Period of Oscillation of Underdamped System is Regular

Theorem
Consider a physical system $S$ whose behaviour can be described with the second order ODE in the form:
 * $\dfrac {\mathrm d^2 x} {\mathrm d t^2} + 2 b \dfrac {\mathrm d x} {\mathrm d t} + a^2 x = 0$

for $a, b \in \R_{>0}$.

Let $b < a$, so as to make $S$ underdamped.

Then the period of its movement is well-defined, in the sense that its zeroes are regularly spaced, and given by:
 * $T = \dfrac {2 \pi} {\sqrt {a^2 - b^2} }$

Proof
Let the position of $S$ be described in the canonical form:
 * $(1): \quad x = \dfrac {x_0 \, a} \alpha e^{-b t} \cos \left({\alpha t - \theta}\right)$

where $\alpha = \sqrt {a^2 - b^2}$.

The zeroes of $(1)$ occur exactly where:
 * $\cos \left({\alpha t - \theta}\right) = 0$

Thus the period $T$ of $\cos \left({\alpha t - \theta}\right)$ is defined by:
 * $\alpha T = 2 \pi$

and so:
 * $T = \dfrac {2 \pi} {\sqrt {a^2 - b^2} }$