Sound is the only thing that carries well

Why light and radio fail underwater, and what is used instead.

Sound is the only thing that carries well underwater because light is absorbed and scattered by water itself within a short distance and radio waves are damped out almost immediately by the same seawater that would otherwise carry them, while a sound wave, a physical vibration passed from one molecule to the next through a medium far denser and stiffer than air, travels through that same water with remarkably little loss, which is why every underwater vehicle that needs to sense its surroundings, communicate, or work out where it actually is ends up leaning on sound rather than on the senses that work perfectly well everywhere else.

The physics of underwater acoustics

Light fails underwater because water molecules, and whatever fine particles happen to be suspended among them, absorb and scatter light far more aggressively than air does, stripping out the longer wavelengths first and leaving the fading blue-green light divers recognise from only a modest depth before even that gives way to complete darkness. Radio waves fail for an entirely different reason, since seawater conducts electricity well enough that it behaves, from a radio wave's point of view, almost like a sheet of metal, draining the wave's energy away into circulating currents within the water itself over a distance of only a few metres rather than letting it travel freely the way it does through air. Sound is not an electromagnetic wave at all, it is a mechanical disturbance, a pattern of compression passed from one molecule to its neighbour, and water's own density and stiffness, the very properties that make it feel so much heavier to push through than air, are exactly what let it carry that compression onward efficiently rather than swallowing it the way it swallows light and radio. Those same properties also let sound travel through water noticeably faster than it manages through air, a detail with no real bearing on how far the sound reaches but a useful reminder that water is not simply a denser obstacle for sound, it is, in its own way, a better medium for carrying it than air ever was.

The tapping-underwater comparison

Two swimmers at opposite ends of a pool can hear a fingernail tapped against the pool wall with startling clarity, a small, precise sound arriving almost undiminished across the whole length of the pool, while the same two swimmers trying to talk to each other underwater find their voices muffled, distorted, and barely intelligible past a few metres, and trying to simply see each other clearly much beyond that distance fails just as quickly. The tap travels well because it is transmitted efficiently into the water through a rigid, well-coupled surface and carries as a clean mechanical vibration, while a voice underwater is fighting both the awkward transfer of sound out of the air in the throat and mouth into the surrounding water and the greater complexity of speech itself, yet even a garbled underwater voice still reaches further than the two swimmers could ever hope to see one another once the water clouds even slightly. Divers who have spent time signalling this way learn quickly to trust a tapped signal on a tank or a rail over a shouted one, not because the tap is a cleverer sound but because it couples directly into the water rather than first fighting its way out of a mouth full of air, the same practical lesson every sonar system since has taken to heart.

One figure worth keeping in mind

Light in clear seawater loses the overwhelming majority of its intensity within the first few tens of metres of depth, while sound pitched in the right range can travel many kilometres, sometimes far more, before fading past the point of being useful, a gap of scale so large that the two senses are effectively operating on entirely different maps of the same ocean. A vehicle relying on light alone is, in practice, working with a bubble of visibility a few body-lengths across, while the same vehicle listening acoustically is working across a horizon many, many times larger, which is precisely why sound rather than sight ends up carrying the weight of sensing, communicating, and navigating across any real underwater distance. Radio's own reach shrinks even faster than light's does, mattering only in the first metre or so of depth, which leaves sound not merely the best of three imperfect options but the only one still functioning by the time a vehicle has descended any real distance at all.

What this changes in practice

Because sound is the one sense with real range, submarines and underwater vehicles use it for jobs that would otherwise fall to sight or radio on land, sending out a pulse and timing its echo to build a picture of what lies ahead in water too dark or too turbid for any camera to help, and passively listening for the sound other vessels or machinery make rather than watching for them. The same acoustic sense also answers the open question the previous article in this set left standing, since a vehicle that has drifted off its intended line with no visual reference to check against can still recover its position by timing sound pulses to and from fixed beacons or the seabed itself, turning the one sense that reaches any distance underwater into the vehicle's primary way of knowing where it actually is. Even a cavitating propeller's telltale rattle, the sound described earlier in this set as a warning of damage building on the blade, is only useful as a warning at all because sound is the one signal reliably available to hear it with in the first place.

Where this stops being true

Close to the surface, in shallow, clear water, or over the short ranges divers and cameras actually work at, light remains genuinely useful and radio still functions perfectly well the instant any part of an antenna breaks the surface into open air, so sound's dominance is specifically a feature of real depth and real distance rather than a universal rule about water itself. It is exactly the conditions this whole set has been built around, a vehicle working alone, out of sight, and far from any surface, where sound stops being one option among several and becomes the only one still standing.