Expedition 12 Hot Topic: The Scoop on Sound
By Kristen Kusek
Under water, it's difficult to communicate!
(Photo by Amy Nevala, WHOI)
The basic components of a sound wave are frequency, wavelength, and amplitude. In this example of a sound wave, the period of one cycle of this wave is 0.5 seconds, and the frequency of this wave is 2 cycles per second or 2 Hertz (Hz). Click images for a larger view.
These two waves have the same frequency but different amplitudes.
These two waves have the same amplitude but different frequencies.
Images above based on NOAA's Ocean Explorer
Have you ever tried to talk to a friend in a pool—while you were both under water? If so, whatever you said probably sounded like an inaudible string of garbly “gloughs” that made you giggle so hard you had to pop up for a breath of air. Although a pool conversation (if you can call it that) proves that sound travels through water, it also proves that our vocal chords don’t transmit sound that well in the water. That’s why scuba divers usually use hand signals to communicate with each other.
Scientists studying the ocean use much more sophisticated methods to communicate under water using sound. Sound travels in the form of waves, as light does. The neat thing about sound is that scientists can transform information into sound waves and send it over great distances. Scientists use sound to send and retrieve information from undersea robots, buoys, and other ocean tools, and they are always working hard to improve sound-based technology. Ocean acoustics, the study of sound in the sea, is a rapidly growing field of research today.
The history of using sound to “communicate” in the sea goes all the way back to Leonardo da Vinci, the famous Renaissance Italian scientist (and painter, and writer, and inventor…the list goes on) who died nearly 500 years ago. Historians say he once stuck a long tube in the ocean to listen for a faraway ship and later designed an underwater breathing apparatus. In modern times, studying how sound travels in the sea really took off during World War II, when the navies used sound to track submarines.
Catch a wave
When an object vibrates under water, it creates pressure waves that make water molecules either squeeze together (compress) or stretch out (decompress). Sound waves radiate out from the source of the sound like ripples on a pond after you throw a stone into it. Sound travels much faster in water (1,500 meters/second) than it does in the air (340 meters/second).
Picture a long roller-coaster-like sound wave, with top-rounded peaks and U-shaped troughs (bottoms). Sound waves have three main characteristics: frequency, wavelength, and amplitude. Wavelength is the distance between two peaks of the sound wave. Amplitude is the height of the “bump,” or the “loudness” of the sound (Think “amp” on a stereo!).
The third part of a sound wave—frequency—is important for our autonomous underwater vehicles (AUVs), Puma and Jaguar, on this expedition. Frequency is the number of pressure waves that pass by a reference point per unit time; it is measured in Hertz per second (Hz/sec). Musicians call this “pitch.” The shorter the wavelength (distance between the peaks) is, the more pressure waves you can fit into a given time slot, so the higher the frequency—and vice versa. Humans hear sound waves with frequencies between 20 to 20,000 Hz. Anything above 20,000 Hz is called “supersonic,” which means it is even faster than the speed of sound. Dogs can hear these frequencies, whereas humans cannot. Cats such as pumas and jaguars—the names of our AUVs on this cruise—can hear even higher frequencies, enabling them to hear even the slightest squeak from a mouse in the wild. Lots of military aircraft and even the Space Shuttle can travel at supersonic speeds—literally “breaking the sound barrier.”
Depending on their frequency, sound waves can travel over short distances or all the way around the world. The lower the frequency (and longer the wavelength), the farther sound waves can travel through water. For our purposes of communicating with Puma and Jaguar, that distance may be several thousands of meters away, and scientists aboard the Knorr will use sound waves at a frequency of 8 to 12 kHz—which is what a bunch of car keys sound like if you jingle it.
The SOFAR Channel
The ocean has a feature called the thermocline, an area where the water temperature changes very quickly as you go deeper. To understand why the SOFAR channel exists, we have to look at pressure, too. Pressure increases by one atmosphere every 10 meters of depth in the water column. The interplay between temperature and pressure creates the SOFAR channel through which sound waves travel great distances at their slowest speed. Notice that as you go deeper in the water, the speed of sound (green) first follows the temperature pattern (red), and then it is more affected by pressure and follows the pressure pattern (yellow).
New hydrophone arrays have been developed to detect and monitor previously "unheard" volcanic and earthquake activity in the oceans. They are deployed in the SOFAR channel, a layer of water in the ocean that channels sound waves generated by seismic events and allows them to be transmitted and detected thousands of miles away. Click the image to view an animation.
The temperature and pressure of the water also affects the speed of sound as it travels in the ocean. Sound travels faster in warmer water and in waters under higher pressure. This phenomenon gives the ocean an interesting ability to trap sound waves at a certain depth in sort of a tube or tunnel, through which sound can travel for many miles. Scientists think that’s how humpback whales get such great mileage out if their songs; they go down to this special zone to sing and send their songs far and wide. Here’s how it works:
The ocean has a feature called the thermocline, an area where the water temperature changes very quickly as you go deeper. Below the thermocline, seawater temperature remains constant and cold. Most of the world’s ocean lies below the thermocline.
Sound waves travel more slowly as you go deeper into colder water. They reach their slowest speed at the bottom of the thermocline. Then below the thermocline, another factor becomes important: pressure. Under the pressure from the weight of all the water above, sound waves speed up again.
So you have two areas where sound waves travel fast, in warmer waters near the surface and in waters at depth where pressure is higher. In between, you get a thin channel between 500 to 1000 meters wide, where sound waves travel more slowly. Scientists call this the SOFAR channel (SOund Fixing and Ranging). The ceiling and floor of this channel are the water layers above and below, where sound waves speed up. In low to middle latitudes, the SOFAR channel is between 600 and 1200 meters below the sea surface. It is closer to the surface in high latitudes.
SOFAR, so good?
When sound waves enter the SOFAR channel, they slow down. When they hit the channel’s ceiling or floor, they do not keep going into the “fast lanes,” but are refracted off the ceiling or floor. In this way, sound waves are “channeled” in the SOFAR channel, so that their energy can propel them horizontally over long distances. Listening in the SOFAR channel, scientists or naval personnel can record sounds whose source is thousands of kilometers away.