Echoes

When an acoustic wave bounces off an object and returns to its source, that is known as an echo.  Echoes enable animals to sense and translate information on their surroundings.  The bat emits short, high frequency “chirps” and listens for the echo.  A similar process is used by some cave-dwelling birds to navigate in the dark, and by dolphins, porpoises, and whales to locate objects under water.  People use sonic echoes through special equipment to find and track gamefish and submarines, and to map the ocean floor.

Animals
 

We have chosen one of our favorite animals, the Bottlenose Dolphin (Tursiops truncatus), for our sound exhibit.  Known for their inshore habits, playfulness around vessels and star performances at oceanariums, bottlenose dolphins, are probably the most popular of all cetacean species.

All dolphins are toothed whales belonging to the sub-order odontocetes, of the order cetacea.  As a group, dolphins are often referred to as “small” cetaceans, even though some of them are quite large, attaining lengths of over 20 feet.  In addition, although the terms dolphins and porpoises are often used interchangeably, they really refer to two different types of animals.  Dolphins possess a distinct beak. Their teeth are conical in shape. Most species of dolphins are larger than porpoises, with the males usually being larger than the females.

Dolphins of some kind occupy virtually all oceans and major seas as well as some large river systems.  Their distribution, however, is not random.  Each species has become specialized to fit into a particular niche.  A niche relates to all aspects of a species’ way of life, including not only its physical home but also its food, behavior, predators and physical environmental factors necessary for its survival.  In short, the niche defines a species’ role within an ecosystem.  For each dolphin species this role is unique.

All dolphins are carnivores.  Some feed exclusively on either fish or cephalopods (the class of marine invertebrates including squid, octopus and cuttlefish), while others have a more varied diet including fish, squid, crabs, shrimps and lobsters.

Dolphins have become marvelously adapted to life in the sea.  Anatomically, their bodies have become streamlined to move effectively in their aquatic environment.  Their hind limbs have disappeared, their front limbs have developed into flippers, and their powerful tail provides their chief means of propulsion.

Another factor that increases dolphins’ swimming speed by reducing their drag in the water is the smooth skin they possess.  Unlike most mammals, a dolphin's skin is hairless, thick and lacks glands.  It is also kept smooth by constantly being sloughed off and replaced.  A bottlenose dolphin, replaces its outermost layer of skin every two hours.  This is nine times the rate of human skin renewal.  A drawback of their smoothness, however, is that their skin is easily scarred.  Virtually all adult dolphins have an array of scars, notches and nicks that they acquired through interactions with companions, enemies or the environment.  Scars on dolphins are so prevalent in fact that researchers often rely on them as a means of identifying individual animals.  Like all other marine mammals, below the skin, dolphins have developed a thick layer of blubber to insulate them from heat loss.

Although dolphins are believed to have fairly good eyesight, their dark and murky environment often limits their visibility.  Dolphins and whales have come up with an efficient way to combat this problem.  They tend to rely chiefly on their sense of hearing to understand the world around them, much as humans rely on a combination of sight, sound and smell.

Echolocation

Dolphins and many species of toothed whales use their sense of hearing in a very sophisticated behavior known as echolocation.  Echolocation is a process where a dolphin emits a steady series of split-second “clicks” through its blowhole.  The “clicks” are pulses of ultrasonic sound (sounds repeated as rapidly as 800 times/second) produced in a dolphin’s nasal passages and focused in a large, lens-shaped organ in the forehead known as the melon.  The melon concentrates the sound pulses into a directional beam.  When the outgoing sound waves or “clicks” bounce off objects in their path, a portion of the signal is reflected back to the dolphin.  The bony lower jaw of the dolphin receives the incoming sound waves and transmits them to the inner ear where they are converted into nerve impulses and then transmitted to the brain.
 

Through echolocation, a dolphin is able to determine the distance of a target on a continuing basis by measuring the time between emitting the clicks and their return.  Dolphins regulate their rate of click production to allow the returning “echo” to be heard between outgoing clicks.  Using this amazing skill, a dolphin can create an acoustic picture of its surroundings and can determine the size, shape, and direction of movement and distance of objects in the water.  This permits dolphins to hunt prey over a greater range than the limits of visibility allow.
 
Dolphin Sonar Exhibit

This exhibit deals with how sound travels in water compared to how sound travels in air.  Through Dolphin Sonar, students explore the science behind the cetaceans’ remarkable ability: echolocation.  Dolphins use their ears to see.  Dolphins send out sounds and listen for the echoes that come back from objects in the water.  A dolphin can tell a great deal about an object by the echo it makes—that’s like seeing with your ears.

Concept:  Echolocation

Objective:  Students will gain an appreciation of the dolphin’s ability to echolocate.  Students will make the connection that aquatic animals rely on senses other than sight to help them know their environment.

How to use: Aim the dolphin at each visible object in the tank while holding down the button.  Listen carefully to the echo each object makes.  Identify the hidden objects in the tank.  Place object markers in the tank map to help remember what and where the hidden objects are.  Then look at the tank from the side to see the hidden objects and check against the markers.

Explanation:  You are hearing a slowed down version of a dolphin’s sonar “clicks”.  Dolphins send out sonar clicks 250 times faster than these.  On top of that, sound travels faster and farther in water than in air

Dolphins can hear much higher pitched (higher frequency) sounds than humans can.  Dolphins can hear sounds up to 150 kHz (thousands of Hertz or thousands of cycles/sec).  Humans hear sounds up to 20 kHz.  In this exhibit we have changed the pitch so your ears can hear the sounds and their echoes.

The first short “thump” you hear is the sound being sent into the water from a transducer (sonar device).  The transducer is under the dolphin shape at the front of the tank.  The secondary sounds you hear are echoes bouncing back from different objects in the tank.  The objects in our tank might be hollow or solid, metal or plastic, round or flat.  Depending on what material you point the dolphin toward, and how far away that object is, the echoes you hear will sound different.

Dolphins use their sonar to help them find or avoid obstacles in dark or cloudy waters.  This is called echolocation.

Physics-Biology Connection: Dolphins echolocate to help them know their surroundings.  Dolphins send out sonar clicks and listen for the echoes that come back from objects in the water.  This helps them detect things like obstacles, dangers to avoid, or a tasty meal!  By listening to the echoes, a dolphin can tell the distance to an object, how fast the object is moving, its direction, and even what it is made of and its shape.

Dolphin Information

• Dolphins: Their Life History and Biology
• U. S. Department of Commerce Ocean Acoustic Monitoring Program