The Atlas · Chapter 2

OAE Types & Generation Mechanisms

Otoacoustic emissions are sorted first by a simple question — did the ear need a sound to produce the emission, or did it produce one on its own? — and then by the intracochlear mechanism that generates them.

Spontaneous emissions (SOAEs)

Not all otoacoustic emissions are the same. They are sorted first by a simple question: did the ear need a sound to produce the emission, or did it produce one on its own?

Some healthy ears produce a faint, continuous tone with no stimulus at all — a spontaneous otoacoustic emission. They are a normal finding, not a sign of disease. Occasionally a person can even perceive their own SOAE as a soft ringing[1].

Evoked emissions (EOAEs)

Far more useful in the clinic are evoked emissions — the ear's response when we deliberately play a sound into it. The two we record routinely are the transient-evoked OAE (TEOAE), the ear's echo after a brief click, and the distortion-product OAE (DPOAE), produced when two tones are played together.

The four classical types

The conventional classification recognises four emission types. SOAEs occur without stimulation. The three evoked types differ by stimulus: a TEOAE follows a brief click or tone-burst; a stimulus-frequency OAE (SFOAE) is the response to a single sustained pure tone; and a DPOAE is evoked by two simultaneous tones, the primaries f1 and f2. SFOAEs are difficult to separate from the stimulus itself and so see little routine clinical use, while TEOAEs and DPOAEs dominate practice[5].

The DPOAE is the most elegant of the four to measure, because the ear generates energy at a frequency that was never presented. With two primaries close together, the largest distortion product in the human ear appears at the frequency 2f1−f2. Because that frequency differs from both stimulus tones, the emission is easy to isolate from the stimulus in the recording.

on the basilar membranewaves overlap → nonlinear mixingbase · high freqapex · low freqf₂f₁2f₁−f₂in the frequency spectrumfrequency →f₁f₂2f₁−f₂
Two primary tones, f₁ (3279 Hz) and f₂ (4000 Hz), set up traveling waves that overlap near the base of the cochlea. Because the outer-hair-cell amplifier is nonlinear, that overlap generates energy at a frequency neither tone contains — the cubic difference tone 2f₁−f₂ (2557 Hz) — which builds its own traveling wave at a more apical place and is emitted back out of the ear. In the spectrum it sits clear of both stimulus tones, which is what makes the DPOAE straightforward to measure. Simplified educational model (Greenwood map + asymmetric envelope) — not to scale.

A historical note worth keeping: David Kemp first demonstrated in 1978 that the cochlea emits measurable sound energy back into the ear canal, overturning the view of the ear as a purely passive receiver[1]. The discovery that isolated outer hair cells physically change length — electromotility — supplied the mechanism a few years later[4].

The dual-source model

The clinically important reframing is not the four-type list but the mechanism-based taxonomy of Shera and Guinan. Rather than every emission arising from a single nonlinear process, evoked emissions are generated by two fundamentally different intracochlear mechanisms: nonlinear distortion and linear coherent reflection[2].

base · high frequencyapex · low frequencydistortion sourcewhere f1 & f2 waves overlap — moves with frequencyreflection sourcefixed micromechanical irregularities — place-fixedemission travels back out
The dual-source model. A distortion-source emission is generated where the two stimulus traveling waves overlap and shifts position as stimulus frequency changes (wave-fixed). A reflection-source emission arises from coherent backscatter off fixed irregularities of the cochlear partition (place-fixed). Schematic — not to scale.

A distortion-source emission is created where the traveling waves of the stimulus tones overlap and the cochlear response saturates. Because that overlap region moves when stimulus frequency changes, this is termed a wave-fixed source. A reflection-source emission instead arises from coherent backscatter of the traveling wave off fixed micromechanical irregularities distributed along the cochlear partition — a place-fixed source[2].

This matters clinically because the DPOAE recorded at 2f1−f2 is not a pure distortion-source emission: it is the vector sum of a distortion component generated near the f2 place and a reflection component from the 2f1−f2 characteristic place. Their interference produces the fine structure seen in a DP-gram, and the two components can carry non-redundant information about cochlear health and may differ in their sensitivity to pathology and to ageing[3][7].

The practical takeaway: the TEOAE and click-evoked emission behave predominantly as reflection-source emissions, while the DPOAE is a mixed measure. Awareness of the dual-source model explains why DP-gram fine structure exists, why component-separation techniques are used in research protocols, and why no single emission type is a complete probe of cochlear function. For full treatment, see the standard text by Dhar and Hall[6].

Types in summary. Emissions are either spontaneous (SOAE) or evoked (TEOAE, SFOAE, DPOAE). Beneath that list sits the dual-source model: a wave-fixed distortion source and a place-fixed reflection source. The TEOAE is predominantly a reflection-source emission; the DPOAE is a mixed measure — which is why no single emission type is a complete probe of cochlear function.