Hearing (or audition) is one of the traditional five senses. Senses are the physiological methods of Perception. The senses and their operation classification and theory are overlapping topics studied by a variety of fields It is the ability to perceive sound by detecting vibrations via an organ such as the ear. Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies The ear is the sense organ that detects Sounds The Vertebrate ear shows a common biology from Fish to Humans with variations The inability to hear is called deafness.
In humans and other vertebrates, hearing is performed primarily by the auditory system: vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain. The auditory system is the Sensory system for the sense of hearing. Vibration refers to mechanical Oscillations about an equilibrium point. The ear is the sense organ that detects Sounds The Vertebrate ear shows a common biology from Fish to Humans with variations The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain Like touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation. 
Hearing can be measured by behavioral tests using an audiometer. A hearing test is an evaluation of the sensitivity of a person's sense of hearing, most often performed by an Audiologist using an Audiometer. An audiometer is a machine used for evaluating hearing loss The invention of this machine is generally credited to Dr Electrophysiological tests of hearing can provide accurate measurements of hearing thresholds even in unconscious subjects. Such tests include auditory brainstem evoked potentials (ABR), otoacoustic emissions (OAE) and electrocochleography (EchoG). Auditory brainstem response (ABR is an electrical signal evoked from the brainstem of a human or other mammal by the presentation of a sound such as a click An otoacoustic emission (OAE is a Sound which is generated from within the Inner ear. Technical advances in these tests have allowed hearing screening for infants to become widespread.
Hearing threshold and the ability to localize sound sources are reduced underwater, in which the speed of sound is faster than in air. Underwater hearing is by bone conduction, and localization of sound appears to depend on differences in amplitude detected by bone conduction. Bone conduction is the conduction of Sound to the inner Ear through the Bones of the Skull. 
Not all sounds are normally audible to all animals. Each species has a range of normal hearing for both loudness (amplitude) and pitch (frequency). Frequency is a measure of the number of occurrences of a repeating event per unit Time. Many animals use sound to communicate with each other, and hearing in these species is particularly important for survival and reproduction. In species that use sound as a primary means of communication, hearing is typically most acute for the range of pitches produced in calls and speech.
Frequencies capable of being heard by humans are called audio or sonic. An audio frequency (abbreviation AF) or audible frequency is characterized as a periodic vibration whose Frequency is audible to the average human The range is typically considered to be between 15Hz and 20,000Hz.  Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic. Not to be confused with Supersonic. Ultrasound is cyclic Sound pressure with a Frequency greater than the upper Infrasound is Sound with a Frequency too low to be heard by the human Ear. Some bats use ultrasound for echo location while in flight. The microbats constitute the suborder Microchiroptera within the order Chiroptera ( Bats. Echolocation, also called biosonar, is the biological Sonar used by several Animals such as Dolphins Shrews most Bats Dogs are able to hear ultrasound, which is the principle of 'silent' dog whistles. The dog ( Canis lupus familiaris) is a domesticated Subspecies of the gray wolf, a Mammal of the Canidae family of the order For the use of political code words see Dog-whistle politics. Snakes sense infrasound through their bellies, and whales, giraffes and elephants use it for communication. A snake is an elongate Reptile of the suborder Serpentes Like all reptiles snakes are covered in scales. Whales are marine mammals which are neither Dolphins (ie members of the families Delphinidae or Platanistoidae) nor Porpoises Orcas The giraffe ( Giraffa camelopardalis) is an African Even-toed ungulate Mammal, the tallest of all land-living Animal Species Elephants ( family: Elephantidae) are large land Mammals of the order Proboscidea.
An audiogram is a standard way of representing a person's hearing loss Audiometry is the testing of hearing ability Typically audiometric tests determine a subject's hearing levels with the help of an Audiometer, but may also measure An auditory illusion is an Illusion of hearing, the aural equivalent of an optical illusion the listener hears either sounds which are not present in the stimulus Auditory brainstem response (ABR is an electrical signal evoked from the brainstem of a human or other mammal by the presentation of a sound such as a click In Psychophysics, auditory scene analysis (ASA is the process by which the human auditory system organizes sound into perceptually meaningful elements The auditory system is the Sensory system for the sense of hearing. A cochlear implant (CI is a surgically implanted electronic device that provides a sense of Sound to a person who is profoundly deaf or severely hard of hearing An equal-loudness contour is a measure of sound pressure ( dB SPL) over the Frequency spectrum for which a listener perceives a constant loudness for more detail on human hearing see Audiogram, Equal loudness contours and Hearing impairment. A sound is said to have a missing fundamental, suppressed fundamental, or phantom fundamental when its Overtones suggest a fundamental frequency but the Music is an Art form in which the medium is Sound organized in Time. Sounds and noises are only separated by the experience of the listener Presbycusis, or age-related Hearing loss, is the cumulative effect of aging on hearing. Tinnitus (tɪˈnaɪtəs or /ˈtɪnɪtəs/ from the Latin word for " Ringing " is the perception of sound within the human ear in the absence of corresponding