The Physics of Underwater Sound

Sound is a mechanical wave that travels through a medium as alternating compressions and rarefactions. In water, the density is roughly 800 times greater than air, which dramatically alters how sound behaves. The speed of sound in seawater is not constant; it depends primarily on temperature, salinity, and pressure (depth). Typical values range from about 1,450 m/s in cold, shallow water to over 1,550 m/s in warm, deep water. The general formula for sound speed (c) in seawater is approximated by:

c = 1449.2 + 4.6T − 0.055T² + 0.00029T³ + (1.34 − 0.010T)(S − 35) + 0.016D

where T is temperature in °C, S is salinity in PSU, and D is depth in meters. This variability creates layers and channels that focus or scatter sound, a key concept for marine biology and naval operations.

Sound Propagation and Attenuation

As sound travels underwater, its intensity decreases due to spreading and absorption. Spreading loss follows geometrical laws: spherical spreading in deep water (loss of 6 dB per doubling of distance) and cylindrical spreading in shallow water (3 dB per doubling). Absorption is frequency-dependent — low-frequency sound (below 1 kHz) can travel hundreds or thousands of kilometers, while high-frequency sound (above 100 kHz) is absorbed within meters. This is why blue whales communicate with infrasonic calls around 20 Hz, while dolphins use echolocation clicks at 100 kHz or higher.

The Underwater Soundscape

The underwater environment is filled with natural and anthropogenic sounds. Natural sources include wind, rain, breaking waves, ice cracking, seismic activity, and biological sounds from marine animals. Human activities add noise from shipping, sonar, seismic air guns, pile driving, and underwater construction. This composite acoustic environment is called the soundscape, and it plays a critical role in the behavior and ecology of marine organisms.

The SOFAR Channel

One of the most important features of underwater acoustics is the SOFAR (Sound Fixing and Ranging) channel. At intermediate depths (typically 800–1,200 m in mid-latitudes), temperature and pressure create a sound speed minimum. Sound waves that travel within this layer are refracted back toward the axis, allowing them to propagate over global distances with very little loss. Marine mammals such as whales use the SOFAR channel for long-range communication. For example, a blue whale’s call can travel thousands of kilometers when channeled in the SOFAR duct.

Marine Biology and Acoustics: A Deep Connection

Sound is the most efficient way for marine animals to sense their environment because light penetrates only the upper few hundred meters, and chemical cues diffuse slowly. Many species have evolved specialized hearing organs and sound production mechanisms. Studying these adaptations falls under the field of bioacoustics.

Sound Production and Reception in Cetaceans

Cetaceans (whales, dolphins, and porpoises) are divided into two suborders: odontocetes (toothed whales) and mysticetes (baleen whales). Toothed whales produce broadband clicks for echolocation, allowing them to form images of their surroundings. They also produce whistles and burst pulses for social communication. Baleen whales produce low-frequency songs and calls, often used for mating displays and group cohesion. The mechanisms differ: odontocetes have phonic lips in their nasal passages, while mysticetes likely use laryngeal structures or air sacs.

Hearing in cetaceans is highly specialized. Odontocetes receive sound primarily through their lower jaw, which conducts vibrations to the ear bones. Mysticetes may hear through bone conduction or the ear canal itself. This sensitivity enables them to detect faint sounds over large distances, but also makes them vulnerable to noise pollution.

Sound in Fish and Invertebrates

Fish produce a variety of sounds using their swim bladder (drumming muscles), stridulation (rubbing bones or teeth), or fin movements. These sounds serve in courtship, aggression, and alarm. Some fish can hear through their swim bladder, which acts as a pressure transducer, combined with the inner ear. Invertebrates such as snapping shrimp, lobsters, and crabs also produce sounds — the pistol shrimp’s claw snap creates a cavitation bubble that reaches over 200 dB re 1 µPa, one of the loudest biological sounds.

Anthropogenic Noise and Its Ecological Impacts

Human-caused noise has increased dramatically in the oceans over the past century. Shipping noise dominates the low-frequency band (10–500 Hz), while sonar and seismic surveys add high-intensity pulses. Studies have shown that chronic noise exposure can cause:

  • Hearing loss or temporary threshold shifts in marine mammals
  • Behavioral changes such as avoidance of important habitats
  • Masking of communication calls, reducing the range over which animals can interact
  • Physiological stress responses, as measured by stress hormones
  • Disruption of foraging and feeding efficiency

Mass strandings of beaked whales have been closely linked to naval sonar exercises, leading to regulations in many countries. Understanding these impacts is essential for designing mitigation measures, such as seasonal restrictions, reduced vessel speed, and quieter propulsion technologies.

Applications of Underwater Acoustics in Research and Conservation

Modern acoustic technology provides powerful tools for studying and protecting marine life. These include:

Passive Acoustic Monitoring

Passive acoustic monitoring (PAM) uses hydrophones to listen for animal sounds without interfering. Networks of autonomous recorders can track whale migrations, estimate population abundance, and detect rare species. For example, the NOAA Pacific Marine Environmental Laboratory maintains long-term acoustic arrays in the Pacific to monitor blue and fin whales. PAM also helps enforce shipping speed limits in whale habitats.

Active Acoustics: Sonar and Echosounders

Active systems emit sound pulses and analyze echoes. Echosounders are used to map seafloor depth and detect fish schools. Side-scan sonar images the seafloor in high detail, while multibeam sonar provides 3D bathymetry. These tools are essential for habitat mapping, fisheries management, and detecting underwater objects.

Acoustic Deterrents and Mitigation

Acoustic deterrent devices (ADDs) are used to keep marine mammals away from dangerous areas such as fishing nets or construction sites. However, care must be taken to avoid causing hearing damage — this is an active area of research. The IUCN and other organizations publish guidelines for the safe use of acoustic devices in conservation.

Challenges and Future Directions

Despite significant advances, many questions remain. The full impact of chronic noise on entire ecosystems is still poorly understood. Climate change is altering ocean temperature and stratification, which will change sound propagation patterns. New technologies, such as machine learning for automated sound classification, promise to accelerate analysis. Collaborative efforts between acousticians, marine biologists, and oceanographers are needed to address these challenges.

To explore further, see the Discovery of Sound in the Sea project for educational resources, or the Acoustical Society of America for peer-reviewed research.

Understanding the principles of sound in underwater acoustics and marine biology is not just an academic pursuit — it is vital for effective conservation and sustainable use of the ocean. As human activities expand, protecting the acoustic environment will become increasingly crucial for the health of marine ecosystems.