What are the main limitations faced by acoustic communication technology for underwater robots?

Compared to the Wi-Fi or 5G we use daily on our phones, underwater communication is practically in the Stone Age. Here are the main challenges:

1. Snail-like Speed Imagine streaming videos or playing games online; data flies by. But underwater, acoustic waves are the primary communication method, and they travel very slowly, approximately 1500 meters per second. While this might sound decent, it's far slower than the speed of light or electromagnetic waves. This results in extremely low "bandwidth," like a very narrow pipe that can only let a trickle of water through at a time. So, want to transmit an HD video underwater? Forget it. Getting a few words, simple commands, and data back is already good.

2. Frustratingly High Latency Because signals travel slowly, the round-trip latency is very high. For example, if you send a "turn left" command from a ship to an underwater robot several kilometers deep, the signal might take several seconds to reach it. Then, after the robot executes the action and sends back an "I'm done" message, it takes several more seconds. Such latency is fatal for tasks requiring precise operation, feeling like playing a computer game with several seconds of network lag – it's simply unplayable.

3. Echoes and Noise Everywhere The underwater environment is incredibly complex. Part of the acoustic signal travels directly to the receiver, but much of it bounces off the water surface, seabed, fish schools, and rocks before reaching its destination. This is like shouting in a large, empty valley where the listener hears not only your voice but also various delayed echoes from different directions. These "echoes" (technically called multipath effects) scramble the original signal, making it difficult for the robot to distinguish the true command. Moreover, the ocean itself is very noisy, with ship engine sounds, whale and dolphin calls, and even rain. These noises can drown out your signal.

4. Limited Range, Easy to Lose Connection As sound travels through water, its energy is continuously absorbed, becoming weaker over distance, just like your voice gets fainter the further away you are. Higher frequency sounds attenuate faster, but higher frequencies are needed to carry more information (i.e., higher bandwidth). This creates a dilemma: to transmit over long distances, you need low-frequency acoustic waves, but low-frequency waves mean even slower "internet speed." Thus, range and speed are natural adversaries in underwater communication; it's difficult to achieve both.

In summary, the physical properties of water as a medium make acoustic communication slow, laggy, noisy, and short-ranged. This means most underwater robots can only operate autonomously based on pre-programmed instructions or perform non-real-time, simple remote controls, which is a completely different concept from the instant wireless communication we experience on land.