More on damping

At resonance the amount of energy lost due to damping is equal to the rate of energy supply from the driver. The driver is the source of external energy that keeps the oscillations going - for example, the person pushing a kid on a swing. Increasing the damping will reduce the size (amplitude) of the oscillations at resonance, but the amount of damping has next to no effect at all on the resonant frequency.

Damping also has an effect on the 'sharpness' of a resonance; sharpness is a not-very scientific way of describing how sensitively the resonance is tuned, and is sometimes called the 'Q-factor' by engineers. If damping is very small, a system will only oscillate a little if driven even slightly above or below the 'right' frequency - but when the driver hits the resonant frequency 'bang-on', suddenly the oscillations can get very large. Conversely, if damping is large, the amplitude of oscillations at resonance will decrease, but if the driver shakes (excites) the system at the 'wrong' frequency, the system will still respond quite strongly. This means there will be less of a resonant effect, but that it will happen over a larger range of frequencies.

The interactive animation below shows how the suspension of a car can be used to demonstrate resonance and damping.

The 'sharpness' of a resonance is measured by its Q-Factor. Although this does have a precise mathematical definition, it is most easily understood as roughly the number of free oscillations the oscillator will complete before decaying to zero. At the M.O.T. garage, the mechanic tests the damping of your shock absorbers by 'bouncing' the wings of the car...he probably doesn't know he is measuring Q-factor, but he does know that more than a 'bounce-and-a-half' and it fails!

So: a small amount of damping equates to a large Q, and a large amount of damping equates to a small Q.

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