  We define an abstract setting to treat wave equations equipped with
time-dependent acoustic boundary conditions on bounded domains of ${\bf R}^n$.
We prove a well-posedness result and develop a spectral theory which also
allows to prove a conjecture proposed in (Gal-Goldstein-Goldstein, J. Evol.
Equations 3 (2004), 623-636). Concrete problems are also discussed.
