The interaction of two-dimensional standing sonic waves with particles suspended in a fluid is analyzed. The domain considered is bounded below by a plane plate which performs harmonic oscillations, and above by a stationary plate which is slightly curved. The size of the gap at the axis of symmetry exceeds the sonic wavelength, so that there is at least one velocity node of the standing wave inside the region. The domain is filled with a fluid which contains spherical particles. The steady sonic wave causes a steady particle drift. The curvature of the stationary upper wall produces a two-dimensional standing wave, which causes the suspended particles to move in a direction transverse to the wave front of the applied field. It is shown that in addition to the well known particle drift toward the fluid velocity nodes or antinodes there exists a side drift along the nodes and the antinodes. The direction of the drift depends on the sonic wave frequency and on the fluid-to-particle density ratio. The analysis employs a small parameter perturbation method.