The behavior of a solid spherical probe immersed in a warm isotropic collisionless plasma, biased to plasma potential, and excited from an alternating current source, is investigated for frequencies below the ion plasma frequency. A hydrodynamic description of the plasma is used, together with absorptive boundary conditions for the electrons and ions at the probe surface. Expressions are derived for the alternating potential in the plasma, as a function of frequency, of radial distance, of probe radius, and of the plasma parameters. Much of this potential is due to ion acoustic waves, the amplitude of which depends strongly on the ratio Rp of the probe radius to the Debye length. As soon as Rp exceeds the ratio ƒ/ƒpe of the excitation frequency to the electron plasma frequency, the amplitude is reduced much below its value for Rp = 0 corresponding to a point source. It is shown in particular that under some conditions, for values of Rp over a certain limited range, this reduction is independent of Rp. The smaller the ratio of the excitation frequency to the ion plasma frequency, the larger is this range.