Otoacoustic emissions are sounds produced by the inner ear and measured in the ear canal. There are four main types, including spontaneous and transiently evoked emissions produced without or in response to sound. Otoacoustic emissions are used clinically to screen hearing, estimate cochlear sensitivity, and differentiate sensory from neural hearing loss. They provide an objective, noninvasive window into cochlear function and can detect hearing losses as mild as 30-40 dB.

1. Otoacoustic emissions(physiogical
measures of auditory sensitivity)
Otoacustic emissions are acoustic singnals emitted from the cochlea to the middle ear & the external
ear canal where they are recorded.They are generated by active mechanical contraction of outer
hair cells, spontaneously or in response to sound.
There ar four types of OAEs:spontaneous OAEs(SOAE)
Transient evoked OAE(TOAE)
Distortion product OAE(DPOAE)
Stimulus frequency OAE(SFOAE)
All four are recorded with a sensitive ,low noise recording microphone that is placed in the sealed
external ear canal.
When OAE are evoked ,the sealed probe include a sound delivery to external ear canal in addition to
the recording microphone.
Middle ear &inner ear disease might reduce or block acoustic transmission of OAE from the cochlea
to the recording microphone. So tympanometry is done before OAE recording.
Transient-evoked emissions
In human a delay between stimulus offset & onset of the evoked emission varies between 4ms for
high frenquencies,& 20ms for low frequencies.
The TEOAE are typically presented as an amplitude/time plot of the acoustic waveform recorded
from ear canal.
TEOAEs greater than 20db sound pressure level(SPL) can be recorded from newborn,while response
from children & adult range between 10 &15dbSPL.
TEOAEs can be altered in the presence of contralateral stimulation.Typically the effect of
contralateral stimulation is an attenuation of the response which is attributed to efferent effects on
the cochlea.The attenuation is most evident beginning 8ms after stimulus onset suggesting a
brainstem mediated effect.
Distortion product emissions:
DPOAEs are generated in the cochlea in response to two simultaneous pure tone stimuli. The
primary tones (f1&f2) are separated in frequency within 1/3rd octave(typically f2= f1×1.2)& the
distortion product is then typically at a frequency of 2f1-f2.

2. Although DPOAEs are reliably recorded in presence from all normal human,their magnitude is very
small 5-15dbSPL ,approximately 60-70db below the level of the stimuli used to evoke them.DPOAEs
attributed to nonlinearity of motion of the outer hair cells particularly at low stimuli levels.
PDOAEs are typically presented in a magnitude/frequency plot in which frequency is determined by
f2 at low levelor the geometric mean of f1&f2 &magnitude is determined for DPOAEs at the 2f1-f2
frequency bin.Such plot is called a DB-gram.& It has been shown to correlate with functional
integrity of the cochlea.
Otoacoustic emissions(K.j.Lee)
Otoacustic emission(OAEs) are acoustic signals generated by cochlear OHC & transmitted out
through the middle ear to the ear canal where they can be recorded by a sensitive microphone in a
quite ,but not usually sound treated environment.
As a measure of OHC function,OAEs can be used to screen hearing,to estimate cochlear sensitivity by
frequency,& to differentiate between sensory & neural hearing loss.
They cab be used neonates &comatose patients.
Two types of EOAEs are in common clinical use,Transient otoacoustic emission(TOAEs) &distortion
product otoacoustic emission(DPOAEs).Sustained frequency otoacoustic emission(SFOAMs) have no
clinical use.
Characteristic of all OAEs
1) Can be detected as acoustic energy within external auditory canal.
2) Pathway of energy transfer is OHC>basilar membrane>cochlear fluids> oval
window>ossicles> Tympanic membrane,which act as a loud speaker to external canal.
3) OAEs are an epiphenomena, that is , not a process of hearing but a by product of it.
4) Efficient, objective ,noninvasive window into cochlear function.
5) Present OAEs indicate intact OHC function but absent OAEs do not necessarily indicate OHC
malfunction, unless normal middle ear status is confirmed.
Three types of OAEs recorded clinically
SOAEs: about 35-60% of normal hearing individual have SOAEs that is generated with no external
stimulus.
TOAEs: occur in response to transient signal such as click.TOAEs are absent in cochlear lesion, but
present in purely neural lesion.
1)Low level responses below 30dbSPL must be measure in a quite environment.

3. 2) A sign that the cochlea has either normal function through the OHCs or has no more 30-40dbHL
sensorineural hearing impairment.
3) TOAEs can be analyzed by octave band for presence or absence of cochlear response across the
frequence range but only provides a present or absent response for whether cochlear hearing is
better or worse than the 30-40db range at each octave band up through 4000Hz.
DPOAEs: DPOAEs occur in response to two simultaneous pure tones of different frequency(f1&f2).
In response to f1 &f2 stimuli,the healthy cochlea then produces several distortion products (DPs)
at frequencies different from the stimuli.The most prominent DP is usually at the frequency 2f2-f1.
1)A DPOAE is a single tone evoked by two simultaneously presented pure tones.
2)Stimulation levels are typically 55-65 SPL but intensity functions may be tested .
3)usually easiest to obtain a DP from human cochlea when the stimulus or primary frequencies f1
&f2 are separated by ratio of 1:1.2 .for example, 2000 &24000Hz.
4) by using different combination of primary tones different DP frequencies can be generated ,there
by allowing objective assessment of a large portion of the basilar membrane
5)Of the several interaction of the stimulus tones ,the interaction 2f1 –f2 (or cubic difference tone),
usually produces the most detectable DP whose frequency is lower than either of the stimulus
frequencies.
6)Reflects cochlear status nearer f2 as opposed to f1 or the DP.
7) DPOAEs can be obtained in persons with more OHC loss & in response to higher frequency stimuli
than can TOAEs.
Clinical applications of both TOAEs & SPOAEs
1)Neonatal ear specific hearing screening.
2) part of test battery for auditory neuropathy,a rare condition in which there is sensorineural
hearing loss ,abnormal ABR, absent acoustic reflexes & poor recognization ability than expected
based on pure tone audiogram but OAEs are present.
3) useful in patient who are difficult to test.
4)Differentiating between cochlear & eight nerve lesion in sensorineural hearing loss( including
idiopathic sudden hearing loss & candidacy for cochlear implant).Because OAEs are preneural events
,absent EOAEs in losses 40db or greater point to the cochlea as a site of lesion whereas present OAEs
support an eight nerve site of lesion .
5)Monitoring for ototoxic or exposure to high sound level;DPOAEs& TOAEs may be lost or
diminished for high frequencies before changing in Pure Tone Audiometry.

4. 6)In cases of suspected PHA(pseudohypoacusis), Present TOAEs assures no significant conductive
hearing loss & no cochlear loss greater than 40db HL &probably less than 30dbHL.DPOAEs can also
contribute objective information of possible audiometric configuration & cochlear sensitivity.