Ultrasonic welding is facilitated by providing an acoustic weak spot in the joint of the parts called an energy director. This is analogous to a fuse in an electrical circuit. From an acoustic standpoint, what is desired is point contact; ideally the apex of a triangular ridge running around the joint butted up against a flat surface on the mating part is the only contact between the two parts. Relatively large horn (sonotrode) contact area assures that the part couples to the horn and vibrates sypathetically with it. Large fixture contact area on the other part assures that it does not vibrate, and the relative motion of the two parts during application of ultrasound occurs only in the joint area. The apex of the energy director and the material very near it are then put in a state of rapid stress loading and unloading, causing repetitive deflection of the material that causes the molecules in the material to rub against one another and produce the heat necessary to promote melting. The energy director shape is generally accepted to have a 60-degree included angle across the point for semi-crystalline materials and a 90-degree included angle across the point for amorphous materials. Cones, pyramids, cross-hatch patterns, rounded ridges, and other variations are used in certain circumstances. Energy director height is normally in the range of 0.1 to 0.6 mm, but larger or smaller energy directors have been used in certain circumstances.