  An error analysis of trigonometric integrators (or exponential integrators)
applied to spatial semi-discretizations of semilinear wave equations with
periodic boundary conditions in one space dimension is given. In particular,
optimal second-order convergence is shown requiring only that the exact
solution is of finite energy. The analysis is uniform in the spatial
discretization parameter. It covers the impulse method which coincides with the
method of Deuflhard and the mollified impulse method of Garc\'ia-Archilla,
Sanz-Serna & Skeel as well as the trigonometric methods proposed by Hairer &
Lubich and by Grimm & Hochbruck. The analysis can also be used to explain the
convergence behaviour of the St\"ormer-Verlet/leapfrog discretization in time.
