As in the case of light, we can consider rays, for which each sound ray is perpendicular to the sound propagation wavefront. A point sound source can be defined, which produces emanating rays with equal power in all directions. This also results in power reduction at a quadratic rate as a function of distance from the source. Such a point source is useful for modeling, but cannot be easily achieved in the real world. Planar wavefronts can be achieved by vibrating a large, flat plate, which results in the acoustic equivalent of collimated light. An important distinction, however, is the attenuation of sound as it propagates through a medium. Due to energy lost in the vibration of molecules, the sound intensity decreases by a constant factor (or fixed percentage) for every unit of distance from the planar source; this is an example of exponential decay.