In recent years, the availability of high-resolution surface-sensitive experimental techniques such as Helium atom scattering (HAS) and electron energy loss spectroscopy (EELS) has opened new perspectives in the study of the dynamical properties of solid surfaces. In particular, a great deal of experimental data are now available on the phonon dispersions of surfaces of insulator, semiconductors, and metals [1]. Among transition metals, the vibrational properties of the hydrogenated W(llO) surface have attracted much attention because they display unusual and unexpected features which are still poorly understood [2-6]. When a full monolayer of hydrogen is adsorbed, the surface phonon spectrum undergoes a dramatic change and an anomalous behavior appears.