Sound is a mechanical wave. It travels when vibrating particles make neighbouring particles vibrate, passing energy through a material. In air, compressions and rarefactions move from particle to particle. Sound can also travel through liquids and solids, often at different speeds. Most of space is extremely close to a vacuum, with far too few particles to carry ordinary sound between spacecraft. A nearby explosion could produce light, but you would not hear it through empty space. Inside a spacecraft there is air, so sound can travel normally from one astronaut to another.

The same physics matters in theatres, classrooms, studios and hospitals. Designers choose shapes and materials to control reflections and unwanted sound, while engineers use microphones and signal processing to measure or deliberately alter waves.

Waves transfer energy and information without transferring matter from source to receiver overall. Frequency tells us how many oscillations pass each second, wavelength is the spacing between matching points, and wave speed links them through v = fλ. Different frequencies interact with materials differently, which is why engineers can choose particular parts of the electromagnetic spectrum—or particular sound frequencies—for sensing, communication, heating or imaging.