  In this paper, we consider a wave equation on a bounded interval where the
initial conditions are known (are zero) and we are rather interested in
identifying an unknown source term $q(x)$ thanks to the measurement output $y$
which is the Neumann derivative on one of the boundaries. We use a
back-and-forth iterative procedure and construct well-chosen observers which
allow to retrieve $q$ from $y$ in the minimal observation time.
