The direction the meter faces is a function of the meter spec. Some SPL meters specify that they should be pointed directly towards the source of the sound, other specify that they should be pointed perpendicular to a line drawn between the meter and the source of the sound. It depends on the sensitivity map of the microphone element itself. Whether or not it is directional in nature. I.E. Uni-directional or super-cardioid pattern microphones would need to be pointed at the source. Omni-directional and cardioid pattern would be pointed at 90 degrees. The Omin-directional type will take into account ambient sounds more than the uni-directional kind.