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For light damping the logarithmic decrement is approximately

  • A.\( \pi\xi \) with only half the correct constant
  • B.\( 4\pi\xi \) with twice the correct constant
  • C.\( 2\pi\xi \)
  • D.\( \xi/2\pi \) with the constant in the denominator

Answer: C

Correct answer: \( 2\pi\xi \)

The exact relation is

\[ \delta = \frac{2\pi\xi}{\sqrt{1-\xi^2}} \]

and for light damping the square root is close to one, leaving \( \delta \approx 2\pi\xi \).

The approximation is excellent at practical levels: at 5 per cent damping the error is about a tenth of one per cent. Inverting it gives the working formula \( \xi \approx \delta/2\pi \), which is how a decay record is turned into a damping ratio.

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