  A viscoelastic model is established to reveal the relation between alpha-beta
relaxation of glass and the double-peak phenomenon in the experiments of
impulse excited vibration. In the modelling, the normal mode analysis (NMA) of
potential energy landscape (PEL) picture is employed to describe mechanical
alpha and beta relaxations in a glassy material. The model indicates that a
small beta relaxation can lead to an apparent double-peak phenomenon resulted
from the free vibration of a glass beam when the frequency of beta relaxation
peak is close to the natural frequency of specimen. The theoretical prediction
is validated by the acoustic spectrum of a fluorosilicate glass beam excited by
a mid-span impulse. Furthermore, the experimental results indicate a negative
temperature-dependence of the frequency of beta relaxation in the
fluorosilicate glass S-FSL5 which can be explained based on the physical
picture of fragmented oxide-network patches in liquid-like regions.
