The Amazing Ear

by Wendy Sadler

As a music lover, I guess it’s no surprise that I am going to (ear) wax lyrical about why I think the human ear is one of the most amazing pieces of biology I have ever come across. A few years ago, science made simple were approached by Deafness Research UK to develop a show about what can go wrong with your hearing and what scientists are learning about what can be done about it. The (award winning) Bionic Ear Show was born, and we’re proud to say that it has continued to tour across the UK to this day and has now reached over 100,000 people. During the research process I was continually blown away by what the human ear can do, and how it transfers mere physical movements into the music and language that enriches our lives.

bionic_ear1

So what do these unassuming little flaps of skin on your head house that is so amazing? Well for starters they contain the smallest bones in the human body. Inside your middle ear are three tiny bones, known popularly as the hammer, the anvil and the stirrup – together they are ‘the ossicles’ (they sound a bit like a 1960s group!). The smallest of these bones is the stirrup (or stapes) which is just 4 mm long – you can see the size compared to a human hand in this picture:

Ear ossicles.  Image: Eastbourne Ancestors/OsteoArch.wordpress.com

Ear ossicles. Image: Eastbourne Ancestors/OsteoArch.wordpress.com

These tiny little bones take the tiny vibrations that make your eardrum wobble, and amplify the movement to pass on the physical sound information to your inner ear. Through a combination of levers and a concentration of energy, they make the movement around 22 times larger than the movement of your eardrum! Teeny-tiny engineering that allows us to enjoy the full dynamic range of whatever music we’re into.

Treat them with respect though because this amplification effect means that sounds of around 150dB could generate such a sudden movement that it could snap these bones, and fixing them is not as easy a putting a cast on a broken leg! Scientists have however developed replacement (or bionic!) ossicles which can be transplanted if there is a problem.

Another thing that can go wrong in this part of the hearing process is ‘glue ear’. There is a tube called the Eustachian tube that connects the middle ear to the throat. This helps you equalise any pressure differences that can cause problems with hearing. You have probably tried yawning or ‘popping’ your ears manually when flying if you have pressure discomfort in your ears. When you do this you are forcing this tube to open, to make the pressure inside the ear match the pressure in the outside environment. If this tube becomes blocked there is no way for the fluid to drain and the bones become stuck in a thick syrup type fluid. One way to solve this is to insert grommets in the ear drum which allow the fluid to drain out, but most children with glue ear will also naturally grow out of it so this isn’t always necessary.

Once the sound vibrations pass through the middle-ear they are passed onto the inner ear where the physical movement of the sound sets up a standing wave within a tiny snail-shell shaped organ called the cochlea. The shape of the standing wave will vary according to the pitch of the music or sound you are hearing. Depending on where the peaks of this wave are, different hair cells will be excited more or less. The pressure change in the fluid makes the hairs along the cochlea dance!

As these hairs move they generate chemical signals which are sent along the auditory nerve and into your brain. Once they reach the brain, things can get a whole lot more complicated (well – it is the brain, what do you expect!)

To finish off – try this little experiment to illustrate how the sense of hearing can be messed around with by the addition of other sensory information to the brain. This video shows you the McGurk effect.

So finally a little plea to look after your hearing – and of those near and dear to you. Tinnitus (that ringing in your ear sensation) happens when you inflict too much loudness on your ears. Your little inner ear hair cells effectively get stuck in the ‘on’ position and send spurious signals to your brain, even when the source of the sound has gone. Although there are some treatments to mask this perceived effect of ringing in the ears, it currently can’t be cured. If you permanently listen to your music at a loud volume you may damage these hairs beyond repair and then you’ll be stuck with that ringing as you try and fall asleep at night. If you have children you may be interested to know that Apple do offer a limiting setting on iPods and iPhones to prevent the volume being too loud, and you can set a code so that the tech savvy kids can’t switch it off. They may thank you later in life (but probably not right now!).

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