What is Sound?

Most of us take sound for granted. It's always there -- the normal background sounds of city living: Neighbors' lawnmowers, car stereos, air traffic, and the biggest component, vehicle traffic. Since humans rely on sight more than sound, we tend to ignore these normal everyday sounds except when they become annoyingly or painfully loud. If we can't do anything about it, such as turn it down or get away from it, we become stressed and can even be physically injured. If we are repeatedly exposed to loud sound levels (at live rock concerts, for instance), we can permanently damage our ears to the point where things have to be louder and louder for us to hear them, or until we lose our hearing altogether.

Something similar is happening in the world's oceans.

The background noise level has steadily risen over the past four decades, mostly due to increasing commercial, industrial and military ship traffic. Jacques Cousteau's "Silent World" no longer exists; indeed it never truly did. Snapping shrimp, underwater earthquakes, and storms generate incredible levels of noise (see ambient noise chart).

Now manmade (anthropogenic) sound dominates the marine environment.

Because of the qualities of water itself -- its opacity, salinity, constant movement, variable temperature, and the shape of the land beneath it -- the only thing that can "see" into it or through it is sound. This is why whales, dolphins and porpoises, over millions of years of evolution, have developed a unique way of navigating and communicating and finding food -- through the use of sound. The nearly 80 species of cetacean all use varying levels and frequencies of sound, some very high-frequency almost out of the range of human hearing, to extremely low frequency that cannot be heard by humans but can be felt. Generally speaking, the toothed cetaceans (odontocetes) use higher-frequency sounds that we humans can hear, from whistles to series of clicks (called "click trains"). The baleen whales (mysticetes), and especially the largest one such as the blue whale, produce sounds so low in frequency that they can only be heard by specialized listening equipment. Even the oft-recorded humpback whale song has low-frequency components.

The uses and meanings of sounds the different cetaceans produce are still not well understood by the scientists who study them, but all agree that sound is as critical to the survival of these wholly aquatic marine mammals as is air. Without using sound, the cetaceans couldn't find food, couldn't navigate, couldn't attract mates or keep tabs on young, and cannot avoid collisions with unseen obstacles or avoid predators.

Obviously, the louder the ocean becomes, the harder it is for animals to hear or be heard. Given enough time, species can adapt to changes in their environment. When changes happen too quickly and too drastically, species simply cannot adapt and are negatively affected.