Sound speed and the pseudo-Newtonian potential

We propose a special-relativistic correction to the sound speed calculated from the Paczynski and Wiita (1980) pseudo-Newtonian potential. in order to avoid divergence to infinity at the Schwarzschild radius. Under this correction, the re-scaled Sound speed approaches c/root3 at the Schwarzschild radius; otherwise, it diverges there. Spherical flows onto a black hole are briefly discussed.