The SOFAR Channel: How Sound Travels Thousands of Miles in the Ocean

Marine organisms, including fish, use sound as a critical sensory cue for navigation, habitat selection, and communication; however, human activities such as overfishing, climate change, and motor boat noise pollution are degrading ocean soundscapes, threatening marine ecosystems and the life cycles of species like coral reef fish that rely on acoustic cues to find their way home.

This section explores how marine organisms communicate through sound. Bioacoustics, ecoacoustics, and ecotremology are the main scientific branches studying marine sounds. Herring communicate using gas emissions creating microbubbles forming an 'alphabet.' Cetaceans are the most studied marine mammals, with mysticeti producing sounds at 186 dB capable of communicating thousands of kilometers. Toothed whales use biosonar for environmental exploration and can use sounds as weapons. Sperm whales produce sounds up to 190 dB, far exceeding jet engine noise, using combined sounds and vibrations to stun giant squid.

The ocean contains extremely loud biological sounds that can travel vast distances. The sound known as 'Julia' was recorded in 1999 and was so powerful it was heard across the entire equatorial Pacific Ocean. Such extreme marine sounds are believed to originate from large marine animals, potentially being among the loudest sounds ever recorded in nature.

The ocean is an acoustically efficient environment where sound travels 2,000+ miles with minimal energy, enabling marine animals like whales to communicate across vast distances using low-frequency infrasonic calls that travel through the SOFAR channel; however, human-generated noise pollution from shipping, seismic surveys, and wind farms has increased ocean noise tenfold in just 50 years, disrupting marine life communication, navigation, and survival, as evidenced by incidents like the 2000 Bahamas stranding of 16 beaked whales caused by Navy sonar, and research showing stress hormone levels in North Atlantic right whales dropping 10-fold when shipping noise decreased post-9/11.

This final segment explores the diverse sounds animals produce for survival and communication. Willow tits, walruses, midshipman fish, and various marine species demonstrate how different organisms use sound for mating, territory defense, and social interaction. The bioduck sound, mysterious for decades, was identified in 2014 as coming from Antarctic minke whales. The Gabardi damsel fish uses aggressive vocalizations during breeding season. Beluga whales trained by the US Navy demonstrated remarkable vocal learning abilities, even imitating human speech. These examples illustrate the evolutionary importance of sound in animal behavior and the ongoing scientific effort to understand and catalog the rich acoustic world of non-human species.

Anti-submarine warfare (ASW) relies on two primary sonar detection methods—passive sonar, which listens for target-generated noise and provides stealthier detection but requires the helicopter to hover below 50 feet for several seconds, and active sonar, which emits pings to detect objects within 5 nautical miles with 100% certainty; the effectiveness of both depends critically on target speed, as faster-moving targets generate more detectable noise, and the Mark 46 torpedo system offers dual seeker head modes (passive and active) with configurable pitbull ranges to engage submarines within its 12-nautical-mile firing envelope.

Anti-submarine warfare (ASW) is the naval capability to detect, track, classify, and localize submarines using sonar technology, which includes both active sonar (actively transmitting sound into the water) and passive sonar (listening for underwater noises); modern ASW operations employ sophisticated equipment such as hull-mounted sonar arrays, towed sonar arrays, expendable bathythermographs for measuring temperature-depth profiles, and surface torpedo tubes to engage subsurface threats, with sonar technicians using specialized consoles to interpret sensor data and create underwater situational awareness.

Anti-Submarine Warfare (ASW) is a critical military discipline involving aircraft, naval vessels, helicopters, drones, satellites, and submarines to detect and destroy enemy submarines. Modern submarines serve dual roles as offensive weapons and deterrents, making their detection extremely challenging due to minimal acoustic emissions and the ocean's natural acoustic environment filled with geological, biological, and human-generated sounds. Sonar systems are the primary detection tools, divided into passive types that only listen without revealing position, and active types that emit sound pulses and analyze echoes for detailed imaging. Active sonar provides superior precision but faces limitations from underwater obstacles and the asymmetry where transmitted signals can be detected at twice the echo return range, giving submarines tactical advantage.

During World War II, sonar technology created a critical blind spot in anti-submarine warfare known as the 'instantaneous echo' phenomenon. When sonar pulses traveled through water, the speed of sound meant that echo returns from targets at certain ranges merged with the original transmitted pulse, making it impossible for human operators to distinguish between the turning signal and the original pulse. This occurred because the time delay between transmission and reception became too short for human perception to differentiate. The situation created a dangerous scenario where enemy submarines could hide within this range while evading depth charge attacks. This blind spot was particularly problematic for Allied forces hunting German U-boats in the Atlantic, as it allowed submarines to execute evasive maneuvers and leave the scene undetected when getting closer than the sonar's minimum effective range.

The AN/ CRT-1 sonar system, developed by RCA for the US military during WWII, revolutionized anti-submarine warfare by enabling aircraft to detect U-boats through underwater acoustic signals; this technology transformed U-boats from hunters to hunted, with the system's ability to triangulate submarine positions through multiple frequency analysis and its deployment in convoy protection missions ultimately contributing to the decisive Allied victory in the Battle of the Atlantic.

