An otoacoustic emission (OAE) is a sound that is generated from within the inner ear. Having been predicted by Austrian astrophysicist Thomas Gold in 1948, its existence was first demonstrated experimentally by British physicist David Kemp in 1978, and otoacoustic emissions have since been shown to arise through a number of different cellular and mechanical causes within the inner ear. Studies have shown that OAEs disappear after the inner ear has been damaged, so OAEs are often used in the laboratory and the clinic as a measure of inner ear health. Broadly speaking, there are two types of otoacoustic emissions: spontaneous otoacoustic emissions (SOAEs), which occur without external stimulation, and evoked otoacoustic emissions (EOAEs), which require an evoking stimulus. Mechanism of occurrence OAEs are considered to be related to the amplification function of the cochlea. In the absence of external stimulation, the activity of the cochlear amplifier increases, leading to the production of sound. Several lines of evidence suggest that, in mammals, outer hair cells are the elements that enhance cochlear sensitivity and frequency selectivity and hence act as the energy sources for amplification. Types Spontaneous Spontaneous otoacoustic emissions (SOAEs) are sounds that are emitted from the ear without external stimulation and are measurable with sensitive microphones in the external ear canal. At least one SOAE can be detected in approximately 35–50% of the population. The sounds are frequency-stable between 500 Hz and 4,500 Hz and have unstable volumes between -30 dB SPL and +10 dB SPL. The majority of those with SOAEs are unaware of them, however 1–9% perceive a SOAE as an annoying tinnitus. It has been suggested that "The Hum" phenomena are SOAEs. Evoked Evoked otoacoustic emissions are currently evoked using three different methodologies. Stimulus-frequency OAEs (SFOAEs) are measured during the application of a pure-tone stimulus and are detected by the vectorial difference between the stimulus waveform and the recorded waveform (which consists of the sum of the stimulus and the OAE). Transient-evoked OAEs (TEOAEs or TrOAEs) are evoked using a click (broad frequency range) or toneburst (brief duration pure tone) stimulus. The evoked response from a click covers the frequency range up to around 4 kHz, while a toneburst will elicit a response from the region that has the same frequency as the pure tone. Distortion-product OAEs (DPOAEs) are evoked using a pair of primary tones and with particular intensity (usually either 65–55 dB SPL or