Speed of sound
Sound is a trouble of mechanical energy that propagates through matter as a wave (through fluids as a density wave, and through solids as both density and shear waves). Sound is further characterized by the general properties of waves, which are frequency, wavelength, period, amplitude, speed, and direction (sometimes speed and path are combined as a velocity vector, or wavelength and direction are combined as a wave vector).

Humans make out sound by the sense of hearing. By sound, we usually mean the vibrations that travel through air and are audible to people. However, scientists and engineers use a wider meaning of sound that includes low and high frequency vibrations in the air that cannot be heard by humans, and sensations that travel through all forms of matter, gases, liquids, solids, and plasmas.

The substance that supports the sound is called the medium. Sound propagates as waves of alternating pressure, causing limited regions of compression and rarefaction. Particles in the average are displaced by the wave and oscillate. The scientific study of the combination and reflection of sound waves is called acoustics.