This is one of the things you want to do not need.
But they are small enough and cheap enough to get them anyway.
It is an extra loud whistle that you can always carry with you.
Even where people are looking for help, you also have to ask for help for any time, you know it. Moreover, there are many places where the voice is not far enough to hear you.
However, as long as you can blow this whistle, you will be able to draw attention from someone in your ears shot - its a whistle, and probably have more than your voice carried.
If you know the MORSE code, you can also send a message. The easiest messages to remember will be international, s consequential signals - continuous sequence of 3 dots (short blast), 3 dashes (long life) and 3 dots (short explosion again). It is instantly recognizable and will catch the attention of anyone who can listen to it. They will use it to find you and come to your rescue.
I dont have anything to measure decibel levels.