  We study the effect of a periodic density modulation on surface acoustic wave
(SAW) propagation along a 2D electron gas near Landau level filling $\nu=1/2$.
Within the composite fermion theory, the problem is described in terms of
fermions subject to a spatially modulated magnetic field and scattered by a
random magnetic field. We find that a few percent modulation induces a large
peak in the SAW velocity shift, as has been observed recently by Willett et al.
As further support of this theory we find the dc resistivity to be in good
agreement with recent data of Smet et al.
