What is the source of the sound of firecrackers?_?

The air vibrates.

This black powder is called black gunpowder, which is made by mixing sulfur, charcoal powder and potassium nitrate (white saltpetre) in a certain proportion. After lighting the firecracker's powder, the black gunpowder inside burned, and a violent chemical reaction quickly occurred.

Charcoal, sulfur and potassium nitrate react to release a large amount of heat and generate various gases such as sulfur dioxide and carbon dioxide. The volume of gunpowder increased by more than 1,000 times, so that the tightly wrapped layer of papyrus finally couldn't bear it: it exploded to pieces with a "pop" sound - this is why firecrackers go off. Extended information

Every Spring Festival, the areas that love to set off firecrackers sound more and more like battlefields. In some cities where the enthusiasm for firecrackers has declined, even if the sound of firecrackers is neither continuous nor intensive, loud and loud noises will erupt from time to time. This kind of lone wolf firecrackers are more likely to scare passers-by.

When there are no firecrackers to set off, people will try their best to make noise. In 1993, Beijing banned fireworks, and many cities followed suit. As a result, not only "electronic firecrackers" that simulated the noise of firecrackers immediately appeared on the market, but also balloons named "Happy Balls" appeared in large quantities on the eve of the Spring Festival for people to pop for fun when celebrating the New Year.

Even before the invention of gunpowder, people had a soft spot for noise. The origin of "firecrackers" is that ancient people burned bamboo to make explosion sounds, which were used to drive away ghosts and scare away monsters. The louder the noise, the better the effect. After the Song Dynasty, people used newly mass-produced black powder to be packed into paper tubes or bamboo tubes, and the firecrackers and fireworks made were no longer essentially different from modern fireworks.