Whales rely heavily on sound to navigate their underwater world. Unlike land, the ocean depths are often dark, making hearing their primary sense. Sound behaves uniquely in water, enabling whales to communicate, locate food, and perceive their environment across significant distances. Understanding how far their hearing extends is crucial given this dependence on acoustics.
The Physics of Underwater Sound
Sound travels through water much more efficiently than through air due to its higher density. While sound moves at approximately 340 meters per second in air, its speed increases to around 1500 meters per second in water, nearly five times faster. This denser medium also causes sound waves to lose less energy as they propagate, allowing them to travel farther.
The ocean contains a unique phenomenon known as the SOFAR (Sound Fixing and Ranging) channel, a horizontal layer typically found at depths of about 1,000 meters. Within this channel, the speed of sound reaches its minimum due to the interplay of temperature and pressure. Sound waves entering the SOFAR channel become trapped, allowing them to travel thousands of miles without significant loss of signal. This natural waveguide enables long-distance acoustic communication and perception for marine life.
How Far Different Whales Can Hear
Whales are broadly categorized into two groups: baleen whales and toothed whales, each possessing distinct hearing capabilities suited to their lifestyles. Baleen whales, such as blue whales and fin whales, primarily use low-frequency sounds for communication and navigation. These low-frequency vocalizations travel exceptionally long distances underwater.
Blue whales, for example, produce calls that can be heard by other whales up to 1,000 miles (1,600 kilometers) away under optimal oceanographic conditions. Fin whale calls can travel even further, potentially reaching up to 6,000 kilometers (about 3,700 miles) in the deep sound channel, compared to 250 kilometers at the surface. These extensive ranges allow these large whales to communicate across vast ocean basins, which is essential for finding mates and coordinating migrations.
Toothed whales, including sperm whales and orcas, utilize higher-frequency sounds, primarily for echolocation to hunt prey and navigate their environment. Sperm whales, known for producing some of the loudest sounds, use powerful clicks up to 235 decibels to locate prey, detecting them hundreds of meters away. While their echolocation clicks are highly directional and effective for close-range tasks, their communication sounds can still be heard over tens to hundreds of miles, depending on conditions. Orcas, for instance, can hear each other over tens of miles, with some estimates suggesting around 30 miles in optimal conditions. Their hearing range extends from about 0.5 kHz to 125 kHz, with best sensitivity around 15 kHz.
Human Noise and Whale Hearing
Human activities have altered the underwater soundscape, introducing noise that impacts whale hearing and behavior. Common sources of this anthropogenic noise include commercial shipping, seismic surveys for oil and gas exploration, and naval sonar. Shipping, in particular, contributes a pervasive low-frequency din that has caused background noise levels in the ocean to double every decade over the last 50 years.
This increased noise can mask natural sounds, making it difficult for whales to detect crucial acoustic cues for communication, navigation, and foraging. Acoustic masking effectively reduces their functional hearing range, forcing whales to expend more energy to be heard or to avoid noisy areas. Exposure to intense or chronic noise can lead to behavioral disruptions, such as altered migration paths, feeding patterns, or increased stress levels. In extreme cases, loud sounds like military sonar and seismic air guns have been linked to physical harm, including temporary hearing loss or causing whales to strand. These impacts underscore the growing concern for the acoustic health of marine environments and the species that depend on them.