The sound of the sea

Camping close to the beach at the Yuraygir National Park on the New South Wales north coast fills your head with the sound of waves, and when you wake in the early hours sometimes you think you can hear voices in the night through the walls of your tent.  I hear children playing on the beach, and in the dark I feel I have been given the chance to listen to the spirits of the Bandjalung people calling to each other. There’s nothing to do but drift back to sleep – I can’t quite grasp the words, but it all feels right because this is Aboriginal land.

Of course there’s another explanation for the voices in the surf. The human mind is addicted to finding patterns, and it will find them even if there’s nothing actually there. The sound of the surf is ideal because it is made up of a mixture of every possible sound there is, so the brain can find whatever it wants in it. I don’t mind having both ways of looking at it sitting side by side.

Here is the sound of the sea:

We’re talking white noise – a sound spectrum that puts out energy evenly across the audible spectrum. With noise like this there are no sharp peaks showing a stronger output at particular frequencies, as there are in the guitar responses.

This is what artificially-generated white noise sounds like:

The difference is clear when you hear them both, but how do they compare when you look at their spectral signatures? Here is a spectrum of surf compared against artificially generated white noise from 0 to 6,000Hz (6kHz):



The general similarity between the two is pretty clear. The surf noise is at its most intense from about 100 to 400Hz, and then tails away at the higher frequencies. The white noise is pretty even from about 1300Hz up to 6kHz.

Compare these to the ringing wineglass to get a good feel for the difference between noise and sound:

And here’s the spectral signature of the singing wineglass:



The difference between these two types of sound is obvious not just to the ear. Notice the even spacing between the peaks characteristic of a harmonic series.

Perfect white noise, which would be a perfectly flat red line on GRAPH 1, is one of those ideal concepts that physicists love to toss around. In real life, devices that emit sound like loudspeakers, or record sound like microphones (generically called transponders) each have their own frequency response which is imprinted onto any “perfect” sound that might be created by software or electronic jiggerypokery.  Audio engineers can use white noise to map the peculiarities of any transducer.

But much more interestingly, I recorded the surf noise at the same position relative to the breakers on several days, and the frequency spectrum differed on each day:



More interestingly still, you can select out an individual wavebreak from the recording to analyse, and it looks like this:



The sound of the breaking wave is much spikier than the average beach sound, and quite a bit louder in the 100Hz to 600Hz range.

I’ll leave you to ponder what all this means – the main thing is to get a feel for what information you can get from an audio spectrum.

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