The radiation pressure of sound waves on a spherical obstacle is used to determine the distribution of intensity in the sound field of a quartz disk radiating into a liquid medium. The measured distribution of intensity along the axis normal to a crystal face is compared to that to be expected from a plane piston radiating into a semi-infinite medium, and the agreement shows that the theoretical assumptions are fulfilled for the case used and that the "effective area" of the piston is essentially the actual area of the quartz disk.