Sound is an interruption of mechanical energy that propagates through matter as a wave. Sound is characterized by the properties of sound waves which are frequency, wavelength, period, amplitude and velocity or speed. Noise and sound often mean the same thing; when they differ, a noise is a useless sound. In science and engineering, noise is an undesirable component that obscures a signal.
Humans perceive sound by the sense of hearing. By sound, we usually mean the vibrations that travel through air and can be heard by humans. However, scientists and engineers use a wider description of sound that includes low and high frequency vibrations in air that cannot be heard by humans, and vibrations that travel through all forms of matter, gases, liquids and solids. The matter that supports the sound is called the medium. Sound propagates as waves of irregular pressure, causing local regions of compression and rarefaction. Particles in the medium are displaced by the wave and oscillate. The scientific study of sound is called acoustics.