Underwater Sound Propagation

Underwater sound generally travels faster because water resists compression much more strongly than air, and it can travel farther because water absorbs relatively little sound energy, especially at low frequencies. These are distinct properties: sound speed describes how quickly a disturbance travels, while attenuation describes how its strength decreases with distance.

Why sound travels faster in water

Sound is a mechanical wave transmitted through successive compressions and expansions of a medium. Its speed depends on the medium's resistance to compression relative to its density.

• For a fluid, v = √(B/ρ), where B is the bulk modulus, a measure of resistance to compression, and ρ is density.
• Water has a much higher bulk modulus than air. Although its greater density tends to reduce wave speed, its much greater resistance to compression more than offsets this effect.
• Sound travels at approximately 1,500 m/s in seawater, compared with about 340 m/s in air; the precise values depend on conditions.

Why sound can travel farther underwater

Sound becomes weaker through spreading, absorption and scattering. Its higher speed alone does not explain its greater range underwater.

• Absorption converts sound energy into heat. Absorption in seawater is relatively low for low-frequency sound, allowing such signals to remain detectable over long distances.
• Low-frequency sound generally travels farther than high-frequency sound underwater because absorption increases strongly with frequency.
• Ocean sound speed varies with temperature, salinity and pressure. These variations cause refraction, or bending of sound paths.
• A deep sound channel can guide sound around a depth of minimum sound speed, limiting its vertical spreading and allowing long-distance transmission.
• Greater range is not universal: frequency, water depth, seabed properties, bubbles and other sources of scattering influence how far a signal remains detectable.

Why this matters for underwater noise pollution

The same conditions that support animal communication can also spread human-generated noise over large areas. Shipping, seismic surveys and sonar can therefore affect habitats beyond the immediate source.

• Acoustic masking occurs when noise makes biologically important sounds harder to detect, potentially interfering with communication, navigation and finding prey.
• Sufficiently intense sound can damage hearing; biological effects depend on sound intensity, frequency, exposure duration and the species concerned.