Otoacoustic emissions (OAEs) are faint sounds produced by your inner ear. They originate in the cochlea and travel backward through the middle ear into the ear canal, where a tiny microphone can pick them up. These sounds are a byproduct of the ear’s own amplification system, and their presence or absence reveals a great deal about how well your hearing works at its most fundamental level.
How Your Ear Produces Sound
The idea that ears can generate sound, not just receive it, seems counterintuitive. But the cochlea is not a passive receiver. It actively amplifies incoming sound using specialized cells called outer hair cells. These cells respond to sound vibrations by rapidly changing their length, generating force in the process. This rapid shape-shifting is what audiologists call electromotility, and it acts as a built-in amplifier that boosts faint sounds before they reach the brain.
Here’s how the chain works: sound enters the ear and causes a fluid wave to ripple through the cochlea along a structure called the basilar membrane. Different frequencies peak at different points along this membrane, with high-pitched sounds peaking near the base and low-pitched sounds peaking near the tip. When the wave reaches the spot tuned to its frequency, the outer hair cells at that location kick into action. They amplify the signal, making it strong enough for the brain to detect. But this amplification also creates a smaller, secondary wave that travels back the way it came, out through the middle ear and into the ear canal. That backward-traveling wave is the otoacoustic emission.
Because OAEs depend entirely on healthy outer hair cells, they serve as a direct window into cochlear function. If those cells are damaged, the emissions weaken or disappear.
Types of Otoacoustic Emissions
OAEs fall into two broad categories: spontaneous and evoked.
Spontaneous OAEs occur without any external sound stimulus. The outer hair cells generate them on their own. Not everyone produces detectable spontaneous emissions, and they’re more of a curiosity than a clinical tool.
Evoked OAEs are the clinically useful ones. A probe placed in the ear canal delivers a sound, and a microphone in the same probe records the emission that comes back. The two main types used in clinics are:
- Transient evoked OAEs (TEOAEs): A brief click or chirp is played into the ear, and the microphone captures the cochlea’s broadband response. TEOAEs typically measure hearing across frequencies from about 700 Hz to 8,000 Hz. They’re widely used in newborn hearing screening because the test is fast and straightforward.
- Distortion product OAEs (DPOAEs): Two tones at slightly different frequencies are played simultaneously. The cochlea’s nonlinear amplification process creates a third tone, a “distortion product,” that wasn’t in the original stimulus. Clinicians typically use a frequency ratio of 1.22 between the two tones, which produces the strongest emission. DPOAEs can assess outer hair cell function at specific frequencies up to 10,000 Hz, making them especially useful for tracking changes over time.
How the Test Works
The equipment is simple: a small probe that fits into the ear canal containing one or two tiny speakers and a sensitive microphone. The speakers deliver the stimulus sounds, and the microphone picks up the emission coming back from the cochlea. The entire assembly fits into a soft tip similar to an earplug.
For a result to count as a “pass,” the emission needs to be at least 6 dB louder than the background noise at each frequency tested. This signal-to-noise ratio threshold helps ensure the microphone is actually detecting an emission from the cochlea rather than ambient noise. The test requires the person to sit quietly, since movement, breathing, or background sound can interfere with the recording. For DPOAE testing, results can be collected in under a minute per ear.
Newborn Hearing Screening
OAE testing is one of two methods approved for universal newborn hearing screening. The other is auditory brainstem response (ABR) testing, which measures electrical activity along the nerve pathway from the ear to the brain. The two tests check different parts of the hearing system.
OAE screening measures whether the outer hair cells in the cochlea are functioning. It takes 10 to 20 minutes including setup and costs roughly $1 per baby in disposable supplies. The tradeoff is a higher referral rate: about 10% of newborns will fail and need follow-up testing, often because residual birth fluid in the ear canal or middle ear blocks the emission from reaching the microphone. ABR screening has a lower referral rate of around 3%, but takes longer (15 to 30 minutes) and costs about $11 per baby in disposables.
OAE screening can miss certain conditions. It won’t detect auditory neuropathy, where the cochlea works normally but the auditory nerve doesn’t transmit the signal properly. It may also miss mild or low-frequency hearing losses. For this reason, babies who spend more than five days in the NICU require ABR screening before discharge.
Monitoring Medication-Related Hearing Damage
Certain medications, particularly some chemotherapy drugs and powerful antibiotics, can damage the outer hair cells of the cochlea. This damage typically starts at the base of the cochlea, where high-frequency sounds are processed, and progresses toward lower frequencies. DPOAE testing has become the preferred method for catching this damage early.
The key advantage is timing. DPOAEs can reveal changes in outer hair cell function before the person notices any hearing loss and before standard hearing tests show abnormal results. A baseline DPOAE measurement is taken before treatment begins, and subsequent tests during treatment are compared against it. A drop of 4 to 6 dB in emission strength at the highest testable frequencies is generally considered a warning sign of early cochlear damage. Because the test is objective, fast, and doesn’t require the patient to respond to sounds, it can be performed at the bedside or in a treatment room, even with patients who are too young or too ill for traditional hearing tests.
What Can Interfere With Results
OAE testing depends on sound traveling freely in both directions through the middle ear. Anything that blocks that path can weaken or eliminate the recorded emission, even if the cochlea itself is perfectly healthy.
Middle ear fluid is the most common culprit, especially in children. Fluid dampens the vibrations of the eardrum and the tiny bones of the middle ear, preventing the emission from reaching the microphone. Earwax in the canal can cause similar problems. Children with a history of repeated ear infections tend to produce weaker emissions even after the infection clears, likely because of residual changes to the eardrum or the middle ear structures. Some of this may also reflect direct damage to the outer hair cells from toxins associated with chronic middle ear disease.
This sensitivity to middle ear conditions is both a limitation and a strength. It means a “fail” result doesn’t automatically indicate inner ear damage, since the problem could be in the middle ear. But it also means OAE testing can flag subtle middle ear dysfunction that traditional hearing tests might miss.
What OAEs Tell You About Hearing
A present OAE at a given frequency is strong evidence that the outer hair cells at that location in the cochlea are working. This generally corresponds to hearing sensitivity within the normal range at that frequency. An absent OAE suggests outer hair cell dysfunction, which is the most common mechanism behind sensorineural hearing loss.
OAEs do not measure the full hearing pathway. They confirm that sound is being processed at the cochlear level but say nothing about whether the auditory nerve or brain are interpreting that signal correctly. For a complete picture of hearing, OAE results are typically combined with other tests. Still, because outer hair cell damage accounts for the vast majority of acquired hearing loss, OAEs remain one of the most efficient and informative screening tools available.