8Otoacoustic emissions
Here is a fact that sounds impossible: a healthy ear emits sound. Put a sensitive microphone in the ear canal and you can record faint tones the cochlea produces, either on its own or in response to stimulation. These otoacoustic emissions are not an oddity — they are the audible by-product of the active cochlear amplifier, leaking back out through the middle ear. Because they require working outer hair cells and need no response from the patient, they have become one of the most useful objective tests in all of audiology, and the engine of newborn hearing screening.
FSound the ear makes
In 1978 David Kemp reported that the human ear, stimulated with a brief click, sends a faint sound back out into the ear canal a few milliseconds later — an echo the ear itself generates. The finding was initially met with disbelief: a sensory organ that emits energy seemed to violate the idea of a passive detector. It was, in fact, the first direct evidence that the cochlea is active.[1978]
TCThe amplifier's fingerprint
Otoacoustic emissions are the cochlear amplifier of Module 7 heard from outside. As the outer hair cells inject mechanical energy into the travelling wave, some of that energy travels backward — apex to base, through the oval window, across the ossicles, out to the eardrum — where it radiates into the canal as sound. Because the process is nonlinear (Module 6), stimulating with two tones produces emitted energy at new frequencies that were never presented. Emissions are therefore a direct, objective readout of outer-hair-cell function.[2012]
TTypes of emission
| Type | How it is evoked | Clinical use |
|---|---|---|
| Spontaneous (SOAE) | None — emitted with no stimulus, in many normal ears | Mostly a research/normal-variant finding |
| Transient-evoked (TEOAE) | A brief click; the ear's echo is recorded | Newborn screening; quick pass/refer at the screened frequencies |
| Distortion-product (DPOAE) | Two tones f₁ and f₂; the cochlea emits at 2f₁ − f₂ | Frequency-specific test of outer-hair-cell function |
The distortion-product emission is the clearest demonstration that the ear creates new frequencies. Present two tones and listen at 2f₁ − f₂.
TCWhat present or absent means
A present emission is strong evidence that the outer hair cells are working and the middle ear is clear — broadly, that cochlear function down to the amplifier is normal at the tested frequencies. An absent emission is more ambiguous: it can mean outer-hair-cell damage, but it can equally mean a conductive block(wax, effusion) that traps the emission on its way out. Toggle the effusion in the widget to see a normal cochlea read as “absent.” Emissions are thus a sensitive screen but not, on their own, a specific diagnosis.[2012]
One pattern is diagnostically powerful: emissions present but the auditory brainstem response absent. The intact emissions prove the outer hair cells and cochlear mechanics work, so the lesion must lie beyond them — at the inner hair cell, its synapse, or the nerve. This is the hallmark of auditory neuropathy spectrum disorder, and exactly the situation in which a cochlear implant — which replaces the inner-hair-cell-to-nerve step — is often considered.
FTNewborn hearing screening
Emissions transformed paediatric audiology. Because they are objective, fast, cheap, and need no behavioural response, transient-evoked emissions (often paired with the automated ABR) are the basis of universal newborn hearing screening. A sleeping baby can be screened in minutes: a pass at the screened frequencies is strong reassurance, a refer prompts diagnostic follow-up. Catching hearing loss in the first weeks of life — rather than the first years — is one of the biggest reasons modern implanted children do so well, because it lets implantation happen inside the sensitive period for auditory development.[2009]
FEmissions and the cochlear implant
Otoacoustic emissions test precisely the structure a cochlear implant bypasses — the outer-hair-cell amplifier — so they are not part of implant programming, and an implant recipient with the usual hair-cell loss will have no emissions. Their role in the implant story is diagnostic and selective: they help establish the kind and site of hearing loss before implantation (notably flagging auditory neuropathy), and they are the screening tool that gets children to the implant team early enough to matter.[2009]
We have now followed sound to the point where the inner hair cell hands it to the nerve. The rest of the chapter rides with that signal: the auditory nerve, how it codes intensity and pitch, and where it goes.
Why can a normally-hearing ear produce an absent otoacoustic emission at birth?
Otoacoustic emissions are the audible by-product of which structure?
A distortion-product emission appears at 2f₁ − f₂. What does this demonstrate about the cochlea?
An otoacoustic emission is absent. What are the two broad explanations to keep in mind?