by James Morris
Your ears, which allow you to hear sounds – from the babbling of a brook to the wail of a siren to the song of a bird to the words of a friend – also have the remarkable ability to make sounds. That’s right – your ears actually produce sounds. These sounds can be detected and used for many practical and sometimes surprising purposes.
“It is almost as astonishing as if the eye could produce light or the nose produce odors,” explained Dr. William E. Brownell of the Johns Hopkins University School of Medicine.
Called otoacoustic emissions, or OAEs, these sounds were first predicted by Thomas Gold in the 1940s, but it wasn’t until the late 1970s that they were detected for the first time by David Kemp.
The ears produce two types of sounds – spontaneous and evoked. Spontaneous OAEs occur on their own, without any kind of prompting. Evoked OAEs, as their name suggests, occur in response to a stimulus: if you play a specific type of sound, the ear will produce one in response.
The sounds produced by the ears are a by-product of the normal mechanism of hearing. All of our sensory systems take in information from the outside world. For sight, the information is in the form of photons of light; touch is pressure; smell and taste are small molecules; and sounds are pressure waves in the air. All of these are converted by our sensory organs – eyes, skin, tongue, nose, and ears – into electrical signals, the language of the nervous system. These signals are then sent to the brain where we process and interpret them.
The ear has three parts. The outer ear funnels sounds to the ear drum, causing it to move back and forth. The middle ear has three tiny bones – the smallest bones in the human body – that transmit the waves from the ear drum to the inner ear. The inner ear has a snail-like structure called the cochlea with hair cells that vibrate in response to the incoming waves and convert them into electrical signals.
The cochlea can be thought of as a kind of “inverse piano,” as described by James Hudspeth, a neuroscientist at Rockefeller University and recipient of the prestigious Kavli Prize in 2018. A piano has individual strings that combine to produce music. The cochlea, by contrast, takes complex sounds and separates them into individual notes. These notes are sent to the brain for processing, where we make sense of them as sounds.
The cochlea doesn’t just respond passively to incoming sounds. It also amplifies them, allowing us to detect low levels of sound that we otherwise would not be able to hear, and fine tunes them, separating sounds that are very close in frequency. Hair cells in the outer part of the cochlea are responsible for these two functions, and OAEs are a by-product of this active process.
Because OAEs are part of the normal process of hearing, they provide a simple, reliable, and non-invasive test of hearing. For example, they can be used to screen for hearing loss in newborns, young children, or patients who are unable or unwilling to undergo a conventional hearing test. In fact, all US states mandate hearing tests for newborns before hospital discharge. The presence of a normal pattern of OAEs indicates that the cochlea and hair cells are intact and functioning; fewer, reduced, or absent OAEs all point to at least some degree of hearing loss.
OAEs also provide a useful way to test for damaged hair cells resulting from age, exposure to loud noise, and certain medications. These tests are often performed alongside audiometry to fully assess hearing.
There is some evidence to suggest that there is a connection between ringing or noise in the ears (tinnitus) and OAEs. It could be that some people are able to hear their own spontaneous OAEs. Alternatively, the ear could produce abnormal OAEs as a result of an underlying problem that also causes tinnitus.
In addition to their use in medical settings, OAEs have some unexpected applications as well. For example, Stephen Beeby and others have proposed using them like fingerprints, facial ID, and other forms of biometric identification. And the composer Marianne Amacher famously incorporated OAEs into her music, exploring new types of sound. Describing her music, she wrote, “When played at the right sound level, which is quite high and exciting, the tones in this music will cause your ears to act as neurophonic instruments that emit sounds that will seem to be issuing directly from your head.”
Just as an echo or sonar can tell us something about our surroundings, the sounds produced by ears have taught us how we are able to hear and ultimately make sense of sounds.
© James Morris and Science Whys, 2019