Marine Mammal Sounds Lab

What is sound and how is it different underwater?

Sound is caused when objects vibrate. The vibration of an object creates a pressure wave that can travel through any material. Sound waves (like water waves) can be described in terms of their frequency, wavelength, and amplitude.

Frequency is the number of pressure waves that pass by a reference point per unit time and is measured in Hertz (Hz) or cycles per second. To the human ear, an increase in frequency is perceived as a higher pitched sound, while a decrease in frequency is perceived as a lower pitched sound. Humans generally hear sound waves whose frequencies are between 20 and 20,000 Hz. Below 20 Hz, sounds are referred to as infrasonic, and above 20,000 Hz as ultrasonic. The frequency of middle “C” on a piano is 246 Hz.

Wavelength is the distance between two peaks of a sound wave. It is related to frequency because the lower the frequency of the wave, the longer the wavelength.

Amplitude describes the height of the sound pressure wave or the “loudness” of a sound and is often measured using the decibel (dB) scale. Small variations in amplitude (“short” pressure waves) produce weak or quiet sounds, while large variations (“tall” pressure waves) produce strong or loud sounds. Click here to read more.

Generally the more dense a material is the better sound travels through it. Sound travels much faster in the water (1500 meters/sec) than it does in the air (about 340 meters/sec), and because of this sounds travel much further underwater than they do in the air. Water temperature also has an impact on sound waves. Cold water is denser and sound waves travel faster and further in cold water.

The ocean's thermocline can create a channel (called the SOFAR channel) in which sound travels. Sound travels slower in cold water than it does in warm water. Therefore, as depth increases, temperature decreases, and the speed of sound decreases. Conversely, sound travles faster in denser materials. SO, as pressure (and density) increases with depth, the speed of sound increases. The two competing forces create a zone (near the bottom of the thermocline) of minimum sound speed. Sound waves bend, or refract, towards the area of minimum sound speed. Therefore, a sound wave traveling through a thermocline (a region of rapid change in temperature with depth) tends to bend downward as the speed of sound decreases with decreasing water temperature, but then is refracted back upward as the speed of sound increases with increasing depth and pressure. This up-down-up-down bending of low-frequency sound waves allows the sound to travel many thousands of meters without the signal losing significant energy.

Response 1: Explain how sound waves in the water are different than in air . In your explanation explain the impacts of decreasing temperature and increasing pressure on sound travel.

How do we listen to whales?

Bioacoustics is the study sounds produced by animals. To listen to animals underwater scientists use an instrument called a hydrophone. Hydrophones are waterproof devises that produced electricity when subjected to changes in pressure. These electrical impulses are converted back to soundwaves that we can hear by a computer. The sounds produced by large whales are often in a frequency range far lower than the human ear can be perceived. When you listen to some of the sounds below they may have been sped up many times over so that your ears can perceive them. An array is a network of hydrophones set out over a large geographical area. Hydrophone arrays can be useful for tracking or counting marine mammals.

Response 2: Explain how scientists study Bioacoustics ?

Sounds for communication

Baleen Whales : Baleen whales are generally more solitary than toothed whales. However, because of their size and the low frequency (long wavelength) sounds that they use, its possible for large baleen whales to have "conversations" with other whales hundreds, and maybe even thousands of miles away. Most of the baleen whales produce songs as well as relatively simple communication calls. With the exception of humpback whales, we do not yet have a complete understanding of how baleen whales use sound or songs to communicate.

Response 3: Listen to each of the recordings above. Select three of them and describe (in your own words) what they sound like.

Toothed Whales : By their very nature, toothed whales are all highly social. When you listen to their sounds, much of it will sound like "chirps," "whistles," and "sqeeks." These are often whales or dolphins chatting with other members of their pod. Toothed whales have highly sophisticated systems for using sound to locate prety known as echolocation.

Response 4: Listen to each of the recordings above. Select three of them and describe (in your own words) what they sound like. You may want to compare and contrast them to the baleen whales.

Sounds for Mating

Every winter in the waters of Hawaii the water is filled with the sounds of male humpback whales singing. The songs have a basic structure with a series of sounds repeated over time in patterns or phrases. Each phrase is then repeated several times to comprise a "theme".

A typical song is then made up of 5-7 themes that are usually repeated in a sequential order. A song typically lasts 8-15 minutes (although it may range from 5-30 minutes), and then is repeated over and over in a song session that may last several hours. The sounds that comprise a humpback song are varied and can range from high-pitched squeaks to lower frequency roars and ratchets.

A striking feature of the song is that it gradually changes or evolves over time. Each year, different sounds and arrangements of sounds form to create new phrases or themes. These changes are slowly incorporated into the song, while some older patterns are lost completely. The song apparently changes as it is being sung. The change in the song display seems to occur in a collective or common way throughout the population. Usually after a period of several years, the song is virtually unrecognizable from the original version.

Response 5: Explain what you have learned about humpback "singing." In your response you may talk about: why is it that only males sing, how are individual songs similar, how are they different, what is the goal of their songs.

Sounds for Hunting

Sperm whales are by for the most acoustically active whales in the ocean. They have the most dramatic range of sound production and much of it appears to be used for locating prey in the dark, deep ocean where they feed.

Response 6: Describe three disticnt hunting sounds made by sperm whales. What do scientists believe about the ability of sperm whales to stun their prey with sound?

Echolocation Explained

In addition to communication, toothed whales have highly sophisticated systems for using sound to locate prey known as echolocation. These sounds are produced when air is blown through the blow hole causing internal lips to vibrate. These vibrations are then focused by large fluid filled structures in the head. In sperm whales this structure is called the spermiceti organ. It is a huge (a sperm whales head is one third of its overall body length and the spermiceti organ is the larges structure in the head) wax lipid filled structure that was at once prized for its use in perfume. In dolphins and the smaller toothed whales this structure is known as a "melon." When sounds waves travel out into the water they will bounce back when they hit a solid object. Returning sounds are picked up or "heard" in the whales jaw bone since whales and dolphins do not have ears. The jaw bones of most toothed whales are hollow, fluid filled, and have nerve endings for the detection of sound. Echolocation allows these whales (as well as bats and a few birds) to see by sound. Toothed whales perceive their environment much more by sound than by sight.

Response 7: Create your own labeled diagram to show echolocation works. In your diagram be specific to thestructures of the head that produce and recieve sound waves.

Is the ocean getting too noisy for whales to hear themselves?

Response 8: Read the first two pages of Whales Watching us, and explain the controversy presented to the Supreame Court in 2008 about the impacts of naval sonar use on whales and dolphins.