Damping vibration / oscillations

Damping is the name given to methods of removing energy from a vibrating object so that it vibrates less. That is particularly important if there is a possibility of resonance. Here are some common examples.

Soft padding stuck onto the panels of cars, such as the bonnet (hood) absorb vibrational energy.

The energy of a building swaying after an earthquake can be absorbed by friction or by using the ideas of resonance. In between the outer walls, plates held on one floor dip into a bath of oil on the next.  If an earthquake shakes the building the friction between the moving plates and the oil provides damping. These systems are most useful on upper floors where movement is likely to be the greatest.

Reducing earthquake damage by damping

Reducing earthquake damage by damping

Shock absorbing cross brace in a building reduces earthquake damage.

Shock absorbers absorb the energy of the vehicle as it bounces over bumps in the road.   Pictured is a suspension unit upon which the weight of a car rests. The unit has two main functions:

  • The springs are used to absorb the shock of bumps in the road.
  • At the centre is the shock absorber, this is designed to remove energy. Without it the car would continue to bounce up and down.

The car oscillates up and down on the springs, but without something to absorb the energy the bouncing would only die down slowly. The piston is pushed up and down in the cylinder through the oil. Energy is quickly absorbed by the oil being forced through the holes in the piston, quickly reducing the oscillation. This is damping.

Spring around the outside and shock absorber in the centre.

damping graphs

Damping is always more effective when the amplitude of vibration is large and therefore it is especially effective at the resonant frequency.

Graphing degree of oscillation and damping against frequency.

Every mechanical system, such as a swaying building or a vibrating panel, is damped by friction. If there is only a small amount of friction the energy loss is slow and the vibration continues for some time. This may be either uncomfortable or dangerous. A car with poor shock absorbers will continue to bounce up and down long after it has gone over a bump affecting the steering and control as well as being uncomfortable. It therefore makes sense to have heavy damping in most cases although not so heavy that the forces cause a shock to the structure.

Graphing amplitude of oscillation against time for a damped system

 

Notes upon resonance are here: