A DPOAE, or distortion product otoacoustic emission, is a faint sound your inner ear produces in response to two tones played simultaneously. Audiologists measure these tiny sounds to check whether the delicate sensory cells inside your cochlea are working properly. The test is quick, painless, and requires no response from the person being tested, which makes it especially useful for screening newborns and young children.
How Your Inner Ear Creates These Sounds
Your cochlea contains thousands of outer hair cells that amplify incoming sound. These cells physically expand and contract in response to stimulation, a property driven by a protein embedded in their walls. This motion is what makes it possible to hear quiet sounds. But the process isn’t perfectly clean. Like a speaker pushed to its limits, the cochlea introduces small amounts of distortion when it amplifies sound.
During a DPOAE test, two pure tones at slightly different frequencies (called f1 and f2) are played into the ear canal through a small probe. Because the outer hair cells process sound in a nonlinear way, the cochlea generates additional tones at predictable mathematical combinations of the two input frequencies. The strongest of these byproducts occurs at the frequency 2f1 minus f2. This distortion product travels back through the middle ear and into the ear canal, where a sensitive microphone in the probe picks it up.
The energy for this distortion product originates primarily near the region of the cochlea that responds to the higher-pitched input tone, then travels to the cochlear location tuned to the distortion product frequency. Research using genetically modified mice has confirmed that without functioning outer hair cells, these emissions essentially disappear. Mice lacking the protein responsible for outer hair cell motility produce dramatically weaker, flattened responses, and mice with damaged hair cell structures produce no measurable DPOAEs at all.
What Happens During the Test
A small foam or rubber-tipped probe is placed in the ear canal. The probe contains two tiny speakers that deliver the test tones and a microphone that records whatever comes back out. The most important step is getting a tight seal. A snug fit ensures the test tones reach the eardrum at full strength and blocks out background noise that could interfere with the recording.
For newborns, clinicians typically use the largest probe tip that fits, massage the area in front of the ear for 10 to 15 seconds to loosen the “sticky” newborn ear canal, and gently pull back and down on the outer ear while inserting the probe with a quarter-turn twisting motion aimed toward the nose. Once the probe is seated, it should stay in place on its own. Holding it during the test can push it against the canal wall or introduce handling noise. The baby should be quiet or sleeping, and swaddling helps prevent them from knocking the probe loose.
The test itself usually takes just a few minutes. The equipment sweeps through several frequencies, typically covering 1,000 to 8,000 Hz, recording the strength of the distortion product at each one. No button presses or verbal responses are needed from the patient.
Reading the Results: The DP-Gram
Results are displayed on a graph called a DP-gram, which plots the strength of the DPOAE at each test frequency. A response is considered “present” when it rises at least 6 decibels above the background noise floor. This signal-to-noise ratio is the standard cutoff used in clinical practice.
A clearly normal result shows present responses with good signal-to-noise ratios at roughly 70 percent or more of the tested frequencies, with amplitude levels appropriate for the patient’s age. When this pattern appears and the test conditions were reliable, it’s reasonable to conclude that the outer hair cells are functioning well and hearing is most likely normal.
An absent result, assuming good test conditions and no middle ear problems, points to outer hair cell dysfunction. The hearing loss could range from mild to profound, but the DPOAE test alone cannot tell you exactly how much hearing has been lost. It flags that something is wrong, not how severe it is.
There’s also a middle category that challenges even experienced audiologists: DPOAEs that are present but weaker than expected. This might look like low-amplitude responses at some frequencies with absent responses at others, creating “islands” of activity on the DP-gram. For example, emissions might appear from 500 to 2,000 Hz but vanish above that range. This pattern typically suggests mild cochlear dysfunction and some degree of hearing loss, probably mild in nature. It’s worth noting that DPOAEs can still be present in ears with hearing loss up to about 35 to 45 decibels, so a present response doesn’t always mean hearing is perfect.
DPOAE vs. TEOAE
The other common type of otoacoustic emission test is the TEOAE, or transient evoked otoacoustic emission, which uses brief clicks instead of two continuous tones. The key practical difference is frequency coverage. TEOAEs are most effective at sampling the mid-frequency range, while DPOAEs perform better at 4,000 Hz and above. Since many types of hearing loss (noise-induced damage, age-related decline, drug-related toxicity) hit high frequencies first, DPOAEs are often the better tool for catching early changes.
Both tests are used in newborn hearing screening programs, sometimes in combination. The optimal frequency ratio for the two DPOAE input tones is 1.22, meaning the higher tone is about 22 percent above the lower one. This ratio consistently produces the strongest distortion products in human ears.
Clinical Uses Beyond Newborn Screening
One of the most valuable applications of DPOAEs is monitoring for hearing damage caused by certain medications, particularly chemotherapy drugs and some antibiotics known to be toxic to the inner ear. Standard hearing tests typically only cover frequencies up to 8,000 Hz, but drug-related damage often begins at higher frequencies and works its way down. DPOAEs can be monitored at higher frequencies than standard audiometry and may detect toxic changes earlier than transient evoked emissions, though high-frequency audiometry (testing hearing thresholds above 8,000 Hz) generally catches ototoxic damage even sooner than DPOAEs do.
DPOAEs are also used to track hearing in children too young for reliable behavioral testing, to screen for noise-induced hearing loss in workers exposed to loud environments, and as a research tool for studying how the cochlea processes sound.
What Can Affect the Results
Middle ear problems are the biggest source of false results. The test tones must pass through the middle ear to reach the cochlea, and the emissions must travel back out the same way. Fluid behind the eardrum, as seen in ear infections, can attenuate sound by as much as 40 decibels. Even a small amount of fluid (as little as half a milliliter) begins to reduce signal transmission, and the effect increases with fluid volume. This means a child with a middle ear infection might show absent DPOAEs even though their inner ear is perfectly healthy.
Background noise is another common issue. A crying baby, a loud hospital ward, or even the patient’s own breathing can overwhelm the tiny emission signal. That’s why a quiet environment and a sleeping or calm patient produce the most reliable results. Collapsed or very narrow ear canals, particularly in newborns, can also prevent the probe from delivering or recording signals properly.
Because of these limitations, DPOAEs are rarely used as a standalone diagnostic test. They’re typically part of a broader evaluation that may include middle ear pressure testing (tympanometry) and behavioral hearing assessments when the patient’s age allows it.