For communication between devices that are submerged, acoustic communication is generally the practical choice: sound can travel through seawater over useful distances, while ordinary radio-frequency signals are strongly attenuated. Radio is not impossible underwater, but very-low-frequency systems are specialized and constrained—not a substitute for Wi-Fi, cellular service, or a handheld radio. The trade-off is that underwater sound links are slower, lower-bandwidth, and more sensitive to channel conditions than familiar wireless links in air.
How do underwater acoustic communication and radio compare?
| Decision | Acoustic communication | Radio-frequency communication |
|---|---|---|
| Underwater reach | Usually the practical option for submerged devices; achievable range depends on the sound channel, equipment, and deployment. | Strongly frequency-dependent attenuation limits ordinary RF. Lower frequencies can penetrate farther, but require specialized systems. |
| Throughput | Typically constrained, with usable bandwidth and data rate affected by range and conditions. | Do not assume terrestrial RF data rates carry over underwater; frequency and link budget determine whether a link is usable. |
| Latency | Sound travels through seawater at about 1,500 m/s, so propagation alone can add substantial delay over long distances. | Electromagnetic signals propagate much faster, but attenuation can prevent a practical underwater link. |
| Reliability | Multipath, ambient noise, and changing water conditions can affect reception. | Water conductivity and chosen frequency strongly affect attenuation. |
| Deployment | Needs suitable acoustic transducers and modem hardware. | Underwater use calls for specialized low-frequency equipment; ordinary radios are poor substitutes. |
| Reaching the surface | A surface modem or relay commonly bridges the acoustic subsea link to radio above water. | RF can carry data through air from a surface relay to a vessel or shore; crossing the water surface itself needs its own link design. |
For the core comparison, see NOAA’s overview of sound in the ocean and the IEEE survey of underwater communications.
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Why does sound work better underwater?
Seawater attenuates radio-frequency energy, and the amount depends heavily on frequency. Sound, by contrast, can propagate through water over useful distances, which is why acoustic modems are commonly used to communicate with submerged sensors and vehicles. This does not make every acoustic path reliable: temperature, salinity, pressure, depth, and local conditions affect sound speed and propagation. NOAA notes that sound distance varies greatly, depending primarily on water temperature and pressure, and describes sound-speed changes and refraction in the ocean (NOAA).
Sound speed in seawater is about 1,500 m/s, according to NOAA’s page updated in 2026. At that approximate speed, sound takes about five seconds to travel 7.5 km one way; that is arithmetic from the stated speed, not a modem benchmark. Processing, retransmissions, and protocol exchanges can add more delay. A long-distance link may therefore be technically possible yet poorly suited to time-sensitive control.
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What limits an underwater acoustic link?
- Bandwidth and data rate: Underwater acoustic links generally offer constrained bandwidth, and longer range typically means narrower usable bandwidth and a lower data rate.
- Propagation delay: Sound’s speed is far below electromagnetic propagation in air, so distance matters even before device or network processing.
- Multipath and changing channels: Reflections and changing water conditions can make reception vary over time.
- Noise: Ambient noise can interfere with decoding.
These are central challenges identified in the IEEE overview. There is no universal acoustic range or speed: results depend on the equipment, geometry, waterbody, noise, and operating conditions. Sound reaching a distant hydrophone or enabling whale detection does not establish that a digital modem can maintain a decodable data link over the same distance.
Can radio waves travel underwater?
Yes, but the useful reach depends strongly on frequency and system design. The IEEE survey reports approximate examples of up to about 30 m of penetration for very-low-frequency (VLF) signals and several hundred metres for super-low-frequency or extremely-low-frequency (SLF/ELF) signals. These are examples reported in that survey, not guaranteed depths for every antenna, water condition, or link budget (IEEE survey).
Those specialized bands do not make ordinary Wi-Fi, cellular networks, or handheld radios dependable underwater. For a submerged submarine, low-frequency radio is a constrained special case rather than ordinary two-way broadband service. The available figures do not establish a universal depth-and-data-rate curve, so assess a particular system and mission rather than treating any single penetration depth as a general promise.
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How do underwater devices send data to the surface?
A common design uses an acoustic link between submerged equipment and a surface modem or relay, then radio through the air to a vessel or shore. The underwater segment and the surface-to-shore segment are separate links, each with its own equipment and limitations. Simply placing a radio antenna at the surface does not solve propagation across the water itself; the surface transition requires a purpose-designed interface.
Specialist equipment illustrates the approach. Sonardyne describes its Modem 6 family as an acoustic subsea unit for subsea-to-surface data transfer, including sensor-data retrieval and command/control. The manufacturer lists user data rates up to 9,000 bps and depth ratings up to 5,000 m for the specifications shown (Sonardyne Modem 6). These are manufacturer specifications, not independent test results or typical performance; check the exact model and current datasheet, and do not assume maximum rate and maximum depth are available simultaneously.
Quick Recap
How should you choose a communication method?
- Define the mission. Specify the required distance, depth, data volume, update frequency, and acceptable delay.
- Characterize the environment. Account for whether the water is ocean, coastal, or inland, and for noise, geometry, and changing conditions that may affect the sound channel.
- Choose a suitable link technology. For communication between submerged devices, start with acoustic equipment. Consider RF only when a specialized low-frequency system is actually suited to the depth, data needs, and deployment.
- Check the full system specification. Compare range, rate, depth rating, power, transducer or antenna installation, and integration requirements for the specific model and configuration. Treat maximum specifications as separate unless the manufacturer documents them together.
- Plan the surface connection. If data must reach a vessel or shore, include the surface modem or relay and the above-water radio link in the design.
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