I was going through the equipment list and there's no extra information on a signal whistle. I mean, they bother listing that rations would be jerky, dried fruit, and nuts; but they have nothing for signal whistle. I figure this would be rather important information for players and the DM.
I blow my signal whistle. How far does the sound carry? 300 feet? Half a mile? The game doesn't say. Do any other editions provide relevant information on this?
This was a hard question to answer, because companies that sell signal whistles are weirdly reticent about giving distances that you can hear the whistle, probably because there are so many factors that can change exactly how far you can hear a whistle in real life.
There is no description given for signal whistle distances in either 5th ed or 3.5. In both editions, it's on the list of equipment, but doesn't have a detailed description. In general, this means that it's usefulness is entirely up to the DM to determine, which isn't super helpful.
I did some internet research, and it seems that the average signal whistle sounds at 110-120 db, and can be heard at least a half mile away. There isn't a lot more detail I can add here, but I think it would be fair to say that you hear the whistle automatically out to a half mile, and need to make a Wisdom (Perception) check to hear it out to a mile.
If you want to look at other systems, there is a much more detailed write-up of signal whistle mechanics in Pathfinder. The upshot of those rules is that you can easily hear a signal whistle out to a mile and a half, and much, much farther with a good Perception check. This is far more distance than I've seen anyone mention in articles about signal whistles in real life, so it might not be a good idea to use these rules if you'd prefer your game to resemble real life.
I found an interesting test of whistles here that was comparing modern whistles on a clear fall day. Results varied with the type of whistle but fell between 225 meters to 425 meters. After conversion this is around 750-1400ft, or 12-24 rounds of dashing for your average D&D 5e character.
Converting this to 5e is going to be up to personal taste. At my table I'll be saying that in general a signal whistle can be heard with DC10 perception if the character can dash to its location in under two minutes, and raise the DC by 1 for each half minute away beyond that.
Take into account, the area in which the whistle is blown. A whistle outside will be carried in the air without interference. A whistle in a cave will bounce off the walls, causing echoes, if the wall are hard, such as rock. But, a wall of soft dirt will absorb the sound, restricting the range, especially if the cave path is not straight.
If you have a chance to explore a cave, you can test this out. In the USA, there are guided tours of caves within a day's drive from just about anywhere.
Another concern for the player is misdirection. If the hearer responds and comes running, but comes to a fork in the path, which way would they choose? When I DM, If the signaler sends another blaster when they responder is at a fork, it will be more obvious which way the sound came from.
Of course, chances are, someone or something that the signaler might not want to hear the signal, has heard the signal as well.
Another thing to consider, low tones, such as a horn blow, travel farther than high tones, such as a whistle. This link Low ton vs high tone explains this. This is why fog horns are super low.
Another thing to consider, high tones are used often because the din of constant noise, such as machinery, or groups yelling or talking, mute out low tones. This is why factories use higher pitched whistles for lunch break and end of the shift calls.