SOSUS (Sound Surveillance System) is a classified network of underwater hydrophones strung across the ocean floor designed to track Soviet submarines. During the Vela Incident investigation, SOSUS detected an acoustic signal in the South Atlantic consistent with an underwater or near-surface detonation, placing the event in the ocean rather than on land or underground.

Hydroacoustic monitoring involves listening to ocean sounds to detect and identify vessels. Naval personnel learn to distinguish between mechanical sounds (such as submarine propellers) and natural sounds (such as whale songs). This skill requires patience, absolute concentration, and the ability to interpret subtle acoustic patterns in the ocean environment.

The Izu-Bonin-Mariana Arc is a continuous 2,800 km subduction system from Tokyo to Guam. Despite being one geological system, administrative boundaries create different governance structures. Ahyi sits 541 km from Japan and 591 km from Saipan. There is not a single monitoring instrument on Ahyi. Instead, a network of distant ears uses the SOFAR channel (300m-4km deep) to carry low-frequency sound for thousands of kilometers. Hydrophones at Wake Island (2,270 km away), Guam, and Chichijima detect T-waves from volcanic activity. This is not a Mariana arrangement but a regional story.

The United States operates two massive hydroacoustic monitoring systems: the Pacific Ocean Hydroacoustic Behemoth and the Atlantic Ocean Hydroacoustic Leviathan, which span from the Arctic to the Antarctic and are positioned along oceanic rifts for nuclear test ban treaty monitoring and seismic detection purposes.

Hydroacoustic monitoring detects underwater nuclear explosions and other oceanic phenomena using two station types: hydrophone-based stations (like HA11 in the Pacific and HA04 in Crozet Islands) that deploy hydrophones at depth via cables, and T-phase stations (such as those on Socorro Island in Mexico) using three-component seismometers on islands. The IMS has installed and certified 11 hydroacoustic stations providing comprehensive global ocean coverage. These stations detect not only nuclear tests but also tsunamis, whale vocalizations, and atmospheric changes. During the 2015 Chile earthquake, hydroacoustic stations recorded both the seismic event and subsequent tsunami. Recent research using hydrophone data revealed reduced ocean noise levels during pandemic lockdowns, attributed to decreased ship traffic, demonstrating broader scientific applications beyond treaty verification.

The distribution of naval forces of major powers in the Pacific region and how this distribution itself affects the marine environment is rarely discussed but has documented consequences. Naval exercises, torpedo tests, and hydroacoustic submarine detection systems all generate intense noise in the water environment. Medium-frequency active sonars used by naval fleets of several countries operate at frequencies close to those whales use for navigation and communication. The connection between military sonars and mass strandings of whales has been established by several studies, though naval authorities long and stubbornly disputed this. Now this connection is recognized sufficiently to have several countries limit use of certain sonar types in areas of active whale activity. However, noise from naval activity is only part of the acoustic pollution of the Pacific. Commercial shipping - container ships, tankers - also creates constant low-frequency noise background that has doubled in intensity over the past 50 years in most Pacific regions. This means marine animals using sound for finding mates, communicating with offspring, and navigation now do so in an environment significantly noisier than the one they evolved in. Humpback whales have already changed the frequency of their songs over recent decades, presumably to better distinguish their signals against increased anthropogenic noise.

Anthropogenic noise pollution refers to the alteration of the ocean's acoustic environment due to human activity. Ships produce noise across a wide range of frequencies. Military sonars create powerful acoustic impulses. Seismic surveys for oil and gas exploration use air guns that produce explosion-like signals every few seconds for weeks. This fills the ocean with sound that did not exist 100 years ago. This affects marine mammals whose navigation and communication depend on acoustics, and it also creates additional noise in hydrophone data that complicates identification of natural sources.

Human-generated noise has dramatically increased in ocean environments. Ship traffic noise has doubled every decade since the 1960s, and ambient ocean noise is now dominated by anthropogenic (human-made) sound sources. Measurements show that sound levels in the same ocean locations have increased by 10 dB or more over just 30 years. This represents a significant environmental change that affects marine ecosystems and the animals that depend on sound for survival.

Water conducts sound much more effectively than air, especially at low frequencies. The SOFAR (Sound Fixing and Ranging) Channel at 600-1,200 meters depth has minimal sound speed due to combined temperature and pressure effects, allowing sound to propagate over 1,000 km with minimal loss. This channel is used by baleen whales for communication and by military forces for submarine detection. Bioacoustics includes complex humpback whale songs (lasting hours with regional dialects), blue and fin whale calls, dolphin echolocation clicks, and fish choruses during spawning. Anthropogenic noise pollution from ship engines (especially large tankers and container ships) has increased by tens of times over 50 years, creating constant low-frequency hum that masks animal sounds, interferes with communication and food search, and causes chronic stress. Powerful impulsive sounds from sonar and seismic surveys can cause hearing loss and mass strandings.