The difference between audible and inaudible sounds is their frequency ranges; it is relative to the ability of the human ear to only perceive as audible those sounds that fall within a frequency range between 20 and 20,000 hertz. Other organisms, such as insects, dolphins, dogs and bats, are capable of perceiving sounds that are outside of the frequency range that is audible to humans. Certain species of insects, for example, are capable of perceiving sounds that are four times as high as the upper sound frequency limit of human hearing.
What is perceived as sound is a mechanical wave, or vibration, with the properties of displacement and pressure. Unlike electromagnetic waves, sound waves require a medium to travel through and can not propagate through a vacuum. Solids, liquids and gases all carry sound waves, although the physical properties of a substance affect the speed at which they travel.
What is commonly referred to as "sound" is actually the perception of the kinetic energy of sound waves by the brain. The human auditory organs are unable to react to sound waves above or below what is termed the audible range, which represents sound waves with wavelengths falling within the lower frequency range of about 55.75 feet, and at the upper frequency limit, about two-thirds of an inch. The higher-frequency sounds above the audible limit are referred to as ultrasound.