What is the defining characteristic of sound signals on the water?

Sound signals on the water serve as crucial communication tools for boat operators, especially in conditions where visibility is limited. The requirement that they must be audible for at least one-half mile is significant because it ensures that other vessels can hear these signals from a distance, allowing for effective navigation and the prevention of collisions.

This visibility requirement aligns with maritime regulations that prioritize safety and the need for clear communication on the water. When vessels are in close proximity or operating in restricted visibility, the ability for sound signals to be heard over a substantial distance is vital. By ensuring that these signals can reach other boaters, regulations help maintain a safe environment on the water.

The other options do not represent the established requirements for sound signals. For instance, not all sound signals consist of a specific mix of sounds, nor is there a requirement for them to be audible for an entire mile. Additionally, sound signals are not optional; they are mandated for use when visibility is poor, reaffirming the importance of safety on the waterways.