How waves make noise

Why does the sea rustle?

Most people find the sound of the sea pleasant, although it can be as loud as 100 decibels, which corresponds to a jackhammer at a distance of 10 meters. Every noise arises from a superposition of sound waves with different frequencies. But what creates these sound waves? Is it the crash of the waves on the shore? No, because sometimes the sound of the sea can also be heard on the high seas, namely whenever waves break.

Dependence of the waveform on the water depth

If the ratio of wave height to wave length reaches a certain maximum value, the wave becomes unstable. It overturns and forms a white head. If a wave runs towards a rising bank, the speed of the wave decreases with the depth of the water due to the effects of friction with the seabed. The shaft is compressed. So it piles up in the bank area and finally breaks.

Billions of air bubbles mix into the water during the break. And it is ultimately these that are responsible for the noise. The bubbles are deformed by the pressure of the water, i.e. H. deflected from their rest position, around which they then begin to swing. This creates sound waves that propagate first through the water and then through the air. The frequency of the sound wave depends on the size of the respective bubble: the larger the air bubble, the deeper the sound. And because the sizes are very different, there is a noise, to which the noises of bursting bubbles also contribute.

Breaking wave

Scientists at the Scripps Institution of Oceanography in San Diego have examined the size distribution of the bubbles and found that two categories of bubbles can be distinguished. On the one hand, the large bubbles with diameters of one millimeter to a few centimeters arise when the wave bends forward and the tunnel, popular with surfers, forms. If the wave tip hits the surface of the water, on the other hand, small bubbles from a few micrometers to a millimeter in size arise. A precise knowledge of the size distribution is important for climate models, since the oceans have large amounts of CO2 in this way pick up from the air.