Underwater Acoustics

Physics of Sound in the Ocean

Sound speed in seawater depends on temperature, salinity, and pressure, increasing approximately 4.5 meters per second per degree Celsius of warming, 1.3 meters per second per part per thousand salinity increase, and 1.7 meters per second per 100 meters depth increase. These dependencies create a characteristic sound speed profile: speed decreases through the thermocline (cooling effect dominates), reaches a minimum between 700 and 1,200 meters depth, then increases again toward the bottom as pressure effect dominates.

The sound speed minimum creates a natural waveguide called the SOFAR channel (Sound Fixing and Ranging). Sound traveling at this depth is refracted back toward the minimum speed axis whenever it tries to escape upward or downward, trapping acoustic energy in a channel where it can propagate thousands of kilometers with minimal loss. Blue whale calls at 15 to 25 Hertz can theoretically travel the full width of ocean basins through this channel. The SOFAR channel was used during World War II to locate downed pilots and later for submarine detection.

Acoustic absorption in seawater increases with frequency. Low-frequency sounds (below 100 Hertz) lose less than 0.001 decibels per kilometer and can travel transoceanic distances. High-frequency sounds (above 100 kilohertz) lose over 30 decibels per kilometer and are useful only over distances of tens of meters. This frequency-dependent absorption explains why long-range communicators (baleen whales) use low frequencies while short-range echolocators (dolphins, porpoises) use high frequencies.

Biological Acoustics

Marine mammals represent the most sophisticated users of underwater sound. Baleen whales produce low-frequency calls (10 to 1,000 Hertz) for long-range communication, potentially spanning hundreds to thousands of kilometers. Blue whale calls at 188 decibels are among the loudest sounds produced by any animal. Humpback whale songs consist of complex hierarchical sequences lasting up to 30 minutes, repeated for hours, with progressive changes through the breeding season that all males in a population adopt simultaneously.

Toothed whales and dolphins use echolocation (biosonar) to navigate and hunt in turbid or dark water. They produce rapid sequences of broadband clicks at frequencies between 20 and 200 kilohertz, interpreting returning echoes to construct acoustic images of their environment. Dolphin echolocation can detect a tennis ball-sized object at 100 meters, discriminate between objects differing by only 0.3 millimeters in size, and determine the internal structure of objects (distinguishing hollow from solid targets). Sperm whales echolocate at depths exceeding 2,000 meters to locate squid prey in complete darkness.

Fish produce and detect sounds for communication, predator avoidance, and spawning coordination. Many reef fish produce sounds during territorial defense and courtship by vibrating swim bladders or stridulating (rubbing) skeletal elements. Coral reef soundscapes, composed of snapping shrimp, fish chorus, and wave sounds, serve as settlement cues for larval fish and invertebrates navigating from open water to reef habitats. Degraded reefs produce quieter, less complex soundscapes that attract fewer settling larvae.

Anthropogenic Noise Impacts

Commercial shipping represents the dominant source of chronic anthropogenic noise, with roughly 60,000 large vessels continuously generating low-frequency noise (below 300 Hertz) that has raised ambient ocean noise levels by 10 to 12 decibels (a 10 to 16-fold increase in sound intensity) at frequencies used by baleen whales over the past 50 years. This elevated background noise reduces the communication range of blue whales from an estimated 1,600 kilometers in pre-industrial conditions to roughly 100 kilometers today.

Seismic surveys for oil and gas exploration use arrays of air guns that produce sound pulses of 240 to 260 decibels every 10 to 15 seconds for weeks to months. These pulses are detectable thousands of kilometers from the source and have been shown to reduce fish catch rates by 40 to 80 percent within 33 kilometers of seismic operations, displace marine mammals from feeding habitats, and cause physiological stress responses in invertebrates including scallops and lobsters.

Military sonar, particularly mid-frequency active sonar (1 to 10 kilohertz) used for submarine detection, has been linked to mass strandings of beaked whales and other deep-diving cetaceans. The mechanism likely involves behavioral responses (rapid ascent, cessation of diving) that cause decompression-like injuries. Multiple mass strandings correlating with naval sonar exercises have been documented in the Canary Islands, Bahamas, Mediterranean, and elsewhere, leading to some operational restrictions in sensitive habitats.