Naked Science Forum

General Science => General Science => Topic started by: paul.fr on 17/02/2007 21:47:56

Title: Why do seashells produce a sound like the ocean?
Post by: paul.fr on 17/02/2007 21:47:56

Anastasia asked yesterday why she can hear the sound of the ocean in sea shells?

I used to know the answer to this, so i told her i would have to try and remember. To save my blushes and restore her faith, can anyone remind me of the answer.

i will ofcourse tell her i got some help......honest

Paul

Title: Re: Why do seashells produce a sound like the ocean?
Post by: another_someone on 17/02/2007 21:56:59

You do not hear the ocean, you just hear white noise (actually, I suspect it is more pink noise than white noise).  The shells concentrate the sounds of the environment around, isolating some frequencies, so that is what you hear, a sound frequency spectrum that sounds similar to the frequency spectrum of the ocean waves.

Title: Re: Why do seashells produce a sound like the ocean?
Post by: paul.fr on 17/02/2007 22:05:52

Quote from: another_someone on 17/02/2007 21:56:59

You do not hear the ocean, you just hear white noise (actually, I suspect it is more pink noise than white noise).  The shells concentrate the sounds of the environment around, isolating some frequencies, so that is what you hear, a sound frequency spectrum that sounds similar to the frequency spectrum of the ocean waves.

Many thnks, i knew it was not the actual sound of the ocean! it was the "shells concentrate the sounds of the environment around, isolating some frequencies, so that is what you hear" that i could not remember.

The more i come to the forum the brighter she thinks i am, thank you.
:-)

Paul

Title: Re: Why do seashells produce a sound like the ocean?
Post by: chris on 18/02/2007 11:49:15

That's right. The sea shells are effectively behaving like "sound pipes". This is an array of tubes cut to different lengths and arranged in a line. By placing your ear next to the aperture of each you hear different "notes", and if you move your head quickly enough, you can play yourself a tune.

What's going on? The tubes are concentrating certain frequencies from the "white noise" in the environment. The sound coming from each tube corresponds to its "resonant frequency". This is when a wave returns to its starting point just as another wave is about to begin. As a result the two wave crests merge (superpose) and make a bigger (louder) wave. This is a standing wave, which has a node (fixed point) at the open end of the tube and anti anti-node (maximum displacement) at the closed end.

So when noise enters such a tube, or a seashell, sounds of the appropriate wavelength reinforce the standing wave and are amplified, and that's what you hear.

As a result, longer tubes (or larger seashells) will produce lower pitched sounds (because they sustain longer wavelength standing waves), whilst shorter tubes (and smaller shells) will tend to produce higher pitches.

A similar trick produces the human voice.

Chris