There are about 49 species of dolphin known currently. Within these 49 species, they're broken up into distinct families: oceanic dolphins (38 species), porpoise family (7 species) and four distinct species of river dolphins.
One thing that all of these dolphins share is their sense of hearing. Dolphin sounds and hearing, also known as SONAR and echolocation, provide dolphins with sophisticated communication techniques that are similar to how people communicate with each other. The dolphin hearing range is also broader than that of many species, which allows them to hear specific sound sand frequencies that humans cannot.
Dolphins use small ear openings on both sides of their heads to listen to or hear sounds. These small openings are what they would usually use for hearing when they are not underwater. To hear sounds underwater, they make use of their lower jawbone, that conducts sounds to their middle ear.
Dolphin sounds are used for communication between dolphins as well as to locate objects and organisms underwater. There's even evidence that dolphins "talk" to each other assigning certain sounds as names.
Dolphins use echolocation underwater, much like whales do. Echolocation allows dolphins to locate objects underwater by transmitting sound waves. They generate a high-pitched sound pulse or click in their foreheads that send sound signals into the water. The echo produced by the sound bouncing off objects helps the dolphins in locating the objects, even determining how far away the objects are located.
The dolphins sense the returning sound vibration by feeling the pulses on their jaws. Each object or animal underwater sends off different echoes, which the dolphins can differentiate. Echolocation helps dolphins determine not only the distance of an object but also the texture, shape and size of the object. It works because water is an excellent sound transmitter, which can transmit sound five times faster compared to air.
Dolphins use this in order to communicate with each other, understand the location of predators and find/capture food.
Other animals that use echolocation include:
There's also evidence that blind humans can be taught to use echolocation.
SONAR (Sound Navigation And Ranging) is the method that dolphins and whales use in navigating under murky waters. As explained in echolocation, they use the sound transmissions that echo back to locate things. Even when it is dark underwater, they can still find food and avoid dangerous places. Dolphins produce two kinds of sounds, the high-pitched whistling sound and the rattle or clicking sound. The whistles act as communicators while the rattles or clicks act as the SONAR.
To better understand the quality of the dolphin's sense of hearing, it can be compared to the hearing of humans, dogs and whales. Dolphins have sharper hearing senses and a broader range than humans do. The human hearing range is sounds from 20 Hz to 20 KHz while the dolphin hearing range is 20Hz to 150 KHz. This means dolphins can hear seven times better than humans do.
When comparing dogs to humans, dogs can also hear far better than human beings do. Dogs are able to hear high frequencies the humans cannot hear and about two times better. However, the dolphin has a hearing range that far exceeds than that of dogs (about five times better than dogs). Of all mammals, dolphins are able to hear and produce some of the highest frequency sounds.
When compared to whales, dolphin sounds are usually communicated using high frequencies while whales often use low frequencies. The whales can communicate at farther distances (several hundred or kilometers away) than dolphins can, though.