Sound comprises a sequence of audible mechanical waves of oscillating pressure moving through one or more media: air or other gases, liquids, or solids. Like light waves, sound waves can be reflected, refracted, or attenuated by the medium they travel through. Attenuation of sound through still air or water because of viscosity is negligible. But wind or water movement refracts sound, either dispersing or focusing the waves.
Sound travels through gases, plasma, and liquids as longitudinal waves. Sound through solids travels as both longitudinal waves and transverse waves. Longitudinal waves run the same direction of vibration as travel, while transverse waves (in solids) oscillate perpendicular to the direction of propagation.
A sound wave periodically displaces the medium it travels through, the displacement periodicity being the inverse of its frequency. The energy carried by a sound wave converts back and forth between the potential energy or strain of the matter and the kinetic energy of the oscillations of the medium.
As with other waves, sound is a complex phenomenon, with characteristics of frequency, wavelength, amplitude, pressure, intensity (energy density by velocity), and direction. For example, sound can be focused like an optical lens focuses light. A sonic lens can squeeze a sound wave down to a spot smaller than its wavelength, which was long thought impossible.
The ancient Tibetan singing bowl is a bronze vessel that makes a sustained ringing sound when rubbed around the edge by a leather mallet. The Tibetan singing bowl holds a key to sonic fluid dynamics. Rubbing a singing bowl vibrates the water inside the bowl at a frequency that creates Faraday waves, which oscillate at half the speed of the bowl’s vibrations. Water drops that break off from the Faraday waves levitate inside the bowl: sitting on the surface and skipping like stones across water.
Sound frequency is measured in hertz (Hz), coined in honor of German physicist Heinrich Hertz, who proved in 1889 that electromagnetic waves existed.