When an ion beam traverses a weak magnetic field through stationary magnetized electrons, broad band ion acoustic turbulence grows convectively. The k spectrum is investigated with an array of four electrostatic probes, using digital signal processing techiques to show how the noise is distributed in three-dimensional k space. The k vectors are usually spread symmetrically about the beam direction. Under certain conditions, however, they are concentrated at an oblique angle to the beam. This can be explained in terms of an E*B electron drift.