I have witnessed and read about a variety of whistle signal formats used by various whitewater groups and associations so it is always best to have discussed before the event what various whistle signals mean.
The two signals that seem to be nearly universal are one long, loud blast, and three loud, long blasts. One blast serves as a "head's up" signal indicating that there is something that requires attention. This most often will be an indication for boaters to hold their position at least until the situation can be properly assessed.
Three long blasts indicate somebody is in distress. This might mean a swimmer or some sort of entrapment scenario. It is not an indication that everyone should start charging downriver to provide assistance. Those who are in a position to accurately assess the problem and are prepared to provide assistance should do so. Others should stay put.
There are some other signals that have been used by some groups. Some will include a two blast signal. Others avoid two blast signals because they can be confused with three blast signals when boaters think they might have missed the first blast. When two blast signals are used they are generally an indication for the group to halt and maintain position.
Some will also include a short, one blast signal to indicate an "all clear" after another signal has been given earlier.
Got it to work. Takes a stiff blow. I’ll try the sonic blast model. Plenty of places to store one. I’m testing them wet. I’d say only needing a small amount of wind to work is an asset. Image bobbing around in a big lake, exerting energy to right your canoe. I’d likely have trouble with a lot of hard blowing,