Whales are master communicators in the ocean, relying heavily on acoustic signals to navigate and interact. While humans cannot directly perceive these sounds underwater without assistance, this unique environment shapes how whale vocalizations travel and how we can understand their hidden conversations.
How Sound Travels in Water
Sound travels significantly faster in water than in air, at around 1,500 meters per second in seawater compared to 340 meters per second in air. This difference is due to water’s higher density and lower compressibility, allowing vibrations to transfer energy more efficiently.
These physical properties enable sound waves to propagate further and with less energy loss underwater. Temperature, pressure, and salinity also influence how sound travels through the ocean’s depths. Sound travels faster in warmer water and at higher pressures. Variations in these factors create distinct layers where sound speed changes, guiding acoustic energy through the marine environment.
The Sounds Whales Make
Whales produce a diverse range of sounds, each serving specific biological purposes. These vocalizations are broadly categorized into clicks, whistles, and pulsed calls. Toothed whales, such as dolphins and sperm whales, use high-frequency clicks for echolocation, helping them navigate and find prey. Sperm whale clicks range from 500 Hz to 30 kHz, while other dolphins use frequencies up to 130 kHz.
Whistles and pulsed calls, characterized by squeaks, screams, and squawks, are used by toothed whales for social communication and maintaining group cohesion. These sounds can also indicate a whale’s mood or state. Baleen whales, including humpbacks and blue whales, produce lower-frequency sounds, primarily below 5,000 Hz. Their vocalizations, such as moans, grunts, and thumps, are suited for long-range communication and attracting mates. Humpback whales are known for their elaborate, repeating “songs” that can last for minutes or hours. Blue and fin whales emit some of the lowest frequency calls, ranging from 10 to 100 Hz.
Methods of Listening to Whales
Humans cannot directly hear most whale sounds underwater due to our auditory system’s limitations and water’s properties. Our ears are adapted for sound in air, and water’s density makes direct perception difficult. Specialized equipment, primarily hydrophones, is therefore necessary to detect and record underwater acoustics. A hydrophone functions by converting underwater sound pressure waves into electrical signals, much like a microphone does in air.
Hydrophones can be deployed in various configurations to monitor marine environments.
Types of Hydrophone Deployments
Fixed bottom recorders (moorings) are anchored to the seafloor for continuous, long-term recording.
Towed arrays, with multiple hydrophones pulled behind a vessel, allow for real-time acoustic data collection and localization of sound sources.
Autonomous underwater vehicles (AUVs), gliders, and drifting buoys carry hydrophones for mobile, long-duration monitoring.
Once collected, hydrophone signals are processed and visualized as spectrograms to analyze whale vocalizations.
The ocean also contains natural acoustic pathways, such as the SOFAR (SOund Fixing And Ranging) channel, a layer at approximately 1,000 meters depth where sound speed is at its minimum. Sound waves entering this channel are refracted and “trapped,” allowing low-frequency sounds to travel thousands of miles with minimal energy loss. By deploying hydrophones within this channel, researchers can detect whale calls and other sounds from enormous distances. However, increasing ambient ocean noise from human activities like shipping, seismic exploration, and sonar operations poses challenges.
Why Listening to Whales Matters
Monitoring whale sounds provides invaluable insights into their lives and marine ecosystems. Acoustic data helps scientists understand whale behavior, including foraging, social interactions, and migration routes. This information is particularly useful for tracking species difficult to observe visually. Analyzing vocalizations helps estimate population sizes and distribution, which is fundamental for conservation efforts and policy development.
Acoustic monitoring also plays a crucial role in assessing the impact of human-generated noise on marine life. Shipping traffic, oil and gas exploration, and naval operations introduce noise that can mask whale communications, disrupt their behavior, and increase stress levels. Understanding these impacts is important for developing mitigation strategies and protecting sensitive marine habitats. Passive acoustic monitoring allows for continuous, non-invasive data collection, contributing to long-term scientific research.