  We consider non-interacting bosonic excitations in disordered systems,
emphasising generic features of quadratic Hamiltonians in the absence of
Goldstone modes. We discuss relationships between such Hamiltonians and the
symmetry classes established for fermionic systems. We examine the density
\rho(\omega) of excitation frequencies \omega, showing how the universal
behavior \rho(\omega) ~ \omega^4 for small \omega can be obtained both from
general arguments and by detailed calculations for one-dimensional models.
