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Why do you 'hear the ocean' in a seashell?

We live in a sea of sound but can we capture the essence of the ocean in a seashell? Dr Karl visits the seaside to see what he can hear.

It's a lovely experience to walk on the beach, and it can be made even lovelier by finding a large empty seashell and putting it to your ear, and hearing the sounds of the ocean. It also gives grown-ups a feeling of benevolent omnipotence to pass the shell to kids, and to see the amazement on their faces. The ocean can't possibly be inside the shell, so the sounds of the ocean coming from the pink walls of a seashell seem like magic.

So what are you actually hearing in the shell?

The answer is that you are hearing the local noises already around you, but altered by the shell — thanks to some clever physics.

One popular (but wrong) explanation is that you are listening to your own blood coursing through you.

This explanation might be based on the fact that you can sometimes hear the pulsing of blood as you lay your head onto a soft pillow.

It's easy to disprove this theory with a simple experiment. Press your ear to a shell and listen, then run around on the beach for a few minutes to increase the blood flow all through your body, and again listen to your magic shell.

You'll find that the loudness of the 'sound of the sea' is still the same.

So now for a three-part explanation.

The first part of the explanation is that the shell acts like a 'resonator'.

When you blow air strongly through your pursed lips over the mouth of an empty bottle, you will hear a musical note. The sound is resonating in the bottle. You and I might call it a "bottle" — but a physical acoustician would call it a "resonant cavity". Getting back to our seashell, the inside is hard with an almost-glazed finish — so it's an excellent reflector of sound (it's a resonator). It also has quite an irregular shape — so it will resonate at many frequencies.

In the same way, this acoustic filter (or shell-near-the-ear) dampens one frequency (or pitch), and boosts other frequencies. In one study, in a typical noisy room, a cup was held to the ear and a tiny microphone held right next to the eardrum. (They chose a cup, because it has a 'simpler' shape than the internal complexities of a seashell.) The microphone registered 15 decibels louder at the cup's resonant frequency of 648 hertz (as compared to not having the cup there at all). But at double the frequency (1296 hertz), the sound heard was 16 decibels quieter.

But to give you the ocean sound, the shell definitely needs the ambient or background sound. No ambient sound, no 'ocean-in-the-shell' sound. If you go into a soundproof room, and listen to your favourite seashell, you'll hear nothing.

The second part of the explanation is that our human brain is superb at finding subtle patterns in the chaotic world around us. We can find animals in clouds, or the face of Jesus in a potato chip, or the Virgin Mary in a fencepost.

The third part of the explanation is that we live in a sea of sound, but we mostly ignore it. This is similar to the phenomenon of being able to feel our socks and underwear for a few brief moments after we put them on. Our brain then blocks the socks/underwear from our consciousness for the rest of the day. In the same way, our brain usually blocks most of the noise of the background buzz.

So now we can put it all together.

The shell close to your ear acts like the audio equivalent of yellow-tinted sunglasses. It changes the make-up of the sounds that continually assault our ears, and that we continually ignore. For example, it lets through more of one frequency, but less of another frequency. So the combination of 'ear and brain' recognises that something has changed in the incoming noise. The brain tries to put a label on this new noise, and notices that you are near the ocean — so it labels this noise as "ocean".

But some people find different patterns in seashell noise.

There's a strange psychic phenomenon called 'shell scrying'. It encourages you to listen carefully to the shell. First, they say, you should hear fragments of words, then words, and finally, whole segments of conversation.

But good luck if you're hoping to hear next week's winning lottery numbers…

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