In most cases, once radio communication with the bridge has been established, use of the boat and bridge sound signals are not necessary and operation of the sound signals is left up to the captains. Often, boats will not use the loud horns after dark as a courtesy to city residents.
So, most often when you hear the customary horn-blowing sequence of "long-short-short" it is a friendly salute to the Port of Duluth from the ship captain and is responded to by the lift bridge operator with the same sequence.
While the famous sound may be a courtesy, it has long been part of the nautical tradition of this great inland sea port.
General Navigation Signals
There are also some "Rules of the Sea" for mariners that are used world-wide:
The law prescribes signals for vessels in sight of each other to indicate the intended course of a vessel when necessary for safe navigation.
One short blast (1 second) of the horn or whistle will show an intention to direct course of vessel to own starboard (right).
Two short blasts will show intention to direct course of vessel to own port (left).
Three short blasts will indicate the vessel's engines are going astern (in reverse).
Five or more short and rapid blasts is a danger signal used when the other vessel's intentions are not understood or where the other vessel's indicated course is dangerous.
Prolonged blast (4 to 6 seconds) will indicate situations of restricted visibility.