Sound is a compression wave: the air is pushed, compresses, pushes the next air and decompresses. The speed of this wave depends on two properties of the air: how much it resists being compressed and how heavy it is. Newton’s formula In the Principia, of 1687, Newton calculated the speed of sound for the first time. Assuming that, when compressed, air obeys Boyle’s law, that is, that pressure is proportional to density, he obtained