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Cochlear implants don't generate sound like a hearing aid would. Instead, they zap your cochlea.

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Sources:
https://www.nidcd.nih.gov/health/hearing-aids
https://www.nidcd.nih.gov/health/cochlear-implants
http://www.npr.org/sections/health-shots/2015/05/18/406838781/deaf-jam-experiencing-music-through-a-cochlear-implant
One of our Patreon patrons, Captain Nick, recently wanted to know what the world sounds like through cochlear implants, which turned out to be a pretty popular — and interesting — question.

Because if you don’t have the implants yourself, you … can’t really know. Cochlear implants are one way for someone who’s deaf or hard of hearing to hear speech, and to understand what they sound like, you might be tempted to put the business end of one up to a mic and listen.

But if you did that, all you’d get is silence, because cochlear implants don’t actually produce any noise. Confused? Let me explain.

There are two main types of devices that help people hear: hearing aids and cochlear implants. Hearing aids are basically miniaturized amps and speakers, meaning the world sounds a lot like it would to someone whose hearing isn’t impaired. Of course, for a hearing aid to work, the person has to have some amount of hearing in the first place.

If they don’t, a hearing aid won’t help. Cochlear implants allow people with profound hearing loss or even complete deafness to hear speech by bypassing most of the ear altogether. What it puts out are electrical currents, not sound waves—which is why it’s silent.

Cochlear implants consist of a mic, a processor, and a receiver with an electrode array. The electrodes and receiver are surgically implanted into the cochlea—in case the name wasn’t clear. Your cochlea is the part of your inner ear that converts sound waves into electrical signals, which then travel to your brain through the auditory nerve.

It’s a hollow tube that’s coiled in on itself, which makes it look a bit like a snail shell, and it’s lined with special cells called hair cells that trigger nerve signals when they detect sound waves. If these hair cells aren’t working—because they never have, or because of some kind of damage—then it doesn’t matter how loud a speaker you put in your ear: you won’t hear sound well, if at all. So a cochlear implant kind of acts like a hair cell replacement.

The electrode array is implanted so it can stimulate parts of the cochlea directly. The mic takes in sounds from the world and passes them to the processor, which then crunches that sound data and interprets it into about two dozen “channels”, each corresponding to a different frequency of sound. For each channel, there’s an electrode in the array that zaps a certain spot in the cochlea.

So what you “hear” is what your brain interprets from the combination of signals. But those sounds aren’t the same as what the mic picked up. There are thousands of hair cells in a fully-functional cochlea, which is why hearing people can pick up a whole range of frequencies.

With only two dozen channels or less, the sounds from a cochlear implant are so different that a lot of people who get them essentially have to train their brains to hear. It’s impossible to completely replicate what it sounds like without zapping your cochlea, but if you’ve ever heard one of those talking dolls when it’s running low on batteries … it’s a little bit like that. The channels the implant uses are specifically designed to amplify speech and make it interpretable, but they’re not great at replicating the complexity of real world sound.

As one person with a cochlear implant explained to NPR, the implants especially fail at relaying pitch and timbre. Because of that, it can be especially difficult to understand tonal languages like Chinese, or to isolate individual voices talking in a crowd. Researchers are working on ways to make cochlear implants clearer, but for now, even the best implants we have make the world sound very different.

But for those who choose to get them, it’s a way to listen to and understand speech — even if it sounds a bit strange. Thanks to Captain Nick for asking this question, and thanks to all our patrons, who keep these answers coming. If you’d like to submit a question to be answered, you can go to patreon.com/scishow.