Sound in the Sea
Sound waves travel through water at a speed of about 1.5 km/sec (0.9 mi/sec), which is four-and-a-half times as fast as sound traveling through air.
High frequency sounds don't travel far in water. Because of their longer wavelength and greater energy, low frequency sounds travel farther.
Commerson's dolphins probably rely on sound production and reception to navigate, communicate, and hunt in dark or murky waters. Under these conditions, sight is of little use.
A Commerson's dolphin makes sounds by moving air between nasal sacs in the blowhole region.
- In contrast, a human makes sound by forcing air through the larynx. The vocal cords in the larynx vibrate as air flows across them, producing sounds. Our throat, tongue, mouth and lips shape these sounds into speech. The larynx of a Commerson's dolphin does not have vocal cords.
- A tissue complex in a toothed dolphin's nasal region, called the dorsal bursa, is the site of sound production. This complex includes "phonic lips" - structures that project into the nasal passage. Toothed whales make at least some sounds by forcing air through the nasal passage and past the phonic lips: the surrounding tissue vibrates, producing sound.
Although Commerson's sounds are mostly inaudible to us, the dolphins vocalize continually, producing high-frequency, narrow band pulses of 125 to 135 kHz. Not only is this range beyond human hearing, but it's also above the frequency range for other dolphins. Commerson's dolphin vocalizations must be slowed down at least eight times for most people to hear them. These high frequency pulses are primarily used in echolocation.
Pulsed calls are the most common vocalization of Commerson's dolphins.
- Experts think these calls function in group recognition and coordination of behavior.
- Commerson's dolphins make these calls at frequencies of about 0.5 to 25 kHz, with peak energy at 1 to 6 kHz.
The term echolocation refers to an ability that odontocetes (and some other marine mammals and most bats) possess that enables them to locate and discriminate objects by projecting high-frequency sound waves and listening for echoes.
A Commerson's dolphin echolocates by producing sound pulses and then receiving and interpreting the resulting echo.
- The pulse trains pass through the melon (the rounded region of a Commerson's dolphin's forehead), which consists of lipids (fats). The melon acts as an acoustical lens to focus these sound waves into a beam, which is projected forward into water in front of the dolphin.
- These sound waves bounce off objects in the water and return to the dolphin in the form of an echo.
- The major areas of sound reception are the fat-filled cavities of the lower jaw bones. The lower jawbone receives and conducts sounds through the lower jaw to the middle ear, inner ear, and then to hearing centers in the brain via the auditory nerve.
- The brain receives the sound waves in the form of nerve impulses, which relay the messages of sound and enable the dolphin to interpret the sound's meaning.
Most dolphins that are confirmed echolocators, like the bottlenose dolphin, use broadband clicks for their echolocation. The acoustic energy in these brief clicks (about 50 to 128 microseconds) is spread over a wide band of frequencies including the lower part of the human hearing range.
- However, Commerson's dolphins produce directional, narrow band pulses. Each pulse lasts about 100 to 1,200 microseconds. The acoustic energy in these longer pulses is in a narrow band of frequencies very near the top of the known dolphin hearing range.
Dolphins can determine size, shape, speed, distance, direction, and even some of the internal structure of objects in the water. They are able to learn and later recognize the echo signatures returned by preferred prey species.
The high frequency pulses of Commerson's dolphins are good for "seeing" fine details of objects on the seafloor. This correlates with the coastal seafloor food preferences of these dolphins.