The science, engineering, and art of the generation, propagation, and reception of sound waves constitute the subject of acoustic. Although the Greek origin of the term refers to sounds that can be heard, the subject of acoustics ranges from the very slow, subsonic vibrations of the Earth and its atmosphere to the very rapid ultrasonic vibrations that lie far above the range of human hearing. The impact of the study of acoustics is extremely widespread. Acoustics is important, for example, in the fields of speech and hearing, the production of music, the design of theaters, the control of unwanted vibrations and noise in the environment, and medical diagnosis and therapy.
Sound needs an elastic medium, solid, liquid, or gas through which to propagate. Unlike light and other electromagnetic waves, sound cannot exist in a vacuum. When a small region of the elastic medium is compressed, it exerts a tug on its neighboring regions, and they in turn pull on their neighbors, and that push will also propagate through the medium. If the compressions and expansions are alternated in a regular way, regions of alternating high and low density will propagate throughout the medium, the result is a sound wave. As the wave goes by, each particle in the medium oscillates. Neighboring particles do not oscillate in unison, but are slightly out of phase, alternately moving closer together and farther apart, thus creating the density changes. Sound waves are longitudinal, because the motion of the particles is parallel to the direction of propagation of the wave.