I remember the early days of my childhood when I tried in earnest to whistle just like the big boys. It looked so simple by just puckering up your lips and blowing out. With much coaching and encouragement from my sister and friends I finally accomplished my goal. Then there came a time in my early teens when other kids made a shrill and much louder whistle by curling their tongue against their tightened lips. Sometimes people will use their fingers to assist this technique. My friend and I were bent on achieving this and practiced all day and finally succeeded while driving his dad crazy. Some folks have tried these methods without success, so the cavity whistle may be their salvation to a loud portable whistle for signaling.
A cavity whistle uses a small hollow over which there is a small hole or opening. A person can blow over the small hole and a high pitched tone can be heard. Many items can be used for a cavity whistle such as a pen cap, bullet shell, a clean cut reed segment or a pinched straw.
The most recent cavity whistle that I have learned about is the cap of an acorn. Cover the open space by placing your thumbs together and bending the upper knuckles over the outer edge. Your thumb tips will naturally spread apart, thus creating a small hole. Place your lips on your knuckles and blow into the hole. With practice you can create an extremely loud sound. Similar cavities for this “bent thumbs” method are small plastic or metal bottle caps for water, soda, beer or motor oil.
Even if you can whistle with your lips, learning to use a cavity whistle is another tool in your signal arsenal.