In usual circumstances, ships send sound signals at specific frequencies to communicate with nearby vessels or ports. Large and medium ships typically have two air horns installed, one at the bow and one at the stern, with a distance of more than 100 meters between them, and they cannot sound simultaneously. In some cases where the transmission of whistle signals is severely attenuated due to obstructions on board, collision avoidance rules allow for the installation of two whistles in a "combination" for use; and it is explicitly stated that the distance between these two whistles must be within 100 meters, and they sound simultaneously.

A foghorn is a warning signal sent out during heavy fog to alert nearby vessels to take evasive action. Foghorns have shorter wavelengths and produce deep sounds.