Season 1, Episode 7: Did you know your ears produce sound?

We all know that healthy ears detect
sounds. But did you know that they also emit sounds too? Apparently these sounds can be measured and used to test hearing in newborns. Biophysicist Chris Bergevin has the details. Healthy ears detect sounds. That’s their
job, right? It’s a relatively complex process, but to put it simply, your
eardrum is connected to three small bones called the ossicles. These are the
smallest bones in your body. These pass airborne vibrations, ie sound,
to the cochlea, a spiral shaped structure buried inside your skull. The cochlea
transduces signals to the central nervous system. The transducers in the
cochlea are called hair cells. You have a few thousand of these in each ear, and
if you damage them, they don’t grow back, so avoid overly loud sounds when
possible. Did you know that your ears also create sound as well? This is not
the ringing you might hear in your ear sometimes called tinnitus — that’s
something else. These sounds are called otoacoustic emissions, or OAEs and
they’re generated from within the cochlea. We think that the hair cells act
as little amplifiers to boost detection of low-level sounds — kind of like a
microphone connected to a PA system, and that OAE’s are a byproduct of that
process which we still know relatively little about. OAEs can be evoked
using some sort of acoustic stimulus or even arise spontaneously in the absence
of external sound. In general, only healthy ears emit OAEs, and OAEs from
a given ear like yours are also unique in terms of their spectral pattern, kind
of like an acoustic fingerprint. All sorts of critters show emissions as well
including owls, lizards, frogs, and even tigers. OAEs can be measured with a
very sensitive microphone, and this is actually how you give a newborn baby a
hearing test.

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