Unit 2. Lesson 3. Sound Use by Marine Mammals

ACOUSTICAL OCEANOGRAPHYUnit 2. Lesson 3. Sound Use byMarine MammalsLesson Objectives: Upon completion of this lesson, students will have gained the abilityto:• understand the importance of sound to marine mammals.• measure frequency, wavelength and the speed at which sound travels• understand why sound is emitted in different frequencies.Vocabulary: cognition, navigation, maternal, social structure, echolocationFunctions and Uses of SoundSound use by marine mammalshas been studied for manyyears. Generally, marinemammals use sound forcommunication, exploration,advertisement, locating food,maintaining mother-offspringbonds, and to identifyindividuals with their pod orgroup.Echolocation, defined as theability to gain information fromsounds produced by the animalthat bounce off distant objectsand return as echoes, is anexample of how sound is usedby marine mammals. Scientistshave learned that Odontocetesuse echolocation. Often, thisability allows the animal tointerpret their surroundings withor without vision. Echolocationis used over great and smalldistances. These echoesconvey much information aboutthe environment. For example,a dolphin can detect a smallball over a football field away, adistance too great to seen byeither dolphin or human.Only a few animals have beenshown to use echolocation.These include dolphins, bats,and a few species of birds.Some of the sounds theseanimals produce might beabove or below a human’scapability to hear. Althoughpeople can not hear thesounds, the animals are usingthem, and processing them tovisualize objects and targetothers for food, or avoidance.©PROJECT OCEANOGRAPHYSPRING 200075

ACOUSTICAL OCEANOGRAPHYAPPLICATIONS OF SOUND IN THE OCEANThe sounds used by marinemammals provide evidence ofhow sound can be used andwhat the animals can learnfrom them. Humans have usedsounds in the ocean to studywhales, learn migrationpatterns, and study geography.Bluewhalesandotherbaleenwhales have the ability toproduce sound so low infrequency that they are anoctave or more below thelowest sound the human earcan hear. Blue whale soundslast many seconds. Bluewhales commonly emit soundsthat have a frequency ofapproximately 17-hertz (Hz).This corresponds to a soundwave of about 88 meters longin water. A wavelength of thissize can travel ocean basinsand can be heard hundreds ofkilometers away. Most likely, asound of this frequency is usedfor navigation. The wavelengthis very long, and can help todetermine if there arelandmasses in the whales’path.Sounds with very longwavelengths are used todiscriminate large obstacles.However, they can not be usedto find small obstacles or fish.The distancebetween thewavelength is solong, it can easilypass over smallobjects. For an object to bedetected in any sizewavelength it must be 1/5-1/4the length of the soundwavelength emitted. Look atthe following example:The click from a dolphin can beheard at 100kHz. It has awavelength of approximately1.5cm, so it is small enough todiscriminate between rocks andsmall fish. It can even relayinformation about theenvironment: whether or notthere are pilings present, or ifthe dolphin is swimming in anopen channel.©PROJECT OCEANOGRAPHYSPRING 200077

ACOUSTICAL OCEANOGRAPHYActivity 3-1. Target SizeFor animals that use echolocation to detect objects in water or air, itis necessary for them to use a frequency that is most favorable to thesize of the target. Objects, landmasses or features of an object donot reflect sound very well, thus, the information is relayed to theanimal improperly incompletely.Materials:• paper• pen• thinking…..Procedure:1. Solve the following equations.The speed of sound in seawater is approximately 1500m/sec.The wavelength, λ, of a sound equals the speed of sound, c, dividedby the frequency, f.Therefore: λ = c/f .In other words, if λ is 4-5 times the length of the object, the object willnot significantly interrupt or reflect the sound wave.Example: This suggests sound frequencies on the order of 150 kHzor higher must be used to detect targets of a size approximately 1cm.If λ = 1cm = 0.01m and the speed of sound is equal to 1500m/sec.Then, rearranging the equation λ = c/f , to c/λ=f the numbers fit in asfollows:(1500m/sec) /0.01m = 150000Hz or 150kHz.(Remember that there are 1000 hertz in a kilohertz. And hertz aredefined at cycles per second, or in units of 1/sec.)78©PROJECT OCEANOGRAPHYSPRING 2000

