We show that the delivery of fracture work to the tip of an advancing planar crack is strongly reduced by surface phonon emission, leading to forbidden ranges of crack speed. The emission can be interpreted through dispersion of the group velocity, and Rayleigh and Love branches contribute as well as other high frequency branches of the surface wave dispersion relations. We also show that the energy release rate which enters the Griffith criterion for the crack advance can be described as the product of the continuum solution with a function that only depends on the lattice geometry and describes the lattice influence on the phonon emission. Simulations are performed using a new finite element model for simulating elasticity and fractures. The model, built to allow fast and very large three-dimensional simulations, is applied to the simplified case of two-dimensional samples.