Underwater Acoustics from the Whale's Point of View

Paul Arveson

Greetings, fellow cetaceans! Welcome to the ASA. I am Ur-wa, a member of the North Pacific Choral Society, which has a musical tradition going back hundreds of thousands of years. For those of you who are unfamiliar with our culture, I will describe it briefly - who knows how much longer it will last.

I am a so-called humpback, Our songs span a range of 10 octaves and time scales from seconds to days.

We spend the summers in the waters off Alaska: the Beaufort Sea and the Chuckchi Sea, where the krill are abundant and delicious -- with our gulping technique we can swallow them by the tons there -- of course it takes a lot of shrimp to feed a whale!

My pod likes to feed around the area between two seamounts above the abyssal plain north of Anak, one of the Aleutian Islands. Our ancestors have lived in this area between the two sacred mountains for over a million years. We know that area intimately -- every hill and rift in the seafloor, the walls of basalt and the deep muddy sediments, and even the rocks below the sediments, which are transparent to sound. We keep track of the currents, the mesoscale eddies, the warm patches and the salty patches.

In the winter we begin our long vacation trip south to the warm waters of the remote Hawaiian Islands, where we show off our singing and composing talents for the ladies. The Hawaiian Islands have been a favorite meeting place for us for as long as we can remember. There is an ancient tradition that says that the Maker formed the western island first, about 4 million years ago, and the big hot island last. We know that the hot island sometimes is very smelly, and seaquakes are very common there. The Maker is perhaps still at work there.

However, there isn't much to eat there, so we have to leave. On our long journey back north in the Spring, we stay in touch with each other, and help the young ones to stay on course. But it is difficult to get lost; there are so many clues to go by, including the resonances and absorption of different bottom sediments, the bathymetry, the temperature gradients, the salinity gradients, the currents, and even the surface swells from distant storms.

The acoustics in the open sea are very spacious and predictable. Especially in the mixed layer, where the waves mix the water to a constant temperature. Sound gets trapped in this layer and travels reliably through it, especially the middle tones.

Once, I confess, I got lost when I was younger. My parents taught me what to do: descend to the deep sound channel at 1300 meters and listen for their calls. It was somewhat cold - 4 degrees C - and as I descended deeper and deeper into the dark, the pressure squeezed hard on my lungs. But I knew when I had arrived there -- I believe I could hear everything across the whole ocean! If you are at the right depth, you can call each other out to a 100 days' journey. Fortunately, my parents were not too far away and I heard them calling, because it was very noisy there.

Another method they taught me was to simply stop in one place near the surface and wait. Within half a day, I would begin to hear them calling. They called this the convergence zone effect, where the sound gets focused every half-days journey across the sea. This happens all the time, as singers travel in and out and in and out of hearing range.

Well, of course you all are aware that this hearing range is getting shorter all the time, because of the second alien invasion. The first invasion of our planet, you will recall, was when surface creatures began spearing our ancestors and hauling their bodies away. These creatures were mostly silent, blown about by the wind. Fortunately most of those aliens have gone away, although there are still reports of some killings in the Western Sea.

However, they have been replaced by the second species of invaders, the noisy ones. These aliens travel at high speeds on the surface, with a constant, droning, ugly noise. They seem to be everywhere, although most of them follow certain fixed paths across the sea. Their noise has rendered the deep-sound channel communication unreliable or useless. And if one is unfortunate enough to be on the same course as one of them, one has to slow down and wait for them to move away in order to hear anything.

There is of course the natural ambient noise of the sea, caused mostly by surface winds that stir up waves. But these new aliens have raised the noise level by 30 decibels above the normal ambient.

We used to think we were the biggest, strongest and loudest creatures in the sea. But these creatures are even bigger, faster, and much louder than we. They stir up long wakes that smell of oil and chemicals. Worst of all, they disturb some of our prey and make it harder to find them. So some of us are getting hungry and malnourished.

There have also been stories of another new kind of invader who like us lives in the deep sea and is much quieter and slower. They are scarce and only make their presence known when they shout screeching calls that penetrate the mixed layer. They are reported to be rather bumbling, stupid creatures, because they travel straight through the sea without regard for the obvious currents and thermal layers where krill are more abundant. They waste so much energy!

Anyway, the noisy surface creatures are increasing in size and number and loudness. I have already lost track of some old friends in the South Pacific, and I can only keep track of our pod members out to the second or third convergence zone, a few days' journey away. The rest are gone, masked by a curtain of chopping, hacking, jamming noise.

Our researchers have attempted to understand the source of this noise. Up close they see whirling fins, maybe three or four of them. Every time they cross the top, they cut through the water so fast that they open up an empty space. Then the water quickly rushes in to fill it. When the empty cavity opens, and when it closes, loud noises are emitted. It is the most efficient noise source we have ever seen. They call this phenomenon cavitation. It never happens in the natural sea, except perhaps on those rare occasions where we have reports that large stones fell rapidly into the water.

So we call these aliens the cavitators. They come in many varieties, from small ones with high-pitched whines, to giant ones two or three times longer than Big Blue, which pound away with a monotonous drumming sound that reverberates across the ocean.

There have been increasing reports of another species of noisy creatures that sit in fixed positions near the coasts, hammering away day and night, and spewing smelly wakes of mud and oil. Who will invade our planet next? Or what new chemical or noise will we confront?

I appeal to all of you to pray that our planet be protected from the incessant noise of the cavitators, and from the invasions of two waves of aliens within our recent past. This has all happened so suddenly. For a hundred thousand years our culture developed its traditions, and now we are facing a threat so new and strange, none of our wise ones know what to do. We are glad that the killing has largely stopped, but our culture is now under attack. Already, we can tell that our children are becoming tolerant of noise. Sometimes they even seem to prefer it to the sweet music of our past. Where have you gone, our beautiful world, our quiet ocean?


Rear view of a cavitator traveling at 16 knots

"Radiated Noise Characteristics of a Large Cargo Ship", P. Arveson and D. Vendittis, J. Acoust. Soc. Am. Jan. 2000.

Index to the Publications of E. Mercado, http://www.cmbn.rutgers.edu/~mercado/pubs.html

This article is based on a talk given at the 145th Meeting of the Acoustical Society of America, May 2003.