Otoacoustic emissions

An otoacoustic emission (OAE) is a sound which is generated from within the inner ear. Having been predicted by Thomas Gold in 1948, its existence was first demonstrated experimentally by David Kemp in 1978 and otoacoustic emissions have since been shown to arise by a number of different cellular mechanisms 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 can 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. One theory is that they act to increase the discriminability of signal variations in continuous noise by lowering the masking effect of its cochlear ampliﬁcation.

Evoked OAEs
OAEs 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 $$f_1$$ and $$f_2$$ with particular intensity (usually either 65 - 55 dB or 65 for both) and ratio ($$f_1\mbox{ }:\mbox{ }f_2$$). The evoked responses from these stimuli occur at frequencies ($$f_{dp}$$) mathematically related to the primary frequencies, with the two most prominent being $$f_{dp}=2f_1-f_2$$ (the "cubic" distortion tone, most commonly used for hearing screening) and $$f_{dp}=f_2-f_1$$ (the "quadratic" distortion tone, or simple difference tone).

Clinical importance
Otoacoustic emissions are clinically important because they are the basis of a simple, non-invasive, test for hearing defects in newborn babies and in children who are too young to cooperate in conventional hearing tests. Many western countries now have national programmes for the universal hearing screening of newborn babies. The primary screening tool is a test for the presence of a click-evoked OAE. Otoacoustic emissions also assist in differential diagnosis of cochlear and higher level hearing losses (e.g., auditory neuropathy).

Biometric importance
In 2009, Dr Stephen Beeby of The University of Southampton, led research into utilizing otoacoustic emissions for biometric identitification. Devices equipped with a microphone could detect these subsonic emissions and potentially identify an individual, thereby providing access to the device, without the need of a traditional password. (Telegraph.co.uk, April 25, 2009, "Ear noise can be used as identification", http://www.telegraph.co.uk/scienceandtechnology/science/sciencenews/5219233/Ear-noise-can-be-used-as-identification.html). It is speculated, however, that colds, medication, trimming one's ear hair, or recording and playing back a signal to the microphone could subvert the identification process. (IEEE Spectrum Online, April 29, 2009, "Your Ear Noise as Computer Password", http://blogs.spectrum.ieee.org/riskfactor/2009/04/your_ear_noise_as_computer_pas.html)