High School Ocean Lesson Plans: Nature of Underwater Sound

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Sound in the Sea

Topics Covered in this Lecture:

Slideshow1: Whales1
Slideshow2: Whales2


We now focus our study on the cetacean's ability to use sound for hunting and communication. In a world blurred by billions of tiny microscopic particles, the cetaceans have raised "sound-vision" to an art form. Where humans use differential scattering of light waves in the visible spectrum to form a three-dimensional picture of the world about them, whales use a much broader range of sound waves to obtain a similar three-dimensional view. Moreover, their ability to produce sounds has allowed them to develop highly sophisticated "songs" and "calls" which appear to function in communication, group bonding, family ties, and reproduction. Of course, that's only what scientists think. Personally, I believe they also create songs for pleasure, art, and culture, but it's all a matter of perspective, isn't it?

The Nature of Underwater Sound

To understand how organisms use sound to visualize their environment, it is instructive to review a little bit about the nature of sound in the ocean. If you've ever stuck your head under the water or had an opportunity to go scuba diving, you may recall that the oceans are quite noisy. Sound travels in seawater at about 4347 feet per second (in water at a salinity of 34.85 ppt and a temperature of 20 degrees C). Compare this to the speed of sound in air, 1129 feet per second (at sea level and 20 degrees C), and you get the picture. Water transmits sound much better than air.

The reason for this has to do with the nature of sound itself. Unlike water waves and light waves, which transmit as a sine wave, sound waves transmit by compression and expansion of the medium through which they travel. That's why sound vibrates objects whereas light (and wind energy in the form of waves) do not. Thus, the speed of sound through a medium, such as air or water, is determined by the properties of that medium.

As a result, as the density of water increases, the speed of sound increases. Consequently, changes in the density of seawater, either from changes in temperature, salinity, and even pressure, can change the speed of sound through water. For example, a 1 degree increase in temperature causes sound to travel about 13 feet per second faster.

Like light and water waves, sound waves have a wavelength. The wavelength of sound is defined as the distance between successive compressions. However, sound is typically measured in frequency, which is defined as the number of compressions passing a given point each second. A convenient unit for measuring sound waves is the hertz (like the rental car agency that a certain football player used to advertise for, but having nothing to do with cars, football, or lawyers, whatsoever). One hertz is defined as a frequency of one wave per second. Low frequencies sound like groans, grunts, and low rumbles (in the neighborhood of 100 hertz, or 100 waves per second). High frequencies sound like high-pitched whistles and shrieks (in the neighborhood of 18,000 hertz). Ultra-high frequencies "sound" like a dog whistle, which we can't hear anyway! Humans detect sounds in the range of 20 to 20,000 hertz. Whales appear to hear in a range from 10 to 170,000 hertz, considerably better than humans.

Finally, we should note that sound, like temperature, salinity, density, nutrients, and a whole bunch of other things we've studies this semester have a characteristic vertical profile. For reasons beyond the discussion of our course (having to do with refraction, reflection, and scattering of sound in the ocean), sound appears to reach a minimum speed at about 1000 meters in tropical oceans. Low frequency waves that transmit into this channel get "bent" or refracted in such a manner as to stay in the channel. This channel where the speed of sound reaches a minimum and where sound tends to remain is known as the SOFAR (meaning Sonar Fixing and Ranging channel. Within the SOFAR channel, low frequency waves may travel hundreds, if not thousands, of miles before dissipating. In temperate waters, this minimum sound depth is shallower, and it reaches the surface between about 60 degrees N or 60 degrees S. This phenomenon is probably controlled by the presence of a deep thermocline (or central water thermocline) and a seasonal thermocline (or at least an intermediate thermocline) which form a lower and upper barrier, respectively, to "channel" sound waves.

Whales, particularly the humpback whales, have learned to make use of the SOFAR channel. Strange and mysterious low-frequency sounds, attributed to humpbacks and other baleen whales, are a common occurrence here. It should not surprise you that the Navy maintains a constant vigilance on the sounds in this layer.

Echolocation and Sonic Vision

Now let's see how organisms make use of sound in the ocean. Recall that only the toothed whales, which include the dolphins and porpoises, appear to echolocate their food. There has been some speculation that the gray, blue, and minke whales, which are baleen whales, and the Weddell seal, California sea lion and possibly the walrus also echolocate, but the data are sketchy.

The use of sound to locate and identify underwater objects or other organisms is called echolocation. Echolocation works just like SONAR; sound waves are sent out in a particular direction and the time for reflections to return is measured to determine distance. However, in whales and dolphins, the sonar signal and the returning echoes are composed of multiple frequencies, and provide a much more detailed sonic "picture" of objects or organisms in the path of the sound beam. By producing a complex sound signal, the animal can determine not only distance of objects, but also size of the object.

Because the dolphins are the best-studied group of whales, we will use them as an example of how cetaceans produce and detect the sounds used in echolocation. As shown in the figure below, the dolphin produces bursts of sound, called clicks (which is what it sounds like to humans. Individual clicks are composed of a mix of sound frequencies ranging from 10 to 170,000 hertz, and are often emitted in rapid succession, anywhere from five to several hundred per second. To our ears, this rapid succession of clicks resembles the sound of a rusty hinge or creaking door. Slowed down for our hearing, clicks have been said to sound like a "mix of flutes, clarinets, police whistles, factory whistles, French horns, diesel horns, sousaphones, and foghorns." Each click may be considered to represent a mixture of all these sounds. That's quite a symphony!

Clicks are produced within the dolphin's air passages somewhere in its forehead below the blowhole. An flexible, oval, fat-filled organ called a melon appears to focus the clicks, and in conjunction with the specially shaped structure of its skull, allow the dolphin to produce two well-focused beams of sound from its head. Echoes from returning objects, although weaker in intensity, are transmitted to the dolphin's jaw, which triggers the inner ear located at the hinge of the jaw. The melon also appears to absorb and focus sounds and transmit them to the inner ear. From the inner ear, nerve impulses are sent to the brain where the whole picture is put together, just like the visual picture put together as a result of stimulation of our optic nerve by the rods and cones in our eyes. In this way, the dolphin "sees" its world and uses this information to carry out much of its daily tasks of living.

As mentioned above, both low-frequency and high frequency clicks are used by dolphins to identify objects. Low-frequency sounds work more like sonar to determine the presence or absence of an object. High frequency sounds enable the animal to distinguish very fine features. Continuous production of multiple frequency clicks while swimming allows the animal to avoid or pursue objects in real-time (see figures below). Incredibly, dolphins are able to distinguish between two species of fish even if they are the same size. They have also been shown to discriminate different thicknesses of the same object. In short, echolocating cetaceans can distinguish species of organisms, analyze the three-dimensional makeup of objects, distinguish between objects of different materials, tell which objects are nearer or farther, and make other determinations based on sonic "echoes" from objects. This sonic picture can be quite different from the visual picture, but far more useful for life in aquatic environments.

Communication and Songs

As sophisticated as echolocation might seem, it is only a warm-up for the other uses of sound that these incredible animals employ. Without question, the ability of these animals to produce whistles for communication and songs places them among the most intelligent beings on our planet. While scientific progress is slow, and hampered by its own professional interests at times, there is little question that whales and dolphins have developed a very sophisticated "language" for communicating among themselves. Whether this language is purely utilitarian, or whether it involves expression of feeling and abstract thought, may take considerable time for scientists to determine. However, for my part, I prefer to think that they sing for the same reasons that we sing, to express joy or sadness, to celebrate or reflect, and perhaps, to convey complex philosophical opinions on the issues of the day!

In addition to the clicks used in echolocation, cetaceans also emit other sounds, commonly whistles and groans. Whistles can be produced simultaneously with clicks and are believed to function to communicate information, to maintain contact among members of a pod, or as a mating ritual. Although clicks in baleen whales are quite rare, whistles and groans are common. All whales, it appears, are capable of producing sounds for communication.

Bottlenose dolphins appear to be highly adept at communicating information to each other. In one case, two bottlenose dolphins in captivity were shown to be able to communicate to each other through microphones in separate enclosures. One dolphin, Doris, was able to respond to a light signal in her pool, and whistle to her companion, Buzz, to push a lever in his pool. Buzz then whistled back and Doris pushed a lever in her pool that completed the process and rewarded them both with a fish.

Studies of killer whales show that sounds play an important part in herding of prey fish and possibly teaching skills to younger animals. When searching for prey, killer whales will spread out in formation and maintain close contact with other members of the pod using whistles. Upon sighting a school of fish, a different set of whistles will spread through the pod and they will join in a circling formation to concentrate their prey. So distinctive are the communication sounds of killer whales that a trained observer can distinguish one pod from another. Each pod appears to have its own dialect, that is, a different sets of whistles and groans that distinguish them from other pods. For more information on killer whales, their feeding behaviors and communication, check out the video entitled "Killer Whales: Wolves of the Sea."

Communication among bottlenose dolphins has been studied for decades. Although a kind of "language" is definitely established, our attempts to understand that language have not been successful. As an example of the types of studies that have been performed and the kinds of information that has been learned, take a few moments to review the dolphin "shorthand" illustrated below. Whistles are indicated as rising or falling lines, corresponding to a rising or falling whistle. The data below were taken from a study on three juvenile captive bottlenose dolphins introduced to a model porpoise in their tank. Each set of lines, whether rising, flat, or descending, or combinations of the two, appeared to correspond to specific behaviors. While it is difficult to interpret these sounds based on one figure, you should gain some appreciation for the complexity of these sounds and the enormous task of scientists studying them.

Perhaps the most mysterious of sounds are those produced by the humpback whales. The songs of these beautiful animals are legendary for their rich and varied tones and fugues. Each song lasts from 10 to 30 minutes and each population of humpback whales has its own distinct repertoire of songs. Songs of the humpback whale are only produced during mating season and they are only sung by the males. Once a female has chosen a male, he quits singing. At other times of the year, when not breeding, humpbacks produce only occasional sounds.

Male humpbacks assume a head-down posture when singing and the character of the songs can be quite complex. Typically, songs are divided up into repeating phrases called themes. After a couple minutes, a new phrase may be introduced, starting a new theme. Each song may contain two to nine themes that, once finished, are repeated again. These songs may continue uninterrupted for days, repeating over and over at 15-30 minute intervals.

Individuals from a given locality all sing the same song, but songs from humpbacks in different oceans, such as the Atlantic and Pacific populations, can be quite different. Changes in a song may occur as the mating season progresses, but any changes are incorporated by all individuals. Although changes in the songs occur over the years, the population's shared song is still distinct even after many years.

Fin whales, and possibly other whales, appear to be able to communicate over long distances using the SOFAR channel. Low frequency grunts have been tracked for miles by hydrophone stations dispersed throughout the oceans. These sounds can be detected at distances more than 1000 miles and it is hypothesized, but not proven, that finback whales use the SOFAR channel to maintain long-distance communication.

How Intelligent Are Whales?

The question of the intelligence of whales and dolphins has always fascinated humans. With a brain size slightly larger than humans and enough neural connections to place them on an equivalent basis, the question is, are they intelligent?

Research with two captive bottlenose dolphins at the University of Hawaii demonstrated that dolphins could learn languages based on words. The dolphins learned nouns, verbs, and abstract words and demonstrated an ability to synthesize new phrases using learned words, despite never having encountered them in the same exact order. For example:

"...during a workout with Phoenix in which she was responding to familiar commands (such as FRISBEE FETCH HOOP, "bring the frisbee to the hoop), the trainer signaled a new sentence that Phoenix had never seen before: FRISBEE FETCH THROUGH HOOP. Phoenix picked up the frisbee... and swam through [the hoop].

In another example, after signaling HOOP FETCH, seven objects were placed in a pool (including the hoop) and the correctly identified the hoop. Whether the trainers used the same hoop or a material resembling the hoop, the dolphins were still able to distinguish a hoop-shape from another.

In a final example, the dolphins demonstrated an ability to "reason" when circumstances presented themselves. When given the command THROUGH HOOP, with the hoop lying at the bottom of the pool, the dolphins would stand the hoop on edge and dart through it before it settled back down to the bottom.

Other examples of intelligence abound in the dolphin literature. The difficulty with the question of intelligence resides most fundamentally in defining intelligence. If we were absolutely unfamiliar with the language and habits of humans, would we be able to discern if they were intelligent. Without revolutionary new methods for quantifying intelligence, in humans and animals, the question of the intelligence of whales will be unresolved. However, the serene and magnificent nature of these animals will be appreciated, no matter what grade they score on the intelligence scale.

from The Remarkable Ocean World