Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. The intensity range of audible sounds is enormous. A signal, usually both audible and visible, signals the shooter to fire. Oregon prohibits sound systems being "plainly audible" at 50 feet. For audible frequencies above it is relatively constant. The pulse may be audible or implied. SPL, at the listening position intended, over the audible frequency spectrum. It may also be used to mean a fireball which creates audible sounds. Voice Evacuation is the second most common audible in a modern system. There is some debate as to the necessity of an audible utterance of niyyah. These clicks are audible to humans and extend into ultrasound frequencies audible to predators. However, many ESD events occur without a visible or audible spark. Ancient music is designated by the characterization of the basic audible tones and scales. There are values and meanings best expressed by certain visible or audible material. In 2020 Scholastic Audio began releasing an uncut audio version of the series on Audible.