  A close inspection of neutron spectra of glass formers in the frequency
region above the boson peak does not show the constant eigenvalue density
expected for a random dynamical matrix, but a slow decrease towards higher
eigenvalues. In 1,4-polybutadiene and selenium, the slope becomes steeper with
increasing temperature in the undercooled liquid, suggesting an influence of
the vibrational entropy. One can describe the behaviour quantitatively in terms
of a balance between level repulsion (from the randomness of the dynamical
matrix) and vibrational entropy. On the basis of this evidence, the boson peak
must be a crossover from such a balance at higher frequencies to a mixture of
sound waves and additional excitations at low frequency.
