Whistle mix is a general term to refer to any pyrotechnic composition that emits a whistling sound when pressed into a tube and ignited. Whistle mix tends to be very fast burning in a loose state; therefore it is also used as burst charge in small shells. It is friction/impact sensitive and should never be rammed/compacted with a rammer and hammer, only pressed. To work correctly it needs to be very fine, mixed thoroughly and solidly compressed into a thick walled tubed and filled about half full. The remaining half of the tube acts as a resonating cavity, amplifying the whistle sound.
Method Grind the potassium perchlorate by itself in a mortar and pestle or coffee grinder until it is a very fine powder. The same needs to be done to the sodium benzoate. Remember both chemicals need to be very fine, like talcum powder and never grind them together. It is a very high-energy mix and burns extremely fast. Screen the chemicals together by passing the mixture several times through a mesh screen. Once you have done this put the mix into a clip seal bag and close it. Gently shake the mix together until the formula is mixed very well. Be sure to touch an electrical ground before starting to mix to avoid the possible production of static electricity from friction with the bag. Alternatively you can mix this composition via screen method if you wish.