Energy dissipation identity for a linearly damped string
= Energy dissipation identity for a linearly damped string
For fixed endpoints and
$$
E(t)=\frac12\int_0^1(y_t^2+y_x^2)\,dx,
$$
integration by parts and the damped wave equation give
$$
E'(t)=-\int_0^1y_t^2\,dx\leq0.
$$