Submarines detect threats across hundreds of miles of open ocean for a simple reason: sound moves roughly 4.4 times faster through seawater than through air. That speed advantage, combined with water’s ability to carry acoustic energy over vast distances with relatively little loss, gives passive sonar systems aboard modern submarines a reach that no airborne sensor can match. The same physics that makes undersea listening so effective, though, depends on ocean conditions that are not fixed, and shifts in temperature, salinity, and pressure can reshape how far and how clearly a submarine crew can hear.
Why the 4.4-to-1 speed ratio gives submarines an acoustic edge
The core numbers are straightforward. Sound travels at about 1,500 m/s (4,921 ft/s) in seawater and roughly 340 m/s (1,115 ft/s) in air, according to NOAA guidance. That 4.4-to-1 ratio means an acoustic pulse covers nearly a mile of ocean in the same time it would cross roughly a quarter-mile of atmosphere. Speed alone, however, is only part of the story. Water is far denser than air, so it transmits acoustic energy more efficiently, and deep-ocean sound channels can trap that energy and guide it across entire ocean basins with minimal loss.
This is why navies have spent decades building submarines optimized for passive listening. A boat running quietly at depth can pick up engine noise, propeller cavitation, or hull vibrations from surface ships and other submarines at distances that would be physically impossible for an airborne microphone. The ocean itself acts as a waveguide, bending sound waves back toward a minimum-speed layer, commonly called the SOFAR channel, where acoustic signals can propagate for thousands of kilometers.
The practical value of that waveguide depends on knowing the exact sound-speed profile at a given location and time. Temperature is the dominant variable in the upper ocean: warmer water speeds sound up, colder water slows it down. Salinity and hydrostatic pressure add further adjustments at depth. Submarine crews and shore-based analysts build detailed profiles before and during a patrol so they can predict where sound will bend, where shadow zones will form, and at what ranges a contact will fade below the noise floor.
Empirical equations and hydrophone networks that keep the math honest
Two competing families of equations have governed operational sound-speed calculations for decades. The Del Grosso equation, published by V.A. Del Grosso, was derived from laboratory measurements of natural waters and remains a standard reference for deep-ocean work. A rival formulation by Chen and Millero approached the problem from a different experimental baseline. Both equations agree closely near the surface but can diverge by a fraction of a meter per second at extreme depths, a difference that matters when predicting detection ranges over hundreds of miles. A technical review from NIST frames sound speed as a thermodynamic property, providing reference values for water and air that help benchmark these empirical formulas.
Experimental work at institutions such as the University of Miami has measured seawater sound speed across temperature ranges from 0 to 40 degrees Celsius, salinities up to about 40 parts per thousand, and pressures reaching roughly 1,000 bar, conditions that span the full depth range submarines actually patrol. Those measurements fed directly into the empirical relationships that fleet planners still use to convert a temperature-salinity-depth cast into a usable propagation prediction.
On the monitoring side, NOAA’s Pacific Marine Environmental Laboratory operates a network of bottom-mounted hydrophones at fixed coordinates and depths to record ambient ocean noise continuously. That network gives researchers, and by extension naval analysts, a real-time reference against which to compare modeled propagation with actual acoustic conditions. NOAA Fisheries emphasizes that how far sound travels depends strongly on ocean conditions, reinforcing the point that the 1,500 m/s figure is a useful average but not a constant.
Warming oceans and the detection-range question no one has fully answered
The speed of sound in the upper ocean is rising as sea-surface temperatures climb. Warmer surface layers push the minimum-speed axis of the SOFAR channel deeper, altering the geometry of the acoustic waveguide that submarines and fixed listening arrays rely on. If the channel shifts enough, the paths along which sound travels most efficiently will change, and detection ranges for passive sonar could shrink or expand depending on the specific geometry at a given location.
No publicly available study has yet quantified exactly how much median passive-detection ranges will change over the next several decades, and the classified nature of operational sonar performance makes independent verification difficult. What is clear from the physics is that even small shifts in sound-speed profiles, on the order of a few meters per second near the surface, can redirect acoustic energy into different depth bands and create new shadow zones where submarines were previously easy to track.
The same warming trend complicates another growing mission for undersea acoustics: tracking shipping noise and monitoring marine mammals. NOAA’s ocean noise mapping efforts rely on the same sound-speed models to predict how far ship noise or whale calls propagate. If the propagation environment shifts, the acoustic footprints of busy shipping lanes will shift with it, potentially exposing new habitats to chronic noise while reducing exposure elsewhere. Managers who set noise thresholds or define quiet areas will need to account for changing propagation, not just changing traffic volumes.
For navies, these environmental shifts cut in both directions. A submarine operating in a region where the SOFAR channel deepens may find that its own noise couples less efficiently into long-range paths, improving stealth against distant fixed arrays but making it harder to hear targets riding higher in the water column. Conversely, in regions where warming sharpens vertical gradients, surface-generated noise may refract more strongly, filling in shadow zones that once offered a measure of acoustic cover.
Operationally, that means sound-speed profiling becomes even more central to undersea warfare. Modern submarines already carry expendable bathythermographs and other sensors to sample local temperature and salinity. As climate-driven variability increases, crews may need to refresh those measurements more often and rely on dynamic models rather than climatological averages. Shore-based planners, meanwhile, will have to revisit long-standing assumptions about where to place fixed hydrophone arrays and how to weight different ocean basins in global surveillance architectures.
The physics that give submarines their acoustic reach are not going away. Sound will still move much faster and farther in seawater than in air, and the basic 4.4-to-1 speed ratio will remain a useful rule of thumb. But the fine structure of the ocean-its stratified layers, eddies, and evolving temperature fields-will matter more as the climate warms. For undersea forces, the challenge is less about losing an advantage outright and more about keeping up with a moving target: an acoustic environment whose rules are being rewritten, slowly but steadily, by the heat accumulating at the surface.
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*This article was researched with the help of AI, with human editors creating the final content.
