Underwater sounds are classified according to whether they are transient or continuous. Transient sounds
are of short duration, often called pulses, and they may occur singly, irregularly, or as part of a repeating
pattern. For instance, an explosion represents a single transient, whereas the periodic pulses from a ship’s
sonar are patterned transients. Underwater sounds also can be classified as continuous, that is, they occur
without a pause or hiatus. Continuous sounds are further classified as periodic, such as the sound from
rotating machinery or pumps, or aperiodic, such as the sound of a ship breaking ice.