Whistle mix is a general term to refer to any pyrotechnic composition that emits a whistling sound when pressed into a tube and ignited. It is used as a rocket propellant, particularly in small bottle rockets. Whistle mix tends to be fast burning and brisant; therefore it is also used as burst charge in shells.
A typical whistle mix is made from 70% potassium perchlorate and 30% sodium benzoate, measured by weight. The fuel in most whistle compositions is a salt of an organic acid. Compositions with chlorates and gallates were used historically, but have now been abandoned as prone to accidental ignition. These mixtures however, are less sensitive than the formerly used ones, which often contained mixtures with picric acid and potassium picrate.
Whistle compositions tend to be particularly sensitive. To achieve the whistling behavior, the composition must be in a tube and be formed into a solid grain; it otherwise burns too rapidly and just explodes. The composition is typically compressed using an arbor or hydraulic press. Ramming with a mallet (which is acceptable for many other compositions) may result in an explosion.