The Atlas · Chapter 3

Instrumentation & Technique

Recording an otoacoustic emission needs only one piece of hardware in the ear — a small probe — but a valid result depends on getting the seal, the stimulus, and the quality control right.

Getting a good seal

Recording an otoacoustic emission needs only one piece of hardware in the ear: a small probe. Inside it sit a tiny loudspeaker, which plays the stimulus, and a sensitive microphone, which listens for the faint sound the cochlea sends back.

The OAE probe seated in the ear canal. A single probe houses both the loudspeaker that delivers the stimulus and the microphone that records the returning emission. The foam or rubber tip must form a complete acoustic seal — a poor seal is the most common cause of a failed recording. Schematic, not to scale.

The probe is fitted with a soft foam or rubber tip that must seal the ear canal completely. The seal matters more than almost anything else in the test: if it leaks, the stimulus escapes and the quiet emission is lost in room noise. A poor seal is the single most common reason an otherwise healthy ear produces a “refer” result[8].

The test environment should be quiet, and the patient still and relaxed — ideally asleep, in the case of a newborn. Sucking, crying, and movement all add noise that the instrument must reject before it can find the emission.

The TEOAE stimulus

A transient-evoked OAE is elicited with a brief broadband click. In routine practice the click is presented in a non-linear paradigm — a train in which one click is inverted and scaled — which cancels the linear stimulus artifact and leaves the non-linear cochlear response. Non-linear click levels are typically set around 80–84 dB peak-equivalent SPL; the lower-level linear click is used less often because it is more prone to artifact[8].

The response is recorded in a short time window — on the order of 20 ms — with the first few milliseconds blanked to exclude stimulus ringing. Because a single click's emission is buried in noise, the instrument averages many sweeps: the random noise cancels over repeated trials while the repeatable emission builds up.

A single click contains every frequency at once, but the cochlea returns them dispersed in time. High-frequency (basal) places respond and re-emit first — near 6 ms — while low-frequency (apical) places return later, past 15 ms. The response is captured in a ~20 ms window with the first few milliseconds blanked to reject stimulus ringing. The wavefront (top) and the arriving echo (bottom) are the same event in place and in time. Simplified educational model (power-law latency, f^−½) — not to scale.

Scrub

The DPOAE stimulus

A distortion-product OAE is evoked by two pure tones presented together — the primaries f₁ and f₂, with f₂ > f₁. Their spacing and level are not arbitrary; the table below lists the parameters that routine protocols converge on.

Parameter	Typical value	Why
Frequency ratio f₂/f₁	≈ 1.22	The emission is largest near this ratio
Primary levels L₁/L₂	65 / 55 dB SPL	An asymmetric L₁ > L₂ level produces robust, sensitive responses
Distortion product measured	2f₁−f₂	Largest human DP; its frequency differs from both stimuli, so it is easy to isolate
Test frequency range (f₂)	~1–6 kHz; up to 8 kHz	Covers the speech range; extended high frequencies for ototoxicity monitoring

Values reflect commonly recommended protocols[8]; exact settings vary by device and clinical purpose.

Quality control during recording

A valid emission depends on three things being right at once, and a careful clinician watches all of them. Stimulus stability should stay high throughout the run — a stability figure that drifts well below the high-90s percent means the probe moved or the seal changed, and the measurement should be repeated. Noise rejection discards individual sweeps that exceed a set level so they never enter the average; turning it off to save time degrades the data and is not advised. And the stimulus spectrum must be genuinely broadband — a click missing high- or low-frequency energy simply cannot evoke a response from the corresponding region of the cochlea[8].

The result is read as a signal-to-noise ratio: the emission must rise a criterion margin above the noise floor — commonly around 6 dB — and be reproducible. Screening protocols typically require a pass in a set number of frequency bands, for example three of four[9].

Common technical pitfalls

Before attributing an absent emission to cochlear pathology, exclude the avoidable causes. Cerumen or debris occluding the probe tip, vernix in a newborn's canal, a partial seal, a probe pressed against the canal wall, and a noisy or unsettled patient all produce a refer result in an ear that may hear perfectly well. Middle-ear effusion does the same — which is why probe checks, otoscopy, and tympanometry belong alongside the OAE, not after it[6].

Technique in summary. The seal comes first — most refer results in healthy ears trace to a leak, cerumen, or a restless patient. TEOAEs use an averaged non-linear click; DPOAEs use two primaries near an f₂/f₁ ratio of 1.22, reading the 2f₁−f₂ product. A valid response demands stable stimulus, active noise rejection, a broadband spectrum, and an adequate, reproducible signal-to-noise ratio.