Dolphins give off high frequency vocalizations or clicks using their nasal passages and listen for echoes reflecting from other surfaces in the water, as noted by Sea World. They use this ultrasound to help navigate their way in the water.
Vocalizations play a key role in dolphin communication. They can use a wide variety of different sounds to communicate with other dolphins, including whistles, squeaks and clicks. They tend to use low frequency sounds for communicating with each other, as these sounds travel farther underwater. However, they can also produce very high frequency sounds of 40 to 150 kHz. These are useful for navigation purposes.
High frequency sounds can only travel short distances underwater. When dolphins give off these sounds, they bounce off objects located nearby. When these sounds bounce back, dolphins' brains process the nature of the echoes and interpret which kinds of objects are in the area. This process is known as ecolocation. It is useful for hunting, sensing nearby predators and navigating under conditions with limited visibility. Dolphins can project two different high frequency sounds at the same time. This likely helps improve the accuracy of the ecolocation signals, according to a June 2011 article in Science News.