Numerical experiments have been performed in order to investigate the convection in an enclosure of aspect ratios 5:1:1.3 subject to a horizontal temperature gradient and a longitudinal sound field. The governing equations are solved by a spectral element method. Different flow structures appear when increasing (or decreasing) the Grashof number. Without acoustic field (acoustic Froude number Fr = 0), a hysteresis occurs connected to a first steady bifurcation with breaking of symmetry, but for Fr not equal 0, no hysteresis is observed as this symmetry is no more effective. The further transition to oscillatory flow is found to be stabilized by the acoustic field. Depending on Fr, this transition can occur with or without the breaking of the left-right symmetry. (c) 2006 Elsevier Ltd. All rights reserved.