The speed of sound in vacuum is the speed of sound in a perfect vacuum. In a perfect vacuum, there are no molecules to support the propagation of sound, so sound cannot propagate through it. This speed is usually referred to as "the speed of sound" and is usually denoted by c, which is the standard notation for the speed of light. In fact, the speed of light is very close to the speed of sound in a perfect vacuum. The speed of sound in a perfect vacuum is the theoretical upper limit for the speed of sound in any substance.