MOBILE PHONE DISCO

This silly little experiment nicely demonstrates how your tympanic membrane, better known as the eardrum works. Build a tiny cling wrap dance floor and use your mobile phone to start a salt and pepper disco. If you’re in the mood, you can even make some tiny paper dancers to boogie on your creation.

                                                                                                

	1. Turn off the  'vibrate alert' and 'divert to voicemail' on your mobile phone. Select a  rockin' polyphonic ringtone that you can dance to. Now put your mobile phone inside a tall, empty and, most importantly, dry glass. Get some adult help if you're a little whipper-snapper.
	2. Stretch a small piece of cling wrap over the glass. It should be taut, like the skin on a drum.
	3. Sprinkle some salt or pepper, or both on the cling wrap. Now use a second phone to ring the mobile phone inside the glass. When it starts ringing, the  place starts thumpin'. It looks like a flea mosh pit.
	4. If you're feeling particularly inspired,  download and print out these tiny paper dancers, cut and fold as described and set them up on your dance floor. The paper dancers will wiggle around in a slightly embarrassing eighties dance style.

what's going on?

You are witnessing the transformation of sound energy into mechanical energy in the cling wrap and eventually, kinetic energy in the salt and pepper.

The same transformation happens in the auditory canal that leads to your middle ear. But what happens between your eardrum and your brain is absolutely amazing. The sound energy is completely transformed four times before you 'hear' a thing.

Your tympanic membrane, better known as the eardrum, responds to sound just like the cling wrap. This membrane looks like a flattened cone with the apex pointing into the ear. It's stretched across the canal by a ring of bone called the tympanic annulus. The first transformation is from vibrations in the air outside your head to mechanical vibrations of the tympanic membrane.

The centre of your eardrum is attached to one end of a delicate assembly of three tiny bones, called the ossicular chain. The other end of this chain is attached to the oval window of your fluid-filled inner ear, called the cochlea. Together with the two minute muscles that service them, these little bones transmit sound to your cochlea and provide you with an amazing little volume knob and equalizer.

A single bone would provide little or no control over the intensity of the sound energy transmitted to your cochlea. One of the tiny muscles adjusts the tension in your eardrum. The other pulls the smallest bone in your body, called the stapes, in or out of the oval window. Contraction in either of these muscles reduces the sound energy transmitted to your cochlea.

The tiny stapes transmits the eardrum's vibrations into a liquid called perilymph in your baked-bean sized cochlea. The cochlea is a spiral tube coiled around two and a half turns which, if stretched out, is about three centimeters long. As the stapes pushes the oval window inward, another one, called the round window, bulges out. This is important because it allows the liquid vibrations to be transmitted deep into the cochlea where they are transformed a third time.

Bundles of tiny hairs that line the canal are deflected by the waves in the liquid, transforming the energy back into mechanical motion. The membrane and hairs vary in stiffness and length along the canal so that each part responds to different sound frequencies. Exactly how this frequency (pitch) analysis works in not completely understood but the range of audible sounds is staggering. A healthy human ear can detect sound waves ranging from 20 to 20,000 vibrations per second.

Cells at the base of these tiny hairs then perform the fourth and final transformation into electrical energy. This incredible biochemical trick is not yet fully understood but thought to result from differences between the electrical properties of the perilymph and the endolymph. These two fluids bathe different parts of the hairs which creates a tiny voltage.

Deflections in those hair bundles cause the cells below to release chemicals called neurotransmitters which are absorbed by nerve cells. The nerve cells are then stimulated and send an electrical signal to your brain stem via nerves for some initial processing before being forwarded to your cerebral cortex where you finally hear them as 'sound'.

All these transformations happen in microseconds but possibly the most stunning feature of your hearing system is its sensitivity. A single human cell contains trillions of atoms but thanks to the amazing mechanical advantage of your ossicular chain, you can detect back and forth motions in your tympanic membrane of just 0.000 0001 millimeters, the size of a single atom. And that's why you shouldn’t poke anything smaller than your own elbow into your ear!


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