Humpback whales, one of the ocean’s biggest animals, are no exception. Male humpbacks sing using complex combinations of eerie groans, trills, whistles, and growls that travel for miles, perhaps sending signals to impress females and intimidate other males. Humpbacks sometimes copy each other’s songs and even pass songs from one generation to the next.
But noise generated by human activities is increasingly interfering with residents of the underwater world. Scientists have observed migrating humpbacks avoiding noisy areas, making detours that could cost them valuable time and energy. Loud noise also may increase mass strandings of whales and dolphins. Researchers recorded only five of these events from 1900 to 1950, the year naval operations began using super loud, high-power sonar. From 1950 to 2004, they logged more than 1200 mass strandings.
Now, some companies are employing tiny bubbles to stifle ocean noise and protect these marine giants. The technology creates curtains of shimmering air bubbles rising through the water. While individual bubbles are fragile things, walls of them can muffle a major source of loud underwater noise: pile driving.
Piles are supports for bridges, wind turbines, oil and gas platforms, and other large marine structures. They’re driven deep into the seafloor using hydraulic hammers that repeatedly and rapidly strike their tops. It makes a lot of noise. Depending on the pile diameter, hammer size, bottom properties, and the depth needed, driving in a single pile can take several hours and as many as 5000 strikes.
Bubble curtains are formed by forcing air through closely spaced openings in tubes placed in a ring around a stationary source of noise such as pile driving. Air and water have different densities and transmit sound differently, so the bubbles absorb some of the sound energy and some sound bounces off them, essentially becoming trapped inside the curtain.
Quantifying how much difference a bubble curtain makes is tricky, given variables such as turbulence, water depth, and temperature; the structure of the curtain (including size and force of the air); and the complexity of sound itself. Sound moves in waves that, when they pass our ears (and those of whales), cause a change in pressure, known as amplitude. Increasing amplitude makes a sound louder and increases its intensity, expressed in decibels (dB). Each dB level increase of 10 represents multiplying the intensity by 10.
For example, a 10 dB sound is 10 times more intense than a 0 dB one, a 20 dB sound is 100 times more intense, and a 30 dB sound 1000 times more intense. While dB levels in water are not the same as those in air, the general principles are similar.
Decreasing the Decibels
Scientists have measured reductions of up to 30 dB from bubble curtains used around pile driving for a bridge in California [PDF]. German company Hydrotechnik Lübeck claims that its Big Bubble Curtain, currently in wide use in the North and Baltic Seas, can reduce the volume of pile driving noise by up to 95 percent.
Tobias Bohne, a researcher with the Institute of Structural Analysis at Leibniz University in Hannover, Germany, has studied the effect of bubble curtains on pile driving noise. He tells Mental Floss that an advantage of bubble curtains is that they can be placed far enough from the source to be combined with other systems designed to reduce noise, such as cofferdams (insulating sleeves around the pile) and dampeners on the hammers. One study showed that a combination of a bubble curtain and a cofferdam reduced noise levels more than 10dB—a 10-fold decrease [PDF].
Other tweaks to pile driving operations can help as well. For example, starting with lower hammer energy levels and gradually ramping up the force can encourage animals like whales to move away. In areas with continuous activity—installation of multiple wind turbines, for example—taking breaks in pile driving lets animals travel through or briefly return to an area, perhaps to feed.
Our noise in the ocean is causing big problems. But small changes in our behavior, and the use of technology like tiny bubbles, can help protect even the largest creatures that live there.