Okay, Back to the Crackling Sound

8 09 2011

Last week, during our little high-energy swell that drove just about everyone to the bottom at least 10 times per session, there was this bizarre underwater crackling sound that would crescendo for about 4 or 5 seconds with each passing set wave.  Very unusual, at least to me.

After doing some searching, often getting my own notations in my search results, I have picked and chosen some pieces that just might explain it.  I’m open to constructive criticism and correction on this.

Some sites talk about flora and fauna in the tropics, and even Garibaldi in California, that make “crackling” or maybe “cracking” or popping sounds for various dietary or sexual reasons.  Nope–not likely.  Unless, somehow large waves stimulate diet or sexual cravings in small crustaceans or bright orange fish.

So, here’s another theory that takes me back to my graduate hydraulic theory courses.  It sounds like “cavitation.”  You know that popping or “crackling” sound you hear when water is going through a slightly opened valve at way too high of a velocity.  Okay, maybe it’s just something us water nerds have experienced.  But, it also happens when boat props are moving at high speeds.

Without trying to explain Bernoulli’s Equation in a surf blog post, it goes something like this.  The water is moving so fast that the internal pressure of the water momentarily drops below atmospheric pressure, otherwise known as drawing a vacuum.  When that happens, any gas molecules dissolved in the water come out of suspension and form bubbles.  But that state is short lived.  As soon as the water moves away from the area where it was moving fast and the internal pressure of the water reaches atmospheric pressure again, the bubbles “implode” and disappear.  That is what makes that crackling sound when water is cavitating in a pipe or valve:  all those bubbles continuously imploding.  So, here is my theory–not exactly cavitation, but similar concept.

One of our favorite Crowdmembers, Tim Elsner, kept remarking about how “top to bottom” the set waves were breaking.  Got it, a lot of vertical movement in the water.  So, somehow on the set waves the lips were driving the air in the whitewater down further below the surface than normal.  Rather than coalescing like bubbles underwater usually do as they move up toward the surface (you know, like all those diving videos show from the deepwater divers), the bubbles were imploding, making the crackling sound.  Maybe the bubbles were being held down by the whitewater, similar to us humans.  Rather than coalesce, the pressure got to them and they imploded.

Sounds like a research topic, but not sure who would fund it.