations).

In a few simple cases, absorbers can be dimensioned using simple approximate formulas. However, as soon as the excitation is not monofrequency but broadband and not harmonic but transient , more detailed investigations are required in order to determine the optimum attenuation. This is often adaptive; that is, it can automatically adapt to the conditions.

There are also active hydraulic systems on pendulum absorbers that are able to bring the building's vibrations to zero.

A distinction must be made between:

• Ordinary (passive) absorbers,
• actively controlled damper and
• semi-active vibration absorbers, in which the energy required when needed is usually stored in the form of a pretensioned spring (blocking and release only above a certain threshold value).

## Applications

### Buildings

Pendulum absorbers are used to absorb building vibrations generated in particular by wind , but also by earthquakes and human influences. Classic use cases are e.g. B. pedestrian bridges , bridge pylons , steel chimneys or wide-span (steel) stairs or on machines for noise reduction .

Buildings that are in the catchment area of ​​industrial tremors or in earthquake areas are equipped in this way. The best known are the pendulum absorbers used in tall buildings, which prevent building vibrations caused by the wind from building up. The reason for their use is not only building security, but also convenience.

Examples of structures with vibration absorbers:

Four vibration absorbers on an overhead line

An electric overhead lines are Stockbridge damper used to dampen vibrations which are excited by the wind.

### Internal combustion engines

The vibrations excited by the gas and inertia forces in the crankshaft are reduced through the use of torsional vibration dampers , dual mass flywheels or balance shafts .

### vehicle construction

In the area of ​​the chassis, the steering (vibration absorber in the steering wheel), the body and the brakes.