I think all the controversy over pea type whistles are overblown (no pun intended.) I cannot think of one survival scenario where blowing on the whistle and worrying about the pea freezing is realistic. If someone is all that worried that the weather is extreme cold enough to freeze the pea to the inside of the whistle, simply keep it in your pocket, inside of a coat or cupped in the palm of your hand between uses as it would be very unlikely that you would be blowing on the whistle nonstop.

As for the best or loudest type of whistle? Lab and controlled testing cannot come close to duplicating anywhere near real world field conditions of wilderness survival whistle use. For example, wind strength, wind direction, rain, snow, humidity, tree cover, vegetation, open terrain, mountain, hills, lakes, rivers, canyon walls, persons overall physical and physiological condition and a thousand other variables. All these conditions and variables will all produce different results as conditions can change dramatically by the minute or hour. In an urban environment, there are 1000's of more variables to think about how faraway a whistle will be heard...or not heard at all.

In reality, the best whistle is the whistle you have on your person when needed the most...

I carry Fox 40 whistles and not that I think they are the best. Rather that the Ministry of Forests and Lands here give away these whistles for free by the 1000's each year. Needless to say, I have more then a lifetime supply of the Fox 40 whistles.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock