  We report the propagation of high-intensity sound beams in a sonic crystal,
under self-collimation or reduced-divergence conditions. The medium is a fluid
with elastic quadratic nonlinearity, where the dominating nonlinear effect is
harmonic generation. The conditions for the efficient generation of narrow,
non-diverging beam of second harmonic are discussed. Numerical simulations are
in agreement with the analytical predictions made, based on the linear
dispersion characteristics in modulated media and the nonlinear interaction in
a quadratic medium under phase matching conditions.