ACOUSTICAL OCEANOGRAPHYProblem 1:If an object is 2m in length, what wavelength (λ) is needed to detectit? Remember, for an object to be detected using sound, it must be¼ or 1/5 the length of the sound wavelength emitted.Problem 2:Determine the length of a squid that a sperm whale is diving for if itusing a frequency of 1500 cycles/second and the speed of sound inseawater is 1500m/sec. First, you must determine the wavelength (λ)that the sperm whale emits to detect the squid. (λ) = c/fThen, determine the length the squid must be to be detected. Asquid must be ¼ - 1/5 the length of the sound wavelength emitted.Problem 3:Determine the speed of sound in the frigid arctic water where Orcaslive during the summer. The Orca generally uses a sound frequencyof approximately 2kHz to warn others of danger. The wavelength ofthe sound wave is 0.7 meters.Answers to Problems 1-3.1. (2m) x (4) or (2m) x (5). The wavelength must be no more than 8or 10 meters in length to detect an object that is 2 meters inlength.2. (1500m/s)/(1500cycles per second) = 1m or 100cm is the length ofthe sound wavelength emitted. To determine the length of thesquid that is detected, divided the length of the squid by 4 or 5.(100cm)/4 or (100cm)/5 = 25cm or 20cm The squid must bebetween 20 and 25 cm in length for the wavelength of 100 cm todetect it.3. λ=c/f We know f=2000Hz and λ=0.7 m so we can solve for c byrearranging the equation to be c= λf = (0.7m)(2000Hz) = 1400m/sec. The speed of sound in cold water is slightly slower thanin warmer water.DID YOU KNOW?There are currently seven species of cetaceans in U.S. waters thatare protected under the Endangered Species Act. They are the Bluewhale, the Bowhead whale, the Fin whale, the Humpback whale, theNorthern Right Whale, the Sei whale and the Sperm whale. All seven species arelisted as endangered. Scientists use information gained from their calls to trackthem, and monitor they’re well-being.©PROJECT OCEANOGRAPHYSPRING 200079

ACOUSTICAL OCEANOGRAPHYActivity 3-2. Ocean Scramble.There are many different words that are used to describe echolocation, andwho uses it. Correctly unscramble the words below, and solve the mysteryword with the underlined letters.EAHWLCATEINCOMMUCCKLIMMMAALLWAVNGEETHVIRONENMNETOPDSHIFNHLPIODLARPO BREAOODFAONGRNow use the underlined letters to solve the mystery word. You should havethe following letters: NIOTCALOOECH80©PROJECT OCEANOGRAPHYSPRING 2000

ACOUSTICAL OCEANOGRAPHYStudent Information Sheet 3. Sound Useby Marine MammalsThe sounds that marinemammals make havefascinated people andresearchers for years. Peoplelike them so much, thatrecordings of marine mammalsounds, especially whales anddolphins, have become bestsellers.Marine mammals use sound forcommunication, exploration,advertisement, locating food,maintaining pup-motherinteractions, and to identifyindividuals within their pod orgroup. Some of the animalsproduce sounds that are toolow-or too high- pitched forhumans to hear. Some animalsthat use sound includedolphins, bats, fish,invertebrates, polar bears,otter, seals and whales.surroundings when light andvisibility are low.Some sounds used by animalshave very long or very shortwavelengths. Sounds with longwavelengths are used fornavigation, and exploring.Long wavelengths can travelhundreds, even thousands ofkilometers. These kinds ofwaves are so long, they caneasily pass over small objects.Therefore, an animal must alsobe able to emit shorterwavelengths. For an object tointerrupt a wavelength, it mustbe 1/5-1/4 the length of thewavelength. Sounds with shortwavelengths are used to learnabout details in theenvironment, or to find smallobjects.Echolocation is defined as theability to produce highfrequency clicks and to detectechoes that bounce off distantobjects. Marine mammals useecholocation to identify otheranimals, the environment, andmigration paths. Usingecholocation, mammals havethe ability to ‘see’ their©PROJECT OCEANOGRAPHYSPRING 200081