The low-frequency (4-40 Hz) acoustic waves generated by undersea earthquakes are of great importance to monitor the low-level seismic activity associated with seafloor spreading ridges. To better understand the near-source interaction of seismic waves with the seafloor and the resulting generation of low-frequency acoustic waves, the wave propagation in a solid medium (the Earth's crust) and in the overlaying fluid medium (the ocean) were jointly simulated using a three-dimensional (3D) spectral finite-element code (SPECFEM3D). Due to numerical limitations of 3D simulations, the focus was on simple model configurations with a 1 Hz source located below a Gaussian seamount or ridge. The simulated acoustic waves (0-2.5 Hz) propagate as Rayleigh modes and are affected by modal dispersion; their horizontal speed increases away from the source and reaches the sound speed about 140 km away. The amplitude of the generated acoustic waves is affected by the shape of the seafloor topography above the seismic source, as well as their travel times to hydrophones. Consequently, localization of the acoustic sources by trilateration from arrival times may be biased by 3D-effects, and thus the seismic/acoustic conversion zone may not match the epicenter.